How to determine if an audio track is a Dolby Pro Logic II mix. Modern multi-channel systems

Also known as:

Dolby Pro Logic - an advanced decoding scheme used in Dolby Surround. It differs in that it provides a center channel, better signal separation and a more accurate match of sound localization in accordance with what is happening on the screen. Please note that the name Dolby Surround applies to both decoding systems: both Dolby Surround, and to Dolby Pro Logic. Although there are two types of decoders Dolby Surround, the encoding device is manufactured in only one version. Format media Dolby Surround work with both types of decoders. Decoders Dolby Surround were largely replaced by Pro Logic decoders.

In addition to decoding the two-channel output sound signal VCR or player laser discs To obtain the left, center, right, and surround audio signals, the Pro Logic decoder typically generates a sixth output signal for the subwoofer. It is a monophonic signal with a frequency band limited to 100 Hz, which combines information from the front channels. It is not necessary to use this subwoofer signal; many home theater systems to reproduce the full spectrum of audio signals, including low frequencies, use wideband speakers of the left and right channels.

Please note that no dedicated channel is used to transmit the subwoofer signal. Instead, this signal is created by mixing the low-frequency signals from the front channels, so that the left and right speakers do not need to produce bass and can be small in size.

It is important to understand that Pro Logic provides four channels of sound reproduction (left, center, right and mono rear). Although two rear speakers are used, they are fed the same monaural signal.

All Dolby Digital decoders, regardless of whether they are 5.1-compatible or two-channel, have a unique downmixing capability for compatibility with any playback system. At your choice, the decoder can create in real time from Dolby Digital 5.1:

• two-channel Dolby Surround stereo for playback in home theater systems Dolby Surround Pro Logic ;
• two-channel stereo for conventional stereo equipment and headphone listening;
• monophonic signal for output to TV.

Downmixing is not used for laserdiscs since they contain separate audio tracks:

• PCM for playback on conventional stereo equipment or Dolby Surround Pro Logic;
• FM for mono playback; FM in 5.1 format

P. LYUBIMOV, Bobruisk, Mogilev region.
Radio, 2002, No. 1

After installing the TDA3810 processor, for example, in an AU receiver with a decoder Dolby Surround Pro Logic and multi-channel AC, monophonic TV sound acquires noticeable “depth”. This effect occurs due to the presence in the decoder for the rear signal of a time delay of several tens of milliseconds, which is absent in a conventional stereo processor. The author talks about installing the processor in the Technics SC-CH60 audio system, used in conjunction with a TV and VCR.

The article described a device based on the TDA3810 sound processor, which makes it possible to obtain pseudo-stereophonic sound from a monophonic TV broadcast signal. This mode of operation of the microcircuit provides additional features when used in a more complex audio system.

The essence of the proposal is to install the TDA3810 chip in a mini-system, receiver or amplifier capable of reproducing stereo soundtracks through a DSPL (Dolby Surround Pro Logic) decoder with a multi-channel amplifier and a set of speakers. Despite the fact that the pseudo-stereo signal is very far from the DSPL format, the introduction of the TDA3810 into such a path creates a fairly impressive spatial sound picture.

The processor diagram is shown in Fig. 1.

It differs from the typical one given in the absence of elements to provide the “Extended Stereo” mode (this mode is not used in this device), as well as indication LEDs. Pin 12 of the DA1 microcircuit is connected to the common wire, which corresponds to the “Pseudo-stereo” mode.

Sketch printed circuit board The processor and the arrangement of elements on it are shown in Fig. 2. The device uses resistors MLT-0.125, capacitors C10 - C13 - K50-35, the rest - KM-4, KM-5.

The processor board is installed in the "Stereo Sound Processor SH-EH60" block of the mini-system. All processor connections are made to the switch board located in the rear of the case. The Uynp entrance is left free. The processor is powered from a +13 V voltage source of the equipment power supply. The inputs of the left and right channels are connected to the standard “VCR INLUR” connectors. To do this, you first need to unsolder the terminals of elements C706, R710 and C705, R709 (designations on the mini-system board) from the connection points to the connectors. The wires of the inputs of the left and right channels of the processor are soldered into the vacated holes. The soldered pins connect: C706 to R710, and C705 to R709. The wires of the processor outputs of the right and left channels are soldered to the connection points, respectively. The location of the connection holes on the switching board is shown in Fig. 3. Next, the board is secured with a self-tapping screw on the stand on the right side of the case (Fig. 3).

Thus, after the work carried out, the VCR input is now intended for connecting sources with monophonic path (the signal is supplied to the left and right channels) - TV, VCR, etc. Stereophonic signals can be supplied to the VDP and EXT inputs of the minisystem.

If you need to connect a source operating in both mono and stereo modes, for example, a Hi-Fi VCR, then the processor must be supplemented with a two-position switch, which is installed on the rear wall of the SB-EH60 unit housing. In the "Stereo" mode, the switch must close the Ucontrol input to the common wire, in the "Pseudo-stereo" mode - leave this input free. A switch is also necessary if the processor is installed in a device that has only one input.

During listening, the excellent performance of the processor as part of the Technics SC-CH60 mini-system was noted. The sound is especially impressive in DSPL mode. If the decoder does not respond to a monophonic signal, then when the processor is turned on, all speakers create a noticeable spatial distribution of sound.

Using the above method, it is easy to equip a TDA3810 processor and another music Center, receiver, amplifier with DSPL decoder.

