2020-09-22, 03:03 AM
The following is mostly the result of me spending time researching this stuff for my own purposes, and is by no means intended to be in any way definitive or comprehensive enough to be considered a reliable reference. However, I do hope that it will help outline things a little bit for newcomers interested in the subject and can perhaps be refined over time as questions arise, in order to improve understanding of some of the topics discussed.
I'm putting the bulk of the text behind "spoiler" tags purely because this post is massive and you'd be scrolling for days otherwise. You can click the spoilers to expand each section to read a bit at a time, which will make it easier to read in chunks rather than trying to read this entire thing in one go. This feels rather important because it's over 3.5 thousand words in total!
A BRIEF HISTORY OF THE STEREOPHONIC FILM SOUNDTRACK
THE DOLBY NOISE REDUCTION SYSTEM
THE DOLBY MOTION PICTURE MATRIX
70 MM FILM WITH SIX TRACKS OF DOLBY AUDIO
SO, UH, DOLBY... SYSTEM? STEREO? SURROUND? PRO LOGIC? WHAT?
I'm putting the bulk of the text behind "spoiler" tags purely because this post is massive and you'd be scrolling for days otherwise. You can click the spoilers to expand each section to read a bit at a time, which will make it easier to read in chunks rather than trying to read this entire thing in one go. This feels rather important because it's over 3.5 thousand words in total!
A BRIEF HISTORY OF THE STEREOPHONIC FILM SOUNDTRACK
With some notable exceptions, the audio for most early “talkie” films was generally monophonic and rather low-fidelity. Initially, this would have been recorded onto a circular disc not unlike a phonograph record, which was played back simultaneously with the reels of film; later, it was printed onto optical tracks that ran alongside the picture on the film. There were a number of experiments with non-mono sound, the most revolutionary early example of which was Disney’s FANTASIA (1940), which used its own unique audio system (“Fantasound”) wherein a completely separate run of film was used to play back four separate tracks of optical audio containing the left, centre, and right audio channels, with the fourth track being used to dynamically alter the level of the other three as the film played (I believe this may have been an early attempt to manage the signal-to-noise ratio). Unfortunately, FANTASIA was a spectacular failure, so it wasn’t until much later that stereo started to get a foothold in cinemas.
By the 50s, studios were recording their soundtrack masters to magnetic film. In 1952, the prestige Cinerama format used 8 discrete tracks recorded onto a dedicated, fully magnetic-coated run of 35 mm film: 5 fed speakers behind the screen (left, left-centre, centre, right-centre, and right), 2 fed speakers at the back (left surround and right surround), and 1 was a reference to ensure everything played back in sync. There were various other inventions from then onwards, coinciding with different attempts to create widescreen cinema format, but for the purposes of STAR WARS, I’m going to highlight two of them in particular. First, in the early to mid 50s, CinemaScope used 4 thin magnetic strips running up the same run of film as the picture to carry 4 channels of audio: left, centre, right, and a surround (a.k.a. “effects”) channel that triggered the speakers to switch on only when needed to reduce the amount of audible hiss caused by an inherent artefact of the magnetic material when there was no actual audio signal on the channel (I’ll come back to this in a moment when talking about Dolby noise reduction). A little later, the Todd-AO 70 mm format again used magnetic rather than optical audio, this time with 6 magnetic strips running up the same strip of film as the picture rather than CinemaScope’s 4, since the 70 mm film was physically larger than 35 mm and allowed more space for the audio tracks. The Todd-AO layout was 2 left channels, centre, 2 right channels, and 1 surround channel. Note: the “left extra” and “right extra” channels can probably be thought of as analogous to the middle “surround” speakers in a modern home cinema 7.1 (3+2+2.1) speaker layout.
The Todd-AO system worked for 70 mm presentations, but there were some problems to solve in order to shift 35 mm film into the stereophonic realm. The limited space for audio meant that the dynamic range of multiple optical tracks was going to be very small compared to a single (mono) optical track as used on 16 mm, even if only 2 tracks were used (rather than Todd-AO’s 6). Plus, the lack of a centre channel was a concern, since that could make dialogue sound somewhat disconnected from the action happening on the screen, since the audio was coming from the sides rather than the direction of the picture. Both of these problems were solved by Dolby Laboratories.
