In this article Reid Caulfield explores Dolby® Stereo. The format which opened up the world to surround formats. You may work in 5.1 or Dolby Atmos, Dolby Stereo is where it all started!

First things first. There’s a lot of hate out there for Dolby Stereo these days, so let’s get a few things straight right up front:

• Dolby Stereo is an old, outgoing format. It has largely been deprecated in favour of “LoRo” over the last several years in various deliverables specs.

• Some broadcasters and streamers either still require Dolby Stereo as a deliverable or still have it in their deliverables lists, though its actual delivery may be negotiable by producers. I still see arguments arise over it on various sound forums. There seems to be a lot of enmity for Dolby Stereo, but I have nothing but love and respect for it as a format. I will explain why, below.

• Dolby Stereo was incredibly important from the time it was developed and released as a format (early-mid 1970’s) until at least the mid 2000’s, probably longer.

• Dolby Stereo has sonic issues, to the extent that, at least unofficially, Dolby is now cautioning against its use, though Dolby has other fish to fry with Atmos HE (Home Entertainment) adoption and so wants to shift broadcasters attention away from Dolby Stereo and toward Atmos product.

• I am not a paid spokesperson for Dolby or any other entity. These are simply my experiences, as they happened.

In The Beginning, And The Bottom Line

A brief refresher about Dolby Stereo (LtRt) for everyone, but without the loads of math. Dolby Stereo used to be considered magic. It lets us, as re-recording mixers, mix something in “more-than-stereo”; in the 1970’s and ‘80’s, that was 4.0 - no sub and a single center mono surround channel - all the way up through 5.1, and above - and stuff it all into two channels of audio that could be listened to in simple two-channel stereo, or “unfolded” back into its original discrete immersive format.

Dolby Stereo theatrical exhibition - the use-case which its original target market - was really expensive for theatres to get into; it required additional loudspeakers in cinemas and as well as Dolby Stereo hardware decoders, which were also very expensive. These were analog units, so everything needed constant aligning, because most film sound at the time was still optical, and Dolby’s early analog decoders were… inconsistent. Kind of like analog synth oscillators in the 1970’s. Oscillators could easily go out of tune, and Dolby Stereo Decoders could easily go out of proper playback spec in the field. 

*Insert Image of analog Dolby Stereo Decoder* (if you can find one)

My First Dolby Stereo Experience

I was 13 years old in 1975 and a huge Elton John fan. So when the film “Tommy” was released in ‘75, I went solely to see the Elton bit in the film. I was still a bit too young to know yet that it had come from a famous album by ‘The Who’ years earlier. But in 1973, I had decided that I wanted to be a music recording engineer (long story as to why that decision at such a young age) and all of my attention for the next 10 years was focused on that goal.

It was Summer, in the middle of the week in 1975 when I went to a theatre in Montreal to see “Tommy.” Because I was so into sound  - even at 13 years old -  I immediately noticed something I’d never heard before. “Tommy” is a musical and so there is no spoken dialog. Dialog that would normally be spoken was sung, except in this film, mostly by actors who were not singers. The whole film is like an acid trip. Oliver Reed, Ann Margret, Jack Nicholson, The Who… Director Ken Russell was obviously a mad genius.

But this thing I noticed, sitting alone in this really small cinema, center-center seat, was that, in some sections of the film, there was sound coming from behind me. It was “unfolded” four-channel Dolby Stereo and, I believe, the first major film to feature it, even though Dolby had invented it a couple of years earlier.

Why Does Dolby Stereo Even Exist? - Some Background

In the 1970’s & 80’s, premium theatrical-release films featured four available channels - LCRS - but most theatres didn’t convert their equipment for years because of the expense. Film distributors, however, didn’t want to have to ship multiple versions of the same film to different theaters because of the logistical nightmare. They needed a way for any given reel of film to carry the original four channel mix, as well as a simple two-channel’ fold down’ version of that same mix, without taking up precious room on the physical film. To please all parties, Dolby came up with a way to take those four mix channels and - using phase and levels manipulation - “Encode” the four channels down to just two channels. So, while film theatres had Dolby Decoders, mix stages had both encoders and decoders. The idea, basically, was that the film’s mixers would mix the full four-channel premium audio presentation of the film as usual, but then a new step would be added.

I/O Basics For Dolby Stereo

On the mixing stage, a four-channel mix would be fed into the analog Dolby Stereo hardware Encoder and two channels would come out the other side. But it was what happened in the box that was magical. The original four mixed channels would be combined and manipulated in the Dolby Encoder, which would then spit out as a playable, two-channel (“Simple Stereo”) version of the mix for those cinemas which had not yet converted to multichannel sound. For cinemas that did have the Dolby Stereo Decoder and extra speakers, those two channels of delivered audio could be ‘unfolded’ or “De-Matrixed” back into the original four component channels. This also meant that there was no need to find extra room for more audio channels on the actual physical film, which in the 1970’s was still mostly 35mm.  An added bonus: it didn’t hurt that 35mm optical sound was typically very noisy, allowing Dolby to combine their ‘Dolby A-Type’ noise reduction with Dolby Stereo encoding & decoding, selling and leasing more hardware to cinemas in the process. Respect.