LITERATURE
1. Brylov V. Stereo sound in ZUSTST. - Radio, 2001, No. 2, p. 9 - 11, 14; No. 3, p. 10 - 12.
2. Sokolova N. Sound engineering of the “home theater”. - Radio, 1997, No. 5, p. 17 - 19.
3. TDA3810 surround sound processor. - Radio, 2001, No. 2, p. 49 - 51.

2 answers

TL: dr; it is possible possible; this may be easier if you are a programmer.

Since the encoded information is simply a stereo pair, there is no guaranteed way to detect a Dolby Pro Logic II (DPL2) signal unless you specifically store your own metadata saying "this is a DPL2 file." But you can probably make a pretty good guess.

All older analog Dolby Surround formats, including DPL2, store surround information in two channels by inverting the phase of the surround or surround sound and then mixing them with the original left and right channels. Dolby Surround decoders, including DPL2, attempt to recover this information by inverting the phase of one of the two channels, and then look for similarities in these pairs of signals. This is either done trivially, as in Dolby Surround, or these similarities are artificially shifted to push much further left or right, or left or right, as in DPL2.

So the trick is to determine if important data is stored in the surround channel(s). I'll outline a method for you that might work, and I'll try to express it without writing code, but it's up to you to implement it to your liking.

• Trim the first N seconds or so of program content into a stereo file, where N is between one and thirty. Call this file.
• Mix the stereo input channels with the new mono file at -3 dB per channel. Call this file Center.
• Separate left and right input channels into separate files. Call these left and right.
• Invert the right channel. Call this file RightInvert.
• Mix the Left and RightInvert channels with the new mono file at -3 dB per channel. Call this file Surround.
• Determine the RMS and peak dB of the Surround file.
• If the Surround file's RMS or peak DB is below the "tolerance", stop; the source file is either mono or centralized, and therefore contains no surround information. You'll need to experiment with a few DPL2 and non-DPL2 sources to find out what the tolerances are, but after a dozen or so files the numbers should become clear. I'm guessing around -30dB or so.
• Convert the Center file to new file. Call this file CenterInvert.
• Mix the CenterInvert file into a Surround file at 0dB (both CenterInvert and Surround should be mono). Call this new file SurroundInvert.
• Determine the RMS and peak dB of the SurroundInvert file.
• If RMS and/or peak dB SurroundInvert is below "tolerance", stop; your original source contains illuminated left or right front information, not ambient information. You'll need to experiment with a few DPL2 and non-DPL2 sources to find out what the tolerances are, but after a dozen or so files the numbers should become clear - I'm guessing around -35dB or so.
• If you've made it this far, your source input likely contains surround information and is therefore likely a member of the Dolby Surround encoding family.

I have written this algorithm in such a way that you can perform each of these steps with special team V . If you want to be more accommodating, instead of doing the RMS/peak step in sox, you can run the program and check your levels in LUFS against the tolerances. If you want to be even more attractive by creating Surround and Center files, you can filter out all frequencies above 7kHz and emphasize them, just like a real DPL2 decoder.

To keep this algorithm simple, I sketched it entirely in the amplitude domain. The SurroundLevel file calculation will likely be much more accurately done in the frequency domain if you know how to calculate the magnitude and angle of the FFT bits, and you use 30 to 100 ms windows. But this cheapo version above should get you started.

One last word of caution. AAC is a modern psychoacoustic codec, which means it likes to play games with stereo phasing and rendering to achieve compression. So I think it's likely that the simple act of encapsulating DPL2 into an AAC stream will likely encapsulate some of the images present in DPL2. To be frank, neither DPL2 nor AAC belong anywhere in this pipeline. If you must save an analog stream originally encoded with DPL2, do so in a lossless format such as WAV or FLAC rather than AAC.

As of this writing, the operating concepts for Dolby Pro Logic (I) are . These basic concepts still apply to DPL2; operational concepts for DPL2.

If a file has more than one channel, you can assume with some confidence that they are being used for surround purposes, although they could just be multiple tracks. In this case it falls to the game system to do with the channels as it "thinks" best. (if the file header doesn't tell you what to do)

But your file is stereo. If you want to know if this is virtual file surround sound, you can look in the encoder field header to see which encoder was used. This may help somewhat, although not much. Basically the encoder field is left blank and the second is that the encoder doesn't have to be the same as the recorder that mixes the surround data. That is, the recorder will first create raw PCM data and then feed it to some encoder to create compressed file. (AAC or whatever) Also, there are many applications and versions that can change, so the encoder box can keep track of everything, which would be annoying.

However, you can infer with more than 60% confidence whether something is virtual or not by examining the data. It will be an advanced DSP and may even involve machine learning for speed. You will need to find out whether the stereo signals contain certain HRTF (Head Related Transfer Function) features. This can be achieved by studying the differences in intensity and delay between the same sound appearing in the time domain and the harmonic characteristics (characteristic changes in frequency) in the frequency domain. You'll need to do both, because one without the other might just tell you that something is a very good stereo recording rather than a virtual surround. I don't know if there are any HRTF functions that have already been mapped, or if you will need to do this yourself.

This is a very difficult decision and takes a lot of time to get right. Also performance will be problematic.

With this method, you can also turn off the stereo downmix to near-original surround channels. But other methods are used to convert stereo into surround, and they sound good.

If you are determined to perform such a discovery, commit to six months or more of hard work if there are no HRTF functions, several weeks if there are, prepare for a lot of stress, and I wish you good luck. I did something similar. This is a killer.