By the 50s, studios were recording their soundtrack masters to magnetic film. In 1952, the prestige Cinerama format used 8 discrete tracks recorded onto a dedicated, fully magnetic-coated run of 35 mm film: 5 fed speakers behind the screen (left, left-centre, centre, right-centre, and right), 2 fed speakers at the back (left surround and right surround), and 1 was a reference to ensure everything played back in sync. There were various other inventions from then onwards, coinciding with different attempts to create widescreen cinema format, but for the purposes of STAR WARS, I’m going to highlight two of them in particular. First, in the early to mid 50s, CinemaScope used 4 thin magnetic strips running up the same run of film as the picture to carry 4 channels of audio: left, centre, right, and a surround (a.k.a. “effects”) channel that triggered the speakers to switch on only when needed to reduce the amount of audible hiss caused by an inherent artefact of the magnetic material when there was no actual audio signal on the channel (I’ll come back to this in a moment when talking about Dolby noise reduction). A little later, the Todd-AO 70 mm format again used magnetic rather than optical audio, this time with 6 magnetic strips running up the same strip of film as the picture rather than CinemaScope’s 4, since the 70 mm film was physically larger than 35 mm and allowed more space for the audio tracks. The Todd-AO layout was 2 left channels, centre, 2 right channels, and 1 surround channel. Note: the “left extra” and “right extra” channels can probably be thought of as analogous to the middle “surround” speakers in a modern home cinema 7.1 (3+2+2.1) speaker layout.
The Todd-AO system worked for 70 mm presentations, but there were some problems to solve in order to shift 35 mm film into the stereophonic realm. The limited space for audio meant that the dynamic range of multiple optical tracks was going to be very small compared to a single (mono) optical track as used on 16 mm, even if only 2 tracks were used (rather than Todd-AO’s 6). Plus, the lack of a centre channel was a concern, since that could make dialogue sound somewhat disconnected from the action happening on the screen, since the audio was coming from the sides rather than the direction of the picture. Both of these problems were solved by Dolby Laboratories.
THE DOLBY NOISE REDUCTION SYSTEM
As mentioned above, the magnetic material used on tape and some early motion picture film formats has an inherent, audible artefact often referred to as “tape hiss”: a high-frequency hissing sound that is most noticeable in moments where the actual recorded audio signal is less prevalent (e.g. complete silence or quieter moments in the recording). This was undesirable as it was not intended to be heard and was only present due to a technical limitation of the medium.
In 1965, Dolby Laboratories developed a noise reduction system with which recordings of music and other audio could have the signal to noise ratio improved during the recording process in order to reduce the amount of audible “tape hiss” in the resulting recording. The audible hiss is an artefact of the magnetic tape medium rather than part of the recorded sound, so it’s constant, remaining at the same level even when the recorded audio reduces in volume, meaning that the ratio of signal to noise is poor when the recorded signal is quiet (less recorded audio, same amount of noise). At the most basic level, the idea here is that the hiss is more apparent when the actual recorded audio is quieter, but if you were to turn up the level on the recording during those quieter sections as it’s being recorded, you’d not hear the hiss as much (but of course the volume of the signal would be incorrect because signal in what should be quiet sections would instead be loud). If you were to correct for this later by reducing the level back to the intended level during playback, you’d be reducing the amount of audible signal (back to the level it was meant to be heard at) *and* the amount of audible noise at the same time so that the noise isn’t distracting.
Now, tape hiss doesn’t happen evenly across the entire frequency spectrum; it’s heard as a high-pitched noise that mostly occupies frequencies above 1 kHz. Therefore, you don’t need to increase the level of the *entire* audio signal to improve the SNR, you just need to increase the levels in the higher frequencies where the tape hiss resides. To achieve this, the Dolby system split up the audio into a series of specific frequency bands and then applied dynamic range compression only where it was needed in order to increase the amount of signal (i.e. recorded audio) in areas where the original signal level would have otherwise been low, and then on playback, the dynamic range would then be expanded back to its originally intended level (thus reducing the perceived volume of both the signal and the tape hiss noise in the frequencies that were adjusted) by a decoder. The dynamic range compression applied at recording stage is a form of pre-emphasis, and the correction for it during playback is known as de-emphasis. Although you could of course play Dolby audio recordings without applying the correct de-emphasis, the result would be that you’d hear way too much high frequency signal than you should because those frequencies were boosted by the Dolby pre-emphasis process in order to reduce tape hiss, making the sound inappropriately shrill compared to the audio that was originally played into the recorder. Note: there have been various iterations on this system over the years, but the original one used primarily in recording studios (now known as “Dolby A”) is what I’m talking about here.
