35mm cinema film and digital audio

https://everydaything.substack.com/p/35mm-cinema-film-and-digital-audio

by zdw • 4 years ago

110 73 4 years ago

everydaything.substack.com

The analogue soundtrack on 35mm is recorded optically and also used as a fallback in case the digital soundtrack fails for some reason. It often used Dolby Stereo which could be played as a regular stereo track but also as four channels including centre and surround.

70mm film was far superior with 6 discrete magnetic soundtracks. But it was also far more expensive and required entirely new equipment.

Digital soundtracks were a solution to retrofit discrete multi-channel audio to existing 35mm technology. I love how they found space next to and between the perforations. It reminds me of how colour TV works. DTS is one that used the timecodes and came on an external CD which would also be far cheaper and something cinemas could easily upgrade to without replacing existing equipment.

The LFE channel appeared with these formats and is another example of supporting retrofitting. The idea with LFE is cinemas could simply buy and install some subwoofers and feed the LFE channel directly to them. No expensive bass processing required. No need to touch the existing main channels. Easy upgrade. Now every cinema has bass management so it's not really necessary but it's still there.

ghaff2• 4 years ago

As I recall you also had mag strips on some 35mm films but it wasn't very widely used. (And it may have been all films that were originally shot in 70mm.)

davidferguson13• 4 years ago

Yes, a few 35mm films had 4 track mag audio. The only one I can think of off the top of my head was Fantasia.

yuubi• 4 years ago

Dolby digital could be used without replacing much existing equipment: the sound head looked like it bolted right onto the 1940s-vintage projector in the one installation I've seen. would look like it belonged if the projector wasn't gray wrinkle paint and the sound head smooth black finish. yes, several parts have been updated, including the light source and a platter system, but again it looks like none of this involved doing any violence to the actual projector.

lflux3• 4 years ago

Depending on your projector and the amount of space you can fit one or two digital heads in there. We modded our 1950's era Bauer U2's to add DTS readers upstream of the analog sound readers, but at the expense of the 70mm magnetic sound heads. Took about an afternoon.

icodestuff• 4 years ago

There were several projectors (such as the Century JJ) which could show 35 and 70mm that were stuck with only 35mm capability after the Dolby Digital upgrade; the mounting location blocked the 70mm egress slot. There were workarounds that involved hacksaws dremels, and a new roller, but not very good ones.

woodruffw• 4 years ago

Sound on film is fascinating, and demonstrates a funny tension in analog formats: there's visible grain in the frames themselves, and yet a precise light source can encode massive amounts of data in just the spare emulsion between the sprockets.

TedDoesntTalk• 4 years ago

Some old "Super 8" 8mm handheld movie cameras recorded audio magnetically on the side of the film, which is weird because the film itself is not magnetic at all. There's just a thin magnetic strip running along the edge. I've seen it on recordings from the 1970s, but it probably dates to the 1960s.

gwbas1c9• 4 years ago

Theatrical "Stereo" in the 1960s and 1970s was handled via a magnetic strip. I believe it was 4-channel, 3 in the front and one in the rear.

HPsquared• 4 years ago

Makes you wonder what resolution of video could be encoded digitally if you used the entire frame. With compression algorithms and a good encoding method, surely the image quality could be improved over the standard analogue print.

frostburg• 4 years ago

Given that using 0.1 ISO copy film wouldn't be an issue (unlike when shooting live action), you could probably write high bitrate 8k on it.

davidferguson13• 4 years ago

There is actually a reason to do this - and people do. For long term archival film is king - it's store and forget, unlike hard drives that require migration every few years. So people write digital data to film, either in the form of text documents as images, or some digtal encoding of the data, and then it's put into long term storage.

GitHub did this with its Archive Program https://archiveprogram.github.com/, and Piql https://www.piql.com/ is a company that specialise in doing this. They're the company that created the Cinevator, a high speed data to film transfer machine that was originally used for creating film prints, but since film is kinda dead these days, they've pivoted to archival storage.

evan_• 4 years ago

Detail from the comments, about the QR-code-like blocks that appear between sprocket holes:

The blocks also carried software, so that the decoding equipment (such as the DA-10 and DA-20) could have their firmware automatically upgraded in the field, just by playing a newly-released film! Pretty advanced for early 1990 electronics.