If you want to use ready-made solution then your question won't be answered unless the header gives you an encoder field, and the encoder is different and is known to only be used for surround to stereo conversion. I don't think anyone has done this from actual data as I have described, or if they have, it is part of a commercial product. Doing what you need is not usually required, but it can be done.

Ow, BTW, try the HRTF inversion in googling, it might give some help.

Conveying spatial sound sensations is a task with many possible solutions. We have often encountered idealization sound systems, talking about full-fledged multi-channel sound that transports the listener to a given acoustic space with many localized sources. But, alas, this is not realistic yet.

So far, we can only talk about one successful implementation, the closest to our real sensations - binaural. To receive full-fledged volumetric information with its help, the listener must be wearing headphones. The experimental model, on the basis of which the main studies were based, was a mannequin, where microphones were installed instead of the right and left ears. Based on the results, a complex mathematical model was compiled, which made it possible to create virtual sound worlds.

But these studies, although close to ideal, had fairly general results. After all, you must admit that each person has a different shape of the ears, and the distance between the right and left ear is also an individual parameter. In general, the researchers have reached a dead end. Practical use The results of binaural models are now offered in the form of a subculture, developing separately from the main directions used in practice.

Oddly enough, the most important contribution to the field of spatial sound came from the film industry. She faced many challenges during her development, and it cannot be said that her path was smooth. Huge amounts of money were spent on introducing new technologies, and such investments were not always justified. An example is stereo cinema, where viewers were asked to wear special glasses to watch the image in volume. Stereo cinema did not become a standard, and even worse, it did not completely catch on. Therefore, idealized binaural options with headphones are not a technology for film production.

Dolby

Alternatively, this is a multi-channel system. Initially, cinema was silent, then mono sound came, and the next stage was stereo... Stereo burst in with a lot of new ideas and their implementations. A simple turn of the panorama knob delighted viewers and listeners. Interestingly, until the 80s, the concepts “stereo” and “surround” were synonymous. Do you know when they separated? With the transition of the film standard from 35 mm to 70 mm. Initially, manufacturers began to use the so-called effect channel - a loudspeaker located behind the listeners, intended for voicing auxiliary sounds, and often simply silent, since space on 35 mm film was limited. With the introduction of the 70 mm standard, the effects channel began to be used constantly and was renamed the spatial channel (surround channel, henceforth we will call it the S-channel). And from this moment we can talk about the separation of the concepts “stereo” and “surround”. In the 35 mm film version, the Dolby Stereo standard was offered (mid-70s). In 1982, the first Dolby Surround devices entered the market. At that time they cost the same as hi-end equipment, and were available to a very small number of citizens.

In parallel with the film industry, the video industry and cable/ satellite television. The ability to provide surround sound at home is one of the main tasks that has been set for manufacturers. More adequate options for both price and implementation of Dolby Pro Logic and Pro Logic II decoders have appeared. The latter allowed not only to reproduce encoded surround sound, but also to listen to music from a regular CD, laying it out on a surround system. Pro Logic and Pro Logic II are transition bridges between conventional stereo systems and multi-channel surround. Exaggerating some features, we can say that in some ways they (stereo and surround in the Dolby Surround decoder versions - Pro Logic and Pro Logic II) are interchangeable.

With active implementation digital technologies Dolby Labs introduces the Dolby Digital AC-3 (5.1) standard, developed for 35 mm film. A feature of Dolby Digital 5.1 is the presence of two surround channels (right (RS) and left (LS), predecessors had one S-channel, although several speakers were installed) and a subwoofer. If we talk about the abbreviation “5.1”, then it stands for not only five channels and a subwoofer. The fact is that in Dolby Digital, for the first time, the front and S-channels had the same full frequency range. For example, in Dolby Pro Logic the S-channel provided a range from 100 Hz to 7 KHz. Now we are talking about five full-fledged channels with a range of 20 Hz - 20 KHz and higher. The frequency range of the subwoofer, or as it is also called the LFE channel (Low Frequency Effect), is limited and is approximately 0.1 from frequency range five channels, so "5.1" should be deciphered as "five and one tenth".

Dolby Digital 5.1 files have a *.ac3 extension and require a special decoder (software or hardware) for decoding. At the dawn of its development, the technology was not very cheap. Decoders were only hardware and were produced in the form of specialized devices. Now the situation has changed for the better, and the average user can afford to purchase a sound card with built-in decoders or download their software version from the Internet. Thus, the modern cinema hall and home cinema are completely equal in capabilities. It is worth noting one more important aspect.

Along with the development of cinema, video and television, a new range of applications for surround sound technology has opened up - video and computer games. If earlier we talked about the simplest versions of a 3D engine, based on adjustments of phase shifts, delays and simple equalization, now we mean full-fledged 5.1 at a minimum.

Today, Dolby Digital 5.1 is the most universal standard. The sound of AC-3 can be transformed depending on the scale of the equipment. For example, in addition to the available division into five or six channels (depending on the presence/absence of a subwoofer), it can be converted into four channels (Dolby Pro Logic), usually stereo or even mono. Therefore, encoding sound for computer games in Dolby Digital 5.1 there is no need to be afraid that a user with fewer speakers will feel uncomfortable.