The Dolby math for this original magic is well known and the math widely available on the Internet, so I won’t repeat it here. In the late 1990’s, when software came along to supplant the Dolby hardware, Dolby licensed their algorithm to a couple of companies. Now, it’s important to note that various software developers do offer packages that can encode and decode in exactly the same manner as Dolby Stereo - but they can’t call it Dolby Stereo, and if you were (are?) a broadcaster or ‘packaged-media’ distributor or you have created a theatrical film and included Dolby Stereo as a release format and you want to use the Dolby®Stereo logo (or even just the words, “Dolby Stereo”), then you have to have used a Dolby-licensed software package as opposed to any software that did exactly the same thing, but which didn’t license the underlying technology from Dolby. Dolby is an innovative company, but they are also a licensing powerhouse. In fact, it used to be that if you were mixing a film and you were going to be doing a Dolby Stereo version, then you had to pay Dolby to have a certified consultant come and sit with you while you mixed, which every major feature film did, because Dolby Stereo was everywhere back in the day.

Downsides to Dolby Stereo

There are issues with Dolby Stereo, however: it became evident early on that Dolby Stereo - encoded product could be problematic.

• If it was properly encoded at (and on) the mixing stage, it could be badly decoded in theatres if the theatre operators didn’t keep up with proper alignment of their analog Dolby Stereo hardware.

• The reverse could also be true: it was possible to badly encode during the mix (same reason), and thus play out incorrectly in theatres that did have perfectly aligned decoders. This would have been unusual but was not unheard of.

The “issue” we’re talking about are mostly “steering” issues, with random audio events flying off into channels they were not intended for. And even with perfectly aligned hardware everywhere, the 2-channel product sounded “phasey.” You know this sound because every now and then you can still hear it coming out of your 2-channel stereo television.

The “424” Dolby Stereo Mix Method.

The solution for proper encoding on the mix stage was fairly elegant, and a bonus was that it added hours to the mixing process for mix facilities, and Dolby was able to sell yet more expensive hardware to cinemas and mixing theatres.

Here was the process:

1. Mix the four channel version. Get the client to sign off on the mix.

1. Go back to the beginning of the film and mix again, this time employing the Dolby-developed-and-endorsed “424” mix & review methodology (usually with a Dolby consultant present): feed the four mixed output channels into the Dolby Encoder hardware. Remember, the encoder took four channels in, and spit out two “Dolby-Matrixed” audio channels. That two channel product would be fully QC’d to make sure the mix played back properly in only two, “simple-stereo” channels (LoRo).

1. On the mixing stage, those two Dolby-encoded channels - the channels that could be played out as simple two channel stereo - were then fed back into a Dolby Stereo decoder for “unfolding” or “De-Matrixing” the mix back to its four original, component channels.

Four channels into two channels, then back to four, on the mixing stage. 4-2-4.

Clever, no? The four mixed channels, but with only two channels on the final distribution medium - thus taking up less physical space on the film itself - could then be “unfolded” or “Decoded”, or “De-Matrixed” to play back in the original four LCRS channels. This way, one set of deliverables could actually carry two versions of the sound, and theatres could use whichever version they were equipped for.

Using this mixing methodology, mixers could catch any weird Dolby encoded artifacts - steering issues, etc - and correct them before printing the mix and sending it out, and there was often a lot that would need to be corrected. A lot of things could go wrong:

• Sometimes, what seemed to be plain old dialog or sound effects or music in a discrete 4-6 channel discrete mix would cause steering issues or strange artifacts in one or both of the resulting encoded or decoded outputs;

• Any intentional phase manipulation of, say, music, or dialog in a dream sequence was used as a creative effect - e.g. all of “Tommy”, LOL - could cause the Dolby encoded signal - which, remember, was really just phase and levels manipulation - to become confused and not translate properly down to two channels. And if it did translate well and without issue down to two channels, then perhaps the decoding back to the original four channels would present a problem.

Conclusion

And so, with a new dawn ahead of us all - and with massive shifts in content ownership - as I write this, it is May 26, 2021, and on this very day, Amazon has just bought MGM Studios - there may, in fact, be new opportunity for facilities, this time with Dolby® Atmos HE (Home Entertainment). So be on the lookout, and, honestly, be grateful for Dolby’s innovation and their 50-year-old and hugely innovative and influential “stereo-that-could”: Dolby Stereo. Along with their A-Type and then Dolby®SR noise Reduction, they changed the face of sound.