I found another oblique reference to this in the manual for the Dolby DA20 sound processor, page 8-6 here, where it refers to "dynamic loading":

http://www.film-tech.com/warehouse/manuals/DOLBYDA20.pdf

however, I can't find anything else online about this. This raises the possibility of putting a film-bourne virus in a movie or trailer, though I'm not totally sure what you could really do by pwning a sound processor.

p1mrx• 4 years ago

You could replace the THX sound with "never gonna give you up".

davidferguson13• 4 years ago

Dynamic loading was indeed a software update of the processor using the data carried on film. If a film contained an update, it would be contained at the start of a film. The processor would detect this, read the software update, reboot (and revert into mono analogue audio for this) and then continue off using the updated software.

There are 3 blocks every 128 on the film that contain the software update (and/or other data channels, or are unused). They're also used for something called the "Splice cache" which I can discuss if you want. The rest are the audio blocks.

As far as I'm aware, Dolby only used this once - to update from what they called version EC9 to EC11. I've also heard that it didn't work well with the DA10 - their first processor - and this may be part of the reason.

slim• 4 years ago

thank you for sharing your knowledge

gmueckl• 4 years ago

Is the "splice cache" used to cover for discontinuities (e.g. reels spliced together for the entire film, cuts to remove damaged sections...)? I'm just guessing from the name here.

LeoPanthera2• 4 years ago

The Wii used this technique - game discs included the latest version of the system software, so that consoles would get updated by buying new games even if they were never connected to the internet.

ace2358• 4 years ago

I think that was quite common in that era. I’m sure my (banned) 360 would still update from game discs. Not sure on the ps3 though.

babypuncher• 4 years ago

The Dolby Digital track is actually compressed, not uncompressed as the author speculates. Back then, "Dolby Digital" branding was still synonymous with the Dolby AC-3 codec, which offers lossy audio compression.

mrob• 4 years ago

The article miscalculates the data rate as "584KB/s (Kilobytes/s)" when the numbers actually give 554kb/s (using the patent's 5776 bits per square). This is kilobits per second, not kilobytes per second. The incorrect kilobytes number is more than enough for 6 channels of uncompressed CD quality audio, but the real number requires lossy compression.

davidferguson13• 4 years ago

Although interestingly, Dolby Digital on film isn't exactly AC3 - it's an earlier version that's incompatible.

lukeh• 4 years ago

Interestingly AC3 (with Joint Object Coding) is also what Apple Music uses to deliver Spatial Audio, although the OS makes it difficult to decode to any layout beyond 5.1.

davidferguson13• 4 years ago

TLDR: If you know about audio encoding and/or want to help reverse engineer this format, I'd be very interested in hearing from you!

----

Oh, a topic I know a bit about!

To add a few things onto the stuff mentioned in the article: - A lot of places mention Dolby Digital on film being AC3 encoded. This is sadly not true - ish. Dolby were developing AC3 for film and other applications, but the version that ended up going on film isn't quite AC3 - although I suspect it's very close. To quote someone at Dolby:

The audio coding used is actually a pre-cursor of what became AC3, and in fact, two different versions of the codec were used during the lifetime of Dolby Digital on film. We referred internally to these as EC9 and EC11. I don’t remember the exact characteristics of these codecs but unfortunately they are not compatible with AC3

- The article mentions Reed-Solomon encoding being used - this is indeed the case, but more specifically a concatenated code meaning that two Reed-Soloman instances are applied one after the other. However because nothing is nice and simple here, some of the bits are only protected by one of the R-S layers and not both, but I'm not sure which at the moment.

- The article questions how the bytes are stored - this is actually mentioned in the patent. The bytes are arranged in 2x4 groups, ie 2 horizontal, 4 vertical, and therefore each block is 38 bytes wide, and 19 bytes high. Interestingly the picture in the article is rotated - you can tell by the Dolby logo being the wrong way round.

- Something not mentioned in the article or the patent - before the Reed-Solomon decoding is applied, all the bits are XORed against a constant in order to break up large black or white areas which would be hard to detect. Because the same value is used during encoding as decoding, the original data is recovered. Sadly, I don't know what this constant value is.