Naturally, cinemas need to have an advantage over home complexes, so Dolby EX technology was developed specifically for them, implying nothing more than 6.1. In addition to separate S-channels for right and left (RS and LS), it has a center S-channel (CS). Dolby Digital technology allows you to compress data and stream it with a minimum transfer rate of 320 kbps (standard - 384 kbps). Of course, compressed data is more transportable and takes up less space on media, be it film or a regular hard drive. But this technology also has disadvantages - it is designed for only one encoding-decoding cycle and does not solve the problem of relaying and editing compressed data. In addition, although we gain in space, we lose in that the AC-3 audio is not synchronized with the video data, so editing such material is very problematic.

Naturally, multiple encoding/decoding is required. The solution to this problem was the new introduced Dolby-E technology. It allows transmission and multiple encoding/decoding of eight channels with a full frequency range. A total of ten encoding/decoding cycles are allowed. Among other things, the standard provides for the presence of metadata - special instructions describing the basic data. In addition to synchronization, they can tell us the compressor settings for each channel, etc. By the way, you don’t have to use this data, they are nothing more or less, but additional information..

DTS

The majority of users associate the concept of surround with Dolby. This is wrong. Dolby Digital is a more popular brand, nothing more. In the mid-1980s, Terry Beard, president and founder of Nuoptix, teamed up with Jim Ketchum. The tasks that the experimenters set for themselves were the following - to create a single digital audio a new standard for cinemas and solve the issues of high-quality placement of multi-channel audio on film. Initially, experiments were carried out using digital tape recorders (DAT) and a projector. Already at the initial stage of development, synchronization issues were resolved (as we know, in the Dolby version they were resolved only with the introduction of Dolby-E). After some time, regular CD-ROMs began to be used instead of DAT. Using the apt-X100 compression algorithm developed by Stephen Smith and his colleagues at Audio Processing Technology, which allows data to be compressed 4:1, 100 minutes of multi-channel audio were placed on one CD-ROM (data transfer rate 884 kbps). Thus, two discs could store the sound for a three-hour feature film.

After receiving the patent in 1990, she demonstrated the new technology to SMPTE members. By 1992, the technology was shown to Steven Spielberg, albeit based on hard drives, and the director agreed to use it in his new film “Jurassic Park”. Universal Studio, which is the owner of the film, decided to test the technology on several two “less significant” films. The test results were acceptable. Moreover, Universal and Steven Spielberg became co-owners with Terry Beard of a new company called Digital Theater Systems. During the time the company was formed (which took four months), 876 DTS systems were installed in movie theaters. Currently there are more than 19,000 of them around the world. It was planned to implement two technology standards:

DTS-S- DTS stereo, designed for cinemas that want to use digital sound, but cannot afford a full six-channel option.

DTS-6- three front channels (left L, center C, right R), two S-channels (LS and RS) and a subwoofer. It should be noted that the standards are somewhat different from Dolby. For example, the frequency range of a subwoofer in DTS is from 20 to 80 Hz, instead of 3 -120 Hz in Dolby Digital.

One of the main differences between DTS and Dolby Digital is that DTS was developed for 70 mm film. Accordingly, the problem of space was not as pressing as in the Dolby Digital version; the issues of implementing hi-res sound and synchronization were resolved almost immediately from the time the standard emerged.

Now, when we mention DTS, we can talk about eight or more channels of hi-res audio, the latest technologies information compression, and their active practical application.

Dolby vs DTS

DTS standards are widespread just like Dolby. At the same time, competitors manage to quarrel. What else should they do? Dolby has much more powerful support from ordinary consumers and producers of technologies for the user market. Thus, music albums are released in Dolby Surround by record labels Delos, RCA Victor/BMG Classic, and Concord Jazz. Documentation for this standard is constantly in demand by developers of modern video games. Modern satellite television also broadcasts for the most part with sound in Dolby Surround mode. At the last 114th AES Convention in Amsterdam, training workshops took place on recording, mixing and mastering in Dolby 4.0, 5.0, 5.1, 6.0, 6.1, and 10.2.

DTS remains a film production privilege. Today, the issue of hi-res audio has already been successfully resolved with today's fantastic standards of 64 bits and higher, 192 KHz and higher. By the way, Dolby Digital also allows you to use hi-res tracks.

Manufacturers of software and studio equipment have resolved the issue of confrontation very simply - they support both. This can be seen in Steinberg Nuendo - one of the most professional and versatile tools for producing surround sound, and in the Audiotek line of sound interfaces - MAYA 5.1, 7.1, EX, EX7, PRODIGY 7.1. There you will find support for two technologies at once - Dolby Digital and DTS. In addition, standard software applications, designed for DVD audio playback also have decoders for both standards.

If you look at the confrontation between DTS and Dolby, it sometimes gets ridiculous. Dolby issues a press release - DTS responds. For example: “Dolby conducted research at an unknown studio with unknown equipment, while unknown material was used for playback, which was tested by unknown specialists...”. And sometimes, when talking about surround sound and DVD, Dolby is really forgotten, appropriating major achievements, etc. in your favor. In general, an ordinary corporate war.

Other multi-channel systems

In principle, any user, having in his arsenal sound card with four or more outputs and a professional multi-track program, it can feel like the dawn of the 70-80s. It was then that fundamental experiments with multi-channel sound were carried out. Many people know quadraphony, there was even octophony. At the same time, for example, octophony then did not imply modern “7.1”. Then it was assumed that there would be 8 independent channels with a full frequency range, placed around the listener in a circle at equal distances from each other.