This Dolby noise reduction process quickly became commonplace in music recording studios, and it was not long before it was also used in the film industry. Dolby A noise reduction was used for film production purposes starting with A CLOCKWORK ORANGE (1971), but it wasn’t implemented for the audio that was actually printed onto the theatrical film prints until it was used on the optical mono track of prints of CALLAN (1974). The trouble was, since the Dolby process involved applying the aforementioned pre-emphasis to the optical track, cinemas therefore had to install new audio decoders to de-emphasise these tracks back to sound how they were intended to sound. Not every cinema could necessarily afford to do this, and since stereophonic sound was also starting to rise to prominence around the same time, cinema owners didn’t want to spend a bunch of money on Dolby A decoders that could still only do mono anyway. This is one of the main reasons that there were still a large number of cinemas with somewhat antiquated monophonic audio systems when STAR WARS was released in 1977.
That being the case, Dolby came up with a solution to introduce their noise reduction process to an implementation of optical stereophonic film sound that was more cost-effective than the magnetic 6-track surround sound used on formats like Todd-AO and could occupy the limited space available on a 35 mm print by using only 2 tracks instead of 6. The Dolby noise reduction process helped with the dynamic range issue caused by the limited physical space available for the 2 optical tracks, since the pre- and de-emphasis increased the dynamic range during playback, but the problem of there being no centre channel had to be dealt with as well.
In 1965, Dolby Laboratories developed a noise reduction system with which recordings of music and other audio could have the signal to noise ratio improved during the recording process in order to reduce the amount of audible “tape hiss” in the resulting recording. The audible hiss is an artefact of the magnetic tape medium rather than part of the recorded sound, so it’s constant, remaining at the same level even when the recorded audio reduces in volume, meaning that the ratio of signal to noise is poor when the recorded signal is quiet (less recorded audio, same amount of noise). At the most basic level, the idea here is that the hiss is more apparent when the actual recorded audio is quieter, but if you were to turn up the level on the recording during those quieter sections as it’s being recorded, you’d not hear the hiss as much (but of course the volume of the signal would be incorrect because signal in what should be quiet sections would instead be loud). If you were to correct for this later by reducing the level back to the intended level during playback, you’d be reducing the amount of audible signal (back to the level it was meant to be heard at) *and* the amount of audible noise at the same time so that the noise isn’t distracting.
Now, tape hiss doesn’t happen evenly across the entire frequency spectrum; it’s heard as a high-pitched noise that mostly occupies frequencies above 1 kHz. Therefore, you don’t need to increase the level of the *entire* audio signal to improve the SNR, you just need to increase the levels in the higher frequencies where the tape hiss resides. To achieve this, the Dolby system split up the audio into a series of specific frequency bands and then applied dynamic range compression only where it was needed in order to increase the amount of signal (i.e. recorded audio) in areas where the original signal level would have otherwise been low, and then on playback, the dynamic range would then be expanded back to its originally intended level (thus reducing the perceived volume of both the signal and the tape hiss noise in the frequencies that were adjusted) by a decoder. The dynamic range compression applied at recording stage is a form of pre-emphasis, and the correction for it during playback is known as de-emphasis. Although you could of course play Dolby audio recordings without applying the correct de-emphasis, the result would be that you’d hear way too much high frequency signal than you should because those frequencies were boosted by the Dolby pre-emphasis process in order to reduce tape hiss, making the sound inappropriately shrill compared to the audio that was originally played into the recorder. Note: there have been various iterations on this system over the years, but the original one used primarily in recording studios (now known as “Dolby A”) is what I’m talking about here.