I would love there to be a open source decoder for the Dolby Digital on film, but at the moment I'm at the limit of my knowledge on AC3 and audio signal processing. If anyone knows more, or wants to help, I'd be delighted! You can reach me on email `fergusondavid6@gmail.com` or discord `davidferguson#2018`

gwbas1c9• 4 years ago

I suspect the author doesn't know much about digital audio and theatrical audio.

The Dolby Digital signal is AC3, a lossy codec for 5.1 audio. I believe it's the same AC3 that DVDs use. Bitrates (and audio quality) is roughly similar to MP3. Both were developed around the same time.

I suspect audio data is stored uncompressed.

A little bit of math: 96 squares a second, at (assumed) a 48000 / second sample rate correlates to 500 samples / block. If it's 5778 bits a block, (and that's a strong if,) that's ~1000 bits / block / channel, or ~ 2 bits a sample. There's definitely a LOT of compression going on.

I couldn’t find any public references to the dolby matrix encoding format

The author is confusing terms here. "The Dolby Matrix Encoding Format" is what's going on in the analog audio tracks. In audio, "matrix encoding" is how multi-channel audio is derived from two-channel audio. The analog audio is "Dolby Stereo," where a 5.1 mix is "matrixed" into a 2-channel recording, and then "de-matrixed" on playback. (In theatrical audio, "stereo" means what home audio calls "surround" or "multichannel.")

From https://en.wikipedia.org/wiki/Dolby_Digital#Cinema A constant bit rate of 320 kbit/s is used.

Melatonic5• 4 years ago

You sure Dolby Digital is that low of quality? From what I understood it was lossy compression but mainly limited by the bandwith of things like optical cables and could approach some pretty decent levels. Just because it might be at a similar sounding bitrate does not mean the quality is the same.

edit: This explains the differences pretty well - Dolby Digital (original version) can go higher than 320kbs

https://ottverse.com/mp3-aac-ac3-wav-wma-opus-audio-codecs-d...

dylan6044• 4 years ago

Stereo AC3 encodes for DVDs were commonly 128kbps with some in the 224kbps-256kbps range with 5.1 surround in the 320kbps-384kbps range. IIRC, AC3 could go to 512kbps, but rarely did that happen as it took away bandwidth from the video. All of the video/audio/subtitle streams in DVD VOB had a total bandwidth of 9.8Mbps. If you had multiple 5.1 @ 320kbps and a default stereo as 224kbps, you've already robbed just under 1Mbps. Subtitle streams were small, but added up as well when supporting multiple languages. Avg desired bitrate for video was 8.5Mbps, but with overhead and all of the audio, it was often much lower. All of that math is just for the demuxer and does not take runtime into consideration.

klodolph4• 4 years ago

First, I wouldn't call MP3 "low-quality". There are some limitations to MP3, and it has lower fidelity than other codecs at the same bit rate, but MP3s are crystal clear when done right.

AC3 has higher fidelity at the same bit rate, and can be used at higher bit rates, but the difference between AC3 and MP3 is not exactly night and day. AC3 is roughly similar to MP3 in terms of quality and bitrate, but that's not a bad thing.

As mentioned elsewhere in the thread, the audio format in cinema wasn't technically AC3 but something quite similar.

gwbas1c9• 4 years ago

From my experience, "low quality" MP3s had little to do with limitations of the MP3 codec. They came about because whoever encoded them chose low bitrates, used a bad encoder, or chose bad settings when making them. And, typically whoever was encoding an MP3 wasn't a trained professional.

In contrast, AC3 encoding was performed by professionals who understood what they were doing.

lflux3• 4 years ago

Dolby SR, the 2-channel analog format on the film, isn't 5.1. It's 4.0 with Left, Center, Right front channels and Mono surround.

nezgar• 4 years ago

Dolby Surround aka ProLogic, was later enhanced with Prologic II which provided a higher bandwidth and rear left+right matrix.