You can experiment as much as you like. As a result of the ongoing research, there is a need for some standardization in mixing and playback. For example, let’s take the octophony option that we mentioned just above.

The source data is a drum kit recorded on 14 microphones and mixed in such a way that the listener in the center gets a feeling of presence. As the listener's position moves away from the center, the speakers closest to him will be louder, and the balance and sound picture will change. Thus, at one point we hear one thing, at another - another. This is the main drawback of such systems - the listener must be located in the center.

In the surround option, we are given only the spatial effect of presence. That is, sound localization is assigned to a separate channel. Others work as chaperones. That is, we get the emulation of spatial sensations, which is what surround systems are actually designed for.

The most unique multi-channel speaker system is headphones that can provide high-quality 3D.

Although from another point of view, you should never stop experimenting. You have every opportunity for this. If in the mid-90s I had to connect two stereo cards to my PC to experiment with quadraphony, now even the cheapest audio interface has at least four line outputs. Dolby Surround is not a perfect system and there is a lot of room for experimentation.

Standard Dolby Surround encoder and passive decoder

Let's go back a little to history, to the early 80s, when the first Dolby Surround devices appeared. A Dolby Surround encoder is a device that allows you to convert four channels (L-left, C-center, R-right and S-spatial) into two encoded Lt and Rt. Lt and Rt are abbreviations for L-, R-total, which means general. In Dolby Surround documentation, this name refers to encoded channels for older generation Dolby systems. Such Lt and Rt can be recorded on ordinary magnetic film and played back on tape recorders. If a decoder is present, Lt and Rt are decrypted into a multi-channel surround system.

Let's look at the operation of the Dolby Surround encoder in the presented diagram. In Fig. 1 shows how a four-channel system is converted to a two-channel system. Channel symbols and names are standard for all Dolby Surround systems:

• L- front left channel.
• WITH- central channel.
• R- front right channel.
• S- surround channel.
• Lt- common left.
• Rt- common right

Schematic diagram Dolby Surround encoder operation

As we see in the conversion process, the center channel signal is divided equally and mixed with left and right, while its level is reduced by 3 dB. The S signal is processed through a separate path, which includes:
bandpass filter - limiting the frequency band in the range of 100 Hz - 7 KHz (the surround channel has a frequency range of 100 Hz - 7 KHz),
noise reduction Dolby B-type Noise Reduction,
phase correction - the components of the S signal are shifted in phase so that when added to the left and right channels they are in antiphase.

In the passive decoder version, the S-channel signal can be obtained by simply subtracting Lt-Rt. Figure 2 shows a diagram of the operation of the simplest Dolby Surround decoder, at the output of which we have L, R and S.

When decoding, several problems arise, the main one of which is the interpenetration of the front channels and S. For example, when the balance of the levels Lt and Rt changes, the components of both L and R penetrate into the S-channel signal. This issue is solved in two ways - a delay set on the S signal, and limiting its frequency range. With a delay of 10 ms, the Haas effect is eliminated - the signal from the front channels coming with anticipation and source localization is associated with them. Limiting the audio to 7KHz also allows the front channels to be emphasized, as they sound brighter than the S channels.

In this version of passive decoding, it is assumed that the center channel is virtual and corresponds to the panoramic center of the L and R stereo system. This system idealized, since the loudspeakers of the right and left channels must be equidistant from the listener, which is practically unattainable in practice (there are many listeners in the cinema).

Active Dolby Surround decoders. Pro Logic

Active Dolby Surround decoders are a new generation of conversion devices from Dolby Labs, introduced in the mid-80s. Using the same Lt and Rt as input data as in the passive decoder version, they provide for a more complex decoding process taking into account the presence of a physical (rather than a virtual central channel). The first device of this class was Dolby Prologic.

Dolby Pro Logic decoder.

Conventionally, the work scheme of Pro Logic can be divided into two components - passive and active. In the passive part, the S-channel signal is obtained by simply subtracting the left channel signals from the right, and the center channel signal is obtained by adding the right and left. As we know, during encoding the S-channel is recorded in antiphase and with this addition it should disappear. To make the center channel audible, the L and R channel signals are attenuated by 3 dB. Another option is to remove the center channel signal from the left and right channels. This is also done using antiphase addition and mutual compensation of the signal components of the right and left channels. This principle called Cancellation Concept (principle of mutual compensation).

If during the encoding and recording process the balance ratio between Lt and Rt was changed, then this can lead to a number of problems - both L and R components penetrate into the S-channel signal, and so on. In the Pro Logic version, a special Level Control module is provided to equalize the balance between incoming Lt and Rt.

The active part of this decoder allows you to highlight sound images. This system is intended for cinema use only. Along with the sound there is also video. Accordingly, the main task of Pro Logic is to provide an associative series. In the description of Pro Logic there is a term “dominant sound image”. The appearance of this term is not accidental and is directly related to psychoacoustics. When the viewer's attention is focused on the dominant sound image, all other sound objects are “smeared” in space and do not have a clear localization. In the Dolby version this is called the Signal Masking Concept. It is this that is fundamental for active decoder systems. Accordingly, in these devices the speech signal is the control signal, all others are controlled. If movie characters are talking and stereo music is playing at the same time, then their speech is reproduced by the central channel, the gain of L and R is reduced. Music is also played through the S-channel speakers. When speech stops, the sound picture and the balance of channel levels are restored and return to normal. the initial state. Thus, we get a kind of “swinging” called pumping.