This Dolby noise reduction process quickly became commonplace in music recording studios, and it was not long before it was also used in the film industry. Dolby A noise reduction was used for film production purposes starting with A CLOCKWORK ORANGE (1971), but it wasn’t implemented for the audio that was actually printed onto the theatrical film prints until it was used on the optical mono track of prints of CALLAN (1974). The trouble was, since the Dolby process involved applying the aforementioned pre-emphasis to the optical track, cinemas therefore had to install new audio decoders to de-emphasise these tracks back to sound how they were intended to sound. Not every cinema could necessarily afford to do this, and since stereophonic sound was also starting to rise to prominence around the same time, cinema owners didn’t want to spend a bunch of money on Dolby A decoders that could still only do mono anyway. This is one of the main reasons that there were still a large number of cinemas with somewhat antiquated monophonic audio systems when STAR WARS was released in 1977.
That being the case, Dolby came up with a solution to introduce their noise reduction process to an implementation of optical stereophonic film sound that was more cost-effective than the magnetic 6-track surround sound used on formats like Todd-AO and could occupy the limited space available on a 35 mm print by using only 2 tracks instead of 6. The Dolby noise reduction process helped with the dynamic range issue caused by the limited physical space available for the 2 optical tracks, since the pre- and de-emphasis increased the dynamic range during playback, but the problem of there being no centre channel had to be dealt with as well.
THE DOLBY MOTION PICTURE MATRIX
The way that Dolby solved the centre channel problem was to use an existing technique called matrix decoding to turn 2 tracks of optical audio printed onto the film (which would otherwise simply represent the left and right channels) into 3 channels: left, right, and centre. This was originally done with a licenced decoder developed to turn 2 channels into 4 (Sansui’s “Quadrophonic Sound” matrix), although Dolby later developed their own custom matrix in 1979. The 3-channel matrix encoding/decoding algorithm was further enhanced by Dolby using a phase-inversion technique to additionally derive a surround channel for the back of the theatre. The 2 tracks printed on the film are not plain stereo (left and right channels); in addition to having Dolby noise reduction applied, they are also matrix encoded from 4 discrete channels of audio down to 2. In other words, the matrix decoding essentially restores the 4 original channels, at least to the extent that the technology allows. This is therefore sometimes referred to as a 4:2:4 matrix.
The result of this was, essentially, the CinemaScope speaker/channel layout of left, centre, right, and surround, except that it was stored as only 2 optical tracks instead of 4 magnetic tracks on the film. Significantly cheaper, with the added benefit of Dolby’s noise reduction thrown in as well.
It was primarily the massive success of STAR WARS in 1977 that really cemented the “Dolby System” (later branded “Dolby Stereo”) as the de facto standard in cinemas moving forward. Dolby remained relatively unchallenged for a few years after that; other competitors would appear later (see the section on the 1997 Special Edition mix for a brief discussion about DTS, for example).
The result of this was, essentially, the CinemaScope speaker/channel layout of left, centre, right, and surround, except that it was stored as only 2 optical tracks instead of 4 magnetic tracks on the film. Significantly cheaper, with the added benefit of Dolby’s noise reduction thrown in as well.
It was primarily the massive success of STAR WARS in 1977 that really cemented the “Dolby System” (later branded “Dolby Stereo”) as the de facto standard in cinemas moving forward. Dolby remained relatively unchallenged for a few years after that; other competitors would appear later (see the section on the 1997 Special Edition mix for a brief discussion about DTS, for example).
70 MM FILM WITH SIX TRACKS OF DOLBY AUDIO
Dolby initially experimented with using their noise reduction on Todd-AO magnetic soundtracks (e.g. LOGAN’S RUN in 1976), which significantly improved the dynamic range of Todd-AO screenings; later, they refined the Todd-AO format further by changing the two “extra” L+R channels to dedicated low-frequency/bass effects below 250 Hz (colloquially referred to as “baby boom” for... some reason). The resulting speaker layout was: left, centre, right, bass, bass (again), and surround; to use modern syntax, this is effectively “4.2” although the bass information was identical in the two low-frequency channels so it’s perhaps more like 4.1. It was quickly determined that the 250 Hz cutoff for the LFE channels was too high, so this was revised to 125 Hz instead.