gwbas1c9• 4 years ago

From https://en.wikipedia.org/wiki/Dolby_Digital#Cinema

Dolby Digital cinema soundtracks are optically recorded on a 35 mm release print using sequential data blocks placed between every perforation hole on the sound track side of the film. A constant bit rate of 320 kbit/s is used. A charge-coupled device (CCD) scanner in the image projector picks up a scanned video image of this area, and a processor correlates the image area and extracts the digital data as an AC-3 bitstream. The data is then decoded into a 5.1 channel audio source. All film prints with Dolby Digital data also have Dolby Stereo analogue soundtracks using Dolby SR noise reduction and such prints are known as Dolby SR-D prints.

davidferguson13• 4 years ago

All correct (as far as I'm aware), except for the unfortunately common inaccuracy that Dolby Digital on film is AC3. Sadly it's not - it's an earlier iteration of that technology that's incompatible. It must be pretty similar, though.

gwbas1c9• 4 years ago

You can edit the Wikipedia article.

toast0• 4 years ago

I picked up some 35 mm feature film at an auction, thinking I'd be able to find a cheap projector at some point and it could be a fun weird hobby... But then I realized the cheap projectors are not 35 mm. Also, I only got half the film (reels 2, 4 and 6)... so if anyone wants half of Twilight: Breaking Dawn - Part I, my email is in my profile.

milderworkacc• 4 years ago

This is funny, you've got half mathematically, but logically you've got the second, fourth and sixth sixths of the feature!

When I was still working as a projectionist, for big releases (like Twilight) the film courier would often bring three reels on Monday and three reels on Tuesday, so you could make it up Tuesday night and test before the film opened on Thursday (or Wednesday night for midnight releases). I suspect that is why you have what you have...

Thlom• 4 years ago

Most theaters moved to digital around 10 years ago and most of them got rid of their 35mm projectors (some kept one or two just in case)(I worked on the provider side of the migration to digital cinema at the time). In theory there should be an abundance of cheap 35mm projectors on the market, but maybe the demand wasn’t all that so most of the projectors were just tossed? I know some projectors were shipped to 3rd world countries as well.

davidferguson13• 4 years ago

There was no demand, because you can't buy 35mm prints, unlike 16mm and 8mm. And additionally, in order to finance the purchase of digital projectors, many cinema owners went with something called "Virtual Print Fees" (VPFs), where the cost was paid off in time. However, most of these mandated that the film projectors were either removed and destroyed, or disabled so they couldn't run film. Why? Well this ensured that everyone switched to digital and couldn't go back, so the VPFs were still paid. Very sad, hearing about all these machines being damaged.

jackvalentine2• 4 years ago

Used to run films in a past life.

The SDDS soundtrack (blue strips on the outer edge of the film) had severe issues with reliability the older the print got. This is because that part of the film is the bit that touches the rollers while the film is being transported from the platter to the projector and back so unless everything is squeaky clean and perfectly aligned it’ll get scratched and SDDS would begin to drop out (and fallback to other formats, usually analogue depending on your config) resulting in a bad experience.

If you got a second-hand print from somewhere you’d just never bother to run SDDS.

LeoPanthera2• 4 years ago

Since it's digital data, I wonder if anyone ever captured the data the very first time it was played back, and then just played the captured version on subsequent runs.

It would eliminate the problem with it wearing out.

davidferguson13• 4 years ago

I've captured Dolby Digital data to rip the audio from the film before. I don't (yet) have a SDDS reader, but when I get one it's something I want to try.

jackvalentine2• 4 years ago

Commercially? Probably never happened. It is just easier to use more reliable formats like Dolby Digital or DTS (which is shipped with the print on CDs and synchronised using a timecode… some DTS units had HDDs in them you could load the soundtracks in to so you didn’t have to swap the discs over when you changed what movie was showing).

Maybe it was different elsewhere, but SDDS was pretty much a non-starter for us for any kind of effort.

ficklepickle• 4 years ago

I worked as a projectionist in an old theatre in my teens that used 35mm film and the analog sound stripe. This was early 2000s and was very dated for the time. Projectionist used to be a trade, but by this point they gave the job to kids with minimal training for minimum wage. I loved threading the film through the projector and across the room on pulleys to the platter that wound it back up, the clickity clack when you turned it on.

One neat thing I remember: we applied these little foil bits on the edge of the film to control the house lights. They were applied at either end to dim the lights when the movie started and bring them back up at the end.

I think I still have a Star Wars Ep. III trailer on 35mm kicking around.

35mm cinema film and digital audio

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