If there are several characters, their speech differs from other sounds in power, and they are localized in different places, then the active Pro Logic decoder works in the so-called “fast” mode, switching settings from one dominant image to another, the reaction time of the decoding system is minimal. If the sounds do not have significant differences in power, then the decoder goes into “slow” mode.

Adaptive Matrix Dolby Pro Logic.

Speaking about Pro Logic systems, it is worth mentioning an important aspect that distinguishes them from their predecessors - the decoder can determine the direction of localization of sound images. It is enough to take the amplitude of the signal of the central channel and compare it with the amplitude values ​​of the same signal in the right and left channels, and we can determine the direction vector of localization of the sound image. Next is a matter of technology.

The heart of Pro Logic is the adaptive matrix. To analyze the sound picture, four input signals are used - Lt, Rt, Lt+Rt and Lt-Rt, supplied to two differential amplifiers (the first - Lt and Rt, the second - Lt+Rt and Lt-Rt). As a result, two control signals are distinguished at the outputs of these amplifiers, which are nothing more than differences in logarithms of the amplitudes of the incoming signals. Despite the fact that the result is only two control signals, they are bipolar, which allows the device to perform a complete analysis of what is happening. Moreover, this matrix provides a comparison of amplitudes with threshold values, which allows you to switch between “fast” and “slow” modes.

At the output of the adaptive matrix, we have an analysis block that operates on ten signals, adding and subtracting them in a certain weight proportion. As a result, we have four standard signals C, R, L and S.

Pro Logic II

Dolby Surround Pro Logic II is the next stage in the development of analog decoding systems and the last of them. It is much simpler in operating principle compared to Pro Logic; in addition, it allows you to listen in surround mode not only to soundtracks for movies, but also to Dolby Surround music recordings. Starting from this stage, we can talk about cheaper Dolby standards and their entry into the consumer device market.

To understand the simplicity works Pro Logic II will consider two circuits. The first is the implementation of S-channel signal generation.

S-channel signal generation

As we already said in the description of Pro Logic, the absence of penetration of the L and R channel signals into the S channel is very important. But, for example, if the balance between channels is changed and the dominant image is located in the direction, for example, “center-right,” then such penetration will be necessary. In Fig. Figure 5 shows the simplest solution to this problem. Two controlled amplifiers VCA (Voltage Controlled Amplifier) ​​are placed in the circuit between the Lt and Rt inputs and the adder input. The control of both VCAs is interconnected in different polarities. If the Lt signal is more powerful than Rt, then the gain of the Lt signal before the adder decreases, and Rt, on the contrary, increases. Thus, the signals are aligned - penetration is minimal.

How Pro Logic II works.

Pro Logic II is also easy to understand. Signals from both VCAs are fed to FWR (Full-Wave Rectifier) ​​amplitude detector modules. A differential amplifier compares the signals direct current, which are proportional to the signal amplitudes Lt and Rt. Its signal is the control signal for the VCA. The center channel C signal is obtained by adding the equalized Lt and Rt.

Compared to Pro Logic, Pro Logic II does not have a "fast" or "slow" mode - the response time variable changes constantly. Its operation scheme is simple and convenient.

Dolby Digital AC-3

Dolby Digital AC-3 or as it is also called Dolby Digital 5.1 appeared in the late 80s and was developed for 35 mm film. With the advent of this standard, several innovations awaited us. Firstly, a six-channel standard was developed, which made it possible to use a subwoofer, or as it is also called LFE-channel (Low Frequency Effect). Secondly, the S-channel is no longer mono. That is, in a 5.1 system we have SL and SR channels (Surround Left and Surround Right). The loudspeakers of all channels except LFE are full-range. For the first time, the standard offered compression capabilities dynamic range. At the same time, the AC-3 standard provides for data compression. Currently, the Dolby Digital audio data rate is 320 kbit/sec or more.

The main feature of Dolby Digital AC-3 is its scalability.

Dolby Surround 5.1 scaling.

For example, we wrote music in 5.1 standard. At the same time, the user can listen to it without loss in quality in 5.1 (5.0), four-channel Pro Logic version, in ordinary stereo and even in mono. And now imagine how beneficial this is for both users and manufacturers. Computer and video games of the new generation naturally imply the presence of 5.1 sound. At the same time, users who do not have advanced options speaker systems will not feel uncomfortable - the system will scale to the parameters of the existing system. Many satellite TV channels They broadcast in 5.1, but this does not in any way affect the majority of viewers who have TVs with mono sound.

Multi-channel system settings

You may have come across this fragment of this material in the article “Home studio based on PC-2003. Hardware rating”. To maintain the integrity of the topic, I will allow myself some repetition. Moreover, it will allow you to perceive everything more fully.

Using the example of Dolby Digital 5.1, we can consider the main issues regarding the installation of speaker systems and hardware for multi-channel systems.

Correct placement of speaker systems for Dolby 5.1

Dolby Surround 5.1 in expanded form for cinemas and large rooms.

As can be seen from Figure 8, the Dolby Digital 5.1 system is designed based on ideal conditions, where all channels are equidistant from the listener. But such conditions are quite difficult to implement physically, especially in small and large rooms with non-ideal geometry. This problem can be successfully solved electrically. The native documentation for mixing in Dolby Surround (www.dolby.com) provides an interesting option for calculating delay times - by simply converting feet to milliseconds.