In 1978, Dolby slightly rejigged the channel layout to give the two LFE channels a new dual purpose, somewhere in between their original function and the dedicated LFE function. Instead of containing exclusively low-frequency audio (below 125 Hz), they would now also contain unique (stereo) high-frequency effects (above 500 Hz). Appropriately-equipped cinemas would play back stereo surround sound for surround effects pitched at 500 Hz or above, and use the dedicated mono surround channel only for anything below 500 Hz. If a cinema was not set up for this, then only the low-frequency stuff from the new combined LFE/HFE channels would be heard, with the dedicated mono surround channel playing back all the surround effects instead of only those below 500 Hz*. This new layout was used for a very limited release of SUPERMAN in 1978 and later for the wider release of APOCALYPSE NOW in 1979. Since STAR WARS was released before this, though, its 70 mm audio mix only had a mono surround channel (like its Dolby 2-track mix). Even then, many films released after the channel layout was altered didn’t actually contain different information in the two separate surround tracks. I’m guessing that this is probably mostly because creating stereo surround channels would have meant having to create a different mix for 70 mm and 35 mm Dolby Stereo versions of any film released for both formats, but you could instead just start off with 5 tracks (L+C+R+S+LFE, with the LFE possibly just being low-pass filtered out of other channels), duplicate the mono surround for the 70 mm version, and release it for both without having to specifically mix two different versions. When the magnetic masters for the 70 mm version of STREETS OF FIRE (1984) were used as a Blu-ray Disc audio source, for example, the result was a 4.1 track with a mono surround, not a 5.1 track; I’m guessing that’s because they just duplicated a single mono surround track from the master onto 2 tracks on the 70 mm release prints.
Thanks to hairy_hen for the specifics of the revised 1978 Todd-AO/Dolby 6-track channel layout. I had it slightly wrong and thought that they'd just replaced one of the dedicated LFE channels with another mono surround channel (for a total of 2 surround channels), with the tracks on the print being routed to different speakers. The truth is a lot more interesting, and cleverer, because it allows for less messing about from the cinemas' point of view and allows for backwards-compatibility with older Todd-AO prints with the old channel layout as well as cinemas that don't have the required gear for the new layout!
In 1978, Dolby slightly rejigged the channel layout to give the two LFE channels a new dual purpose, somewhere in between their original function and the dedicated LFE function. Instead of containing exclusively low-frequency audio (below 125 Hz), they would now also contain unique (stereo) high-frequency effects (above 500 Hz). Appropriately-equipped cinemas would play back stereo surround sound for surround effects pitched at 500 Hz or above, and use the dedicated mono surround channel only for anything below 500 Hz. If a cinema was not set up for this, then only the low-frequency stuff from the new combined LFE/HFE channels would be heard, with the dedicated mono surround channel playing back all the surround effects instead of only those below 500 Hz*. This new layout was used for a very limited release of SUPERMAN in 1978 and later for the wider release of APOCALYPSE NOW in 1979. Since STAR WARS was released before this, though, its 70 mm audio mix only had a mono surround channel (like its Dolby 2-track mix). Even then, many films released after the channel layout was altered didn’t actually contain different information in the two separate surround tracks. I’m guessing that this is probably mostly because creating stereo surround channels would have meant having to create a different mix for 70 mm and 35 mm Dolby Stereo versions of any film released for both formats, but you could instead just start off with 5 tracks (L+C+R+S+LFE, with the LFE possibly just being low-pass filtered out of other channels), duplicate the mono surround for the 70 mm version, and release it for both without having to specifically mix two different versions. When the magnetic masters for the 70 mm version of STREETS OF FIRE (1984) were used as a Blu-ray Disc audio source, for example, the result was a 4.1 track with a mono surround, not a 5.1 track; I’m guessing that’s because they just duplicated a single mono surround track from the master onto 2 tracks on the 70 mm release prints.
Thanks to hairy_hen for the specifics of the revised 1978 Todd-AO/Dolby 6-track channel layout. I had it slightly wrong and thought that they'd just replaced one of the dedicated LFE channels with another mono surround channel (for a total of 2 surround channels), with the tracks on the print being routed to different speakers. The truth is a lot more interesting, and cleverer, because it allows for less messing about from the cinemas' point of view and allows for backwards-compatibility with older Todd-AO prints with the old channel layout as well as cinemas that don't have the required gear for the new layout!
SO, UH, DOLBY... SYSTEM? STEREO? SURROUND? PRO LOGIC? WHAT?