If DimL is the distance from the listener to the left front channel in feet (30.48 cm), and DimS is the distance from the listener to the nearest surround channel in feet, then the delay in that channel should be equal to the difference between DimL and DimS. In other words: Latency (milliseconds) = DimL (feet) - DimS (feet). If the left channel is 10 feet away and the surround is five feet away, then the delay setting is 5 ms (10-5=5).

This is true not only for calculating the interactions of the front and surround channels, but can also be used in other cases to align the picture. For example, if the center channel is located closer to the listener than the right and left. The formula is like:

• Latency (milliseconds) = DimL (feet) - DimC (feet).

  Accordingly, if we operate a large number of surround channels in large rooms, then the delay parameters for each pair of them are adjusted separately.

What are the new promising developments in surround sound?

Bringing out a new sound for Christmas

The era of establishing standards “for centuries” has long gone into oblivion. And if the path of stereo to cinema took more than half a century - before Mr. Dolby undertook the total transfer of theaters to a multi-channel scheme, this was only a farewell to the slow passage of time. Progress and the acceleration of the pace of life, as well as new techniques to stimulate the market, have led to a truly computer race in the field of home theaters, which has resulted in a rapid (and almost annual) change in sound formats for home use. The passion for completing the home cinema with two additional rear center channels (Dolby Digital EX, DTS ES Discrete/Matrix) has not yet died down; the required number of DVDs with support for these sound systems have not yet been released, as last year was marked by a further expansion of the “model” range - in The series included devices with DTS 96/24 and Dolby Pro Logic II. Below we will try to dot the i's in this issue, telling in detail who and why they can be useful (note that this review does not pretend to compare two standards that are different in essence and purpose, but only deciphers their positive and negative aspects).

The fifth reincarnation of DTS

Ten years have passed since the theatrical introduction of Steven Spielberg's brainchild, DTS sound for cinema, and the collection of home formats has already included: the original DTS 5.1 (with variable bit rates 768-1500 kbps), DTS Neo:6 (which converts two-channel schemes into multi-channel ones), DTS ES 5.1 Matrix (with a rear center signal mixed into the rear) and DTS 6.1 Discrete (with a dedicated rear center). And although the latter, with an accepted bitrate of even 768 kbps, provided the best available sound quality at home, the studio went further, approving DTS 96/24 - a six-channel standard with parameters from DVD-audio discs - 96 kHz sampling, 24 bits, which costs a lot higher than traditional 48 kHz and 20 bits in all formats available for theater.

The company calls the main advantages of DTS 96/24:

• preservation of the original bit depth of studio sound - 20 and 24-bit masters now reach the viewer without data loss, as is the case with DVD-audio;
• superior to conventional CD audio (linear PCM with 16-bit 44.1 kHz parameters) - this statement is controversial, since DTS uses data compression, and PCM manages the original stream with a bit rate of over 3,000 kbps on two channels: while maintaining such characteristics, multi-channel audio will not fit even on several DVDs;
• superior to all theatrical standards (18-20 bit, 48 kHz);
• characteristics equal to DVD-audio without the requirement to purchase a new compatible player - again, a rather controversial statement, since the bitrate of DVD-audio is higher;
• Full compatibility with any source that has the old DTS logo;
• simple digital output via coaxial and/or optical interfaces.

And, although the decoding requirements include the DTS 96/24 logo, that is, the ability of the processor to recognize the new standard, conversion to regular DTS is possible on most DTS-compatible devices, and those that have 96/24 digital-to-analog converters in all channels , can, in most cases, reproduce correctly new sound. This possibility is not advertised, but is easily verified in practice.

The reception of the format in the USA was not the most cordial, but recently it has turned out a certain amount of titles of music of different genres (many releases appeared in April 2003):

Queen: Greatest Video Hits 1
Queen: The Game
Queen: A Night at the Opera
Rhythmedia Tribe
Gorsby/Nash: Another Stoney Evening
Sheila Nicholls: Wake
Megadeth: Peace Sells...But Who's Buying?
Brazilian Jazz
Brazilian Bossa
The Fabulcus Thunderbirds: This Night in L.A.
Frank Zappa: Halloween

Most of them are combined DVD-A releases containing 5.1 MLP DVD-audio tracks, DTS 96/24 for conventional cinema systems and 2.0 PCM for fans of conventional stereo.

So far, only one film has been released in this format - this is "Tomb Raider" of the second region, made in Italy - soundtrack in DTS 96/24 there is only Italian dubbing.

This approach is completely unjustified for the user - DVD-A players today start at $200 - and even 5.1 96/24 sound in DVD-A format is without question better than compressed DTS of the same bit depth. The poor list of released titles and their musical focus makes DTS 96/24 the lot of highly specialized sound maniacs - the advantages of this approach in the theater could seriously concern fans of the “best”, but, probably, the stumbling block was the capabilities of the studios, since remaking the original masters is a matter labor-intensive and thankless, so sound engineers usually limit themselves to only “quick” adaptation of standard DTS/DD film options for home use. The approach of DTS Entertainment, which specializes in such releases, is also quite “unconventional” - often, all the attention is paid to DTS mastering of music, and the DVD-A section is deliberately degraded: the difference between them is eventually leveled out, but it’s worth putting on a high-quality recorded DVD-A disc , and everything falls into place (but more on that below).

The latest good news about the format is its support on a number of digital radio stations - their broadcasting in DTS 96/24 seems to be a very winning move in promoting the market.

Dolby Pro Logic II or the overthrow of Logic 7 from the leader's pedestal

Unlike the next reinvention of the wheel in DTS, Dolby's healthy conservatism did not allow the creation of new discrete standards - and heavy artillery was used in the form of an “upgrade” of Pro Logic to version 2.0. The promises were tempting - a complete conversion of stereo sound to 5.1 with quality (attention!) comparable to Dolby Digital 5.1. Given the availability of a lot of old recordings on video cassettes, LD (of which more than 50,000 titles were released), hundreds of DVDs with DD2.0 sound (anime, classics, TV series), as well as the passionate desire of individual film fans to play CDs through all their speakers, this decision could really shake up the market.

At the beginning, two variations were presented - Dolby Pro Logic II Movie for film programs and Dolby Pro Logic II Music for music, then Car adaptations appeared, and recently Virtual Speaker technology was introduced, which allows you to create a surround field with only two speakers. Dolby Headphone is also based on this principle. There is also a Matrix system included by default in processors, running on mono material. Note that before the release of Pro Logic II (and DTS Neo: 6, which, however, turned out to be very mediocre), the palm in its own decomposition formats 2.0 to 5.1 circuits belonged to Logic 7 from Harman, most successfully implemented in Lexicon processors. However, the highest price for such devices, as well as the generally not the most advanced algorithm, did not do this any favors - devices with Pro Logic II are now available for sale at prices starting from $300.

When working on cinematic material using special Dolby 2.0 algorithms, the sound is laid out in a multi-channel 5.1 scheme with full use of all speakers, and, as stated, is quite close in quality to DD5.1. When choosing music, you can level out the most important defect of Pro Logic’s past - the overestimated level of the center channel. Using the Center Width parameter, this indicator is calibrated to fully match the front speakers. Two more functions - Panorama (perspective adjustment) and Dimension (balance) allow you to quite accurately fine-tune the translation of stereo music into a three-dimensional scene. In this case, Pro Logic has taken a significant step forward compared to the similar DTS Neo:6 content, in which the detrimental influence of the center channel on the music is still strongly felt.

It’s quite nice that the format does not impose any requirements on the source and the sound recorded on it, but official support for the Pro Logic II flag (for example, on games for the Nintendo Game Cube) may mean optimization and better quality. Today, the vast majority of manufactured receivers and processors support Dolby Pro Logic II.

Let's listen...

The comparison was carried out using the following system: Denon A1SR receiver, B&W CDM acoustics, Pioneer 757Ai player.

DTS 96/24 material was used on Queen: The Game, Queen: A Night at the Opera, and Pro Logic II was tested on a variety of CD material (from Stray Cats, Metallica to Wagner and Beethoven) and 2.0 DVDs (“ The Man Who Wasn't There R5, Escaflowne Series 1-4 R1, etc.).

We couldn't test DTS 96/24 in the cinema version, since even if we have the required version of Tomb Raider, the dubbed track can hardly be considered full sound - so we'll wait for the DVD R1 with cinematic material in this format to appear. Of the presented music, DTS 96/24 demonstrated good potential in terms of volume and detail, dramatically surpassing conventional DTS music. However, even a low-quality DVD-A master sounded a little better on these discs, not to mention the new DVD-A discs. Queen's tracks were distinguished by audible sound compression - alas, the disadvantages of compression can only be physically overcome by a radical increase in the bitrate. But even in this form, the potential of the format for home cinema was clearly noticeable - this is where the ground would be for building a new “SuperBit 96” or similar titles - impeccable detail and a stranglehold on localization - exactly what the slightly relaxed modern formats for home versus theatrical SDDS, for example.

As for Dolby Pro Logic II, in music mode on regular CDs it showed rich talents for turning stereo into a multi-channel scheme, subtly and detailing the sound panorama - sometimes a little more carefully than we would like, but very natural and impressive: this is exactly what , what Logic 7 fans wanted, only in a more competent execution. Naturally, this option Not at all for purists, since the timbre colors of the sound are shifted, and the stereo panorama sometimes becomes unstable, but this is still a significant step forward compared to previously proposed algorithms from other studios. The vast majority of owners of multi-channel systems will prefer Pro Logic II as an “add-on” over a regular compact. From the cinema side, this format also impresses even seasoned speakers, almost falling short of DD5.1, but providing a great special effect in all channels - the only criticism is the slightly boomy central channel: creating the required volume, it often muffles the dialogues. However, this can be easily corrected by adjusting the system balance.

Advantages and disadvantages

Format	Advantages	Flaws
DTS 96/24	- high potential for film sound - good quality musical material comparable (but not equal) to some DVD-A discs - ability to play on a non-certified DTS decoder	- a small number of titles, an almost complete absence of films in the format - musical potential does not reach the best examples DVD-A discs - not supported by all modern receivers
Dolby Pro Logic II	- ideal channel-by-channel decomposition of 2.0 sound into 5.1 - both for cinema and music - can be used on any 2.0 and 1.0 material - broadest support from hardware manufacturers	- high-quality DD/DTS 5.1 mastering, still better - center channel boom in theater mode

Summary

It’s unlikely that DTS 96/24 will have a bright future, but it will, of course, win its place in the sun in high-end complexes. Pro Logic II is another matter - when you buy a new receiver, you are doomed to have this opportunity, and after listening to old cassettes with this “feature” you are doomed to continue to use it - the thing is truly nuclear. Once again, Dolby demonstrated its mass approach, and DTS its intimate approach. Well, to each his own.