The terminology around the Dolby MP Matrix is... rather messy.
The very earliest Dolby stereo theatrical releases used slightly inconsistent terms, but for the most part, it looks like posters and the like mostly just called it the “Dolby System” with additional adjectives like “stereophonic” to describe the experience to prospective cinemagoers. This was quickly rebranded as “Dolby Stereo”, but that seems to have happened after STAR WARS (but before THE EMPIRE STRIKES BACK in 1980).
Most of the other terminology around this stuff emerged later because of home video implementations of the matrix decoding technology, none of which was exactly 100% identical to the cinema decoders. If you want to learn about those, keep reading. Otherwise, feel free to skip ahead, but if you’re confused about what I’m calling things later, don’t blame me…
First we got “Dolby Surround” in 1982, which had a less sophisticated decoder than the cinema equivalent and only turned the 2 theatrical audio tracks into 3 (L, C, R) rather than 4 (L, C, R, S). The “missing” centre channel from the matrix was directed equally to the left and right speakers instead of being separated out to feed a centre speaker.
A third party solution soon appeared: Acra-Vector decoding logic was used in Shure home theatre decoders like the HTS-5000, HTS-5200, and HTS-5300, which somewhat confusingly bore the Dolby Surround branding even though they had different circuitry from actual Dolby Surround decoders. These were not only capable of turning the 2 matrix-encoded tracks back into 4, but could alternatively derive 2 surround channels instead of 1 (with a widening effect intended to emulate the sound of multiple surround speakers, as would have been used in cinemas at the time) and a low-pass filtered channel to be fed to a dedicated subwoofer. In other words, Dolby Surround turned 2 channel Dolby Stereo mixes into 3.0 (L+R+S), whereas the Acra-Vector logic could give you a pretty convincing 5.1 (L+C+R+LFE+Ls+Rs).
Despite being widely thought of as superior to any of Dolby’s own home decoders from around that time, the Acra-Vector solutions were quickly out-competed by the 1987 arrival of an official Dolby revision of Dolby Surround called Pro Logic. Both the Acra-Vector and Dolby Pro Logic decoding algorithms were able to turn 2 matrix-encoded L+R+C+S channels back into 4-channel L+R+C+S again, using “active” steering logic rather than the “passive” approach of Dolby Surround.
Since the surround channel is derived from the Lt and Rt (Left Total and Right Total) tracks using a phase-inverted version of the calculation used to derive the front centre channel, there is a risk of sound that should only be in the front centre channel “leaking” out into the back surround channel as well. This is undesirable since characters on screen are directionally in front of the viewer so it’s weird to hear their voices coming from both in front of and behind you. Dolby Surround, Shure/Acra-Vector and Dolby Pro Logic decoders had slightly different ways of dealing with this problem...
Dolby Surround’s single mono surround channel was simply limited to a range of 100 Hz to 7 kHz, but there was very little separation between the front and surround channels so this didn’t necessarily always work that well and there would still occasionally be some dialogue leaking into the surround channel.
The Acra-Vector decoders dealt with this by using active steering to improve separation and applied a delay to the surround channel, which meant that if any dialogue *did* still manage to make it into the surround channel, it would at least be delayed ever so slightly so that it at least sounded as if the sound had travelled from the front to the back of the room rather than emanating from the front and back simultaneously. The amount of delay could be adjusted by the user in increments ranging from 16 to 36 milliseconds. Additional processing was also applied to further differentiate the back of the sound stage from the front. As far as I’m aware, there was no intentional filtering to limit the frequency range of the surround channel as there is in Dolby’s home decoder circuits.
Dolby Pro Logic is mostly an evolution of Dolby Surround, but with the addition of the surround delay that was used in the Acra-Vector ICs, although with less granular control in the consumer hardware; on my Yamaha Pro Logic decoder, for instance, the delay can only be set to either 20 or 30 ms rather than anywhere between 16 and 36 ms, whereas my Dolby SDU4 studio decoder does have granular adjustment via a screw rather than a simple toggle button. There is some additional separation of the surround channel compared to Dolby Surround, and it’s still low-pass filtered to limit it at 7 kHz, but apart from that and the added delay, there’s nothing fancy going on to improve the sound stage like the Acra-Vector logic does. The output is the same 4 channels as the cinema Dolby Stereo decoders: L+C+R+S.
My impression of these three different logic circuits is that the most cinema-accurate one is probably Dolby Pro Logic, although the one that sounds “best” is probably the Acra-Vector (Shure HTS-5300 in particular) due to the additional processing to improve the sound stage beyond what’s actually present in the encoded tracks.
Later iterations on Dolby Pro Logic allowed more than 4 channels to be derived from a 2-track audio stream. Pro Logic II introduced 5.1 decoding in 2000 (many years after the Shure HTS-5000 had already achieved something similar), and the steering logic is significantly more advanced than either Dolby Surround or Dolby Pro Logic, but the way it handles mono surround channels from 2.0 Dolby MP Matrix tracks has the unfortunate effect of clashing somewhat with the front left and right channels since the content is very similar but phase-inverted (this wasn’t really an issue with the older decoders that left the surround channel as mono).
So, let’s see if I can summarise all that:
Side note: After Dolby Pro Logic IIx and Dolby Pro Logic IIz, the next version of the Dolby matrix decoder/upmixer logic released in 2014 (designed to allow steering appropriate for Dolby Atmos compatible 5.1+ speaker arrangements) is called... Dolby Surround. Not Pro Logic III, or Atmos Logic, or anything else that might give you an idea of what it actually does. Just Dolby Surround. Y’know, the same name they already gave the very first home theatre matrix decoder logic back in 1982. So that’s cool.
Oh, and finally: Dolby Stereo isn’t the only 4:2:4 matrix encoding/decoding system used in the film industry, so even if there’s none of the above terminology being used for the stereo mix of a film, it’s probably still matrix encoded anyway. In 1984, Ultra Stereo was developed as a Dolby Stereo competitor, and is compatible with Dolby MP Matrix “stereo” (4:2:4) content. That later got rebranded as part of DTS (Digital Theater Systems), but that’s a whole other story.
The very earliest Dolby stereo theatrical releases used slightly inconsistent terms, but for the most part, it looks like posters and the like mostly just called it the “Dolby System” with additional adjectives like “stereophonic” to describe the experience to prospective cinemagoers. This was quickly rebranded as “Dolby Stereo”, but that seems to have happened after STAR WARS (but before THE EMPIRE STRIKES BACK in 1980).
Most of the other terminology around this stuff emerged later because of home video implementations of the matrix decoding technology, none of which was exactly 100% identical to the cinema decoders. If you want to learn about those, keep reading. Otherwise, feel free to skip ahead, but if you’re confused about what I’m calling things later, don’t blame me…
First we got “Dolby Surround” in 1982, which had a less sophisticated decoder than the cinema equivalent and only turned the 2 theatrical audio tracks into 3 (L, C, R) rather than 4 (L, C, R, S). The “missing” centre channel from the matrix was directed equally to the left and right speakers instead of being separated out to feed a centre speaker.
A third party solution soon appeared: Acra-Vector decoding logic was used in Shure home theatre decoders like the HTS-5000, HTS-5200, and HTS-5300, which somewhat confusingly bore the Dolby Surround branding even though they had different circuitry from actual Dolby Surround decoders. These were not only capable of turning the 2 matrix-encoded tracks back into 4, but could alternatively derive 2 surround channels instead of 1 (with a widening effect intended to emulate the sound of multiple surround speakers, as would have been used in cinemas at the time) and a low-pass filtered channel to be fed to a dedicated subwoofer. In other words, Dolby Surround turned 2 channel Dolby Stereo mixes into 3.0 (L+R+S), whereas the Acra-Vector logic could give you a pretty convincing 5.1 (L+C+R+LFE+Ls+Rs).
Despite being widely thought of as superior to any of Dolby’s own home decoders from around that time, the Acra-Vector solutions were quickly out-competed by the 1987 arrival of an official Dolby revision of Dolby Surround called Pro Logic. Both the Acra-Vector and Dolby Pro Logic decoding algorithms were able to turn 2 matrix-encoded L+R+C+S channels back into 4-channel L+R+C+S again, using “active” steering logic rather than the “passive” approach of Dolby Surround.
Since the surround channel is derived from the Lt and Rt (Left Total and Right Total) tracks using a phase-inverted version of the calculation used to derive the front centre channel, there is a risk of sound that should only be in the front centre channel “leaking” out into the back surround channel as well. This is undesirable since characters on screen are directionally in front of the viewer so it’s weird to hear their voices coming from both in front of and behind you. Dolby Surround, Shure/Acra-Vector and Dolby Pro Logic decoders had slightly different ways of dealing with this problem...
Dolby Surround’s single mono surround channel was simply limited to a range of 100 Hz to 7 kHz, but there was very little separation between the front and surround channels so this didn’t necessarily always work that well and there would still occasionally be some dialogue leaking into the surround channel.
The Acra-Vector decoders dealt with this by using active steering to improve separation and applied a delay to the surround channel, which meant that if any dialogue *did* still manage to make it into the surround channel, it would at least be delayed ever so slightly so that it at least sounded as if the sound had travelled from the front to the back of the room rather than emanating from the front and back simultaneously. The amount of delay could be adjusted by the user in increments ranging from 16 to 36 milliseconds. Additional processing was also applied to further differentiate the back of the sound stage from the front. As far as I’m aware, there was no intentional filtering to limit the frequency range of the surround channel as there is in Dolby’s home decoder circuits.
Dolby Pro Logic is mostly an evolution of Dolby Surround, but with the addition of the surround delay that was used in the Acra-Vector ICs, although with less granular control in the consumer hardware; on my Yamaha Pro Logic decoder, for instance, the delay can only be set to either 20 or 30 ms rather than anywhere between 16 and 36 ms, whereas my Dolby SDU4 studio decoder does have granular adjustment via a screw rather than a simple toggle button. There is some additional separation of the surround channel compared to Dolby Surround, and it’s still low-pass filtered to limit it at 7 kHz, but apart from that and the added delay, there’s nothing fancy going on to improve the sound stage like the Acra-Vector logic does. The output is the same 4 channels as the cinema Dolby Stereo decoders: L+C+R+S.
My impression of these three different logic circuits is that the most cinema-accurate one is probably Dolby Pro Logic, although the one that sounds “best” is probably the Acra-Vector (Shure HTS-5300 in particular) due to the additional processing to improve the sound stage beyond what’s actually present in the encoded tracks.
Later iterations on Dolby Pro Logic allowed more than 4 channels to be derived from a 2-track audio stream. Pro Logic II introduced 5.1 decoding in 2000 (many years after the Shure HTS-5000 had already achieved something similar), and the steering logic is significantly more advanced than either Dolby Surround or Dolby Pro Logic, but the way it handles mono surround channels from 2.0 Dolby MP Matrix tracks has the unfortunate effect of clashing somewhat with the front left and right channels since the content is very similar but phase-inverted (this wasn’t really an issue with the older decoders that left the surround channel as mono).
So, let’s see if I can summarise all that:
- Dolby Stereo is the name Dolby used for its 4:2 Dolby Motion Picture Matrix content when it was printed onto optical tracks on film prints for cinema exhibition;
- Dolby Surround, Dolby Pro Logic and later iterations thereof are names of consumer decoders for home use that decode that matrix-encoded 2-track content back to multiple channels again during playback;
- Dolby Surround branding was also used on Shure’s unique HTS home decoders, despite them actually using Acra-Vector integrated circuits instead of Dolby Surround ones.
Side note: After Dolby Pro Logic IIx and Dolby Pro Logic IIz, the next version of the Dolby matrix decoder/upmixer logic released in 2014 (designed to allow steering appropriate for Dolby Atmos compatible 5.1+ speaker arrangements) is called... Dolby Surround. Not Pro Logic III, or Atmos Logic, or anything else that might give you an idea of what it actually does. Just Dolby Surround. Y’know, the same name they already gave the very first home theatre matrix decoder logic back in 1982. So that’s cool.
Oh, and finally: Dolby Stereo isn’t the only 4:2:4 matrix encoding/decoding system used in the film industry, so even if there’s none of the above terminology being used for the stereo mix of a film, it’s probably still matrix encoded anyway. In 1984, Ultra Stereo was developed as a Dolby Stereo competitor, and is compatible with Dolby MP Matrix “stereo” (4:2:4) content. That later got rebranded as part of DTS (Digital Theater Systems), but that’s a whole other story.
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