9 ms·

35mm cinema film and digital audio

110 points by zdw 5y ago

babypuncher 5y 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 5y 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.
- davidferguson 5y ago
  Although interestingly, Dolby Digital on film isn't exactly AC3 - it's an earlier version that's incompatible.
- lukeh 5y 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.
globular-toast 5y ago
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.
- ghaff 5y 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.)
  - davidferguson 5y 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.
    - ghaff 5y ago
      I think I've seen 2001, maybe Lawrence of Arabia, and perhaps one or two of the other big epic movies of that era on 35mm with mag sound. Even by the late 70s, decent prints were getting hard to find though. I would have thought Wikipedia would have a list but it's not obvious if they do.
- yuubi 5y 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.
  - lflux 5y 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.
    - davidferguson 5y ago
      If you try hard enough you can fit more! I've seen a projector with all of 35mm optical, 35mm mag, DTS, Dolby Digital and SDDS. Was a sight to behold!
woodruffw 5y 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 5y 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.
  - gwbas1c 5y 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.
    - aardvark179 5y ago
      That sounds like you might be thinking of the earliest form of surround sound, as was used for 2001 and some other films. This was developed to become Dolby Stereo in the late 70s, and Dolby Pro Logic for home cinema in the 80s.
      - gwbas1c 5y ago
        You're confusing magnetic tracks and optical tracks. See https://en.wikipedia.org/wiki/Dolby_Stereo https://en.wikipedia.org/wiki/Dolby_Stereo
    - dylan604 5y ago
      There was also stereo optical recordings but it was typically mono. You could hold the film up to a light source and see the inverse of the waveform. There was one place in town that I was aware of that had a stereo optical on their telecine unit. It wasn't that hi-fi though.
evan_ 5y 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 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 5y ago
  You could replace the THX sound with "never gonna give you up".
- davidferguson 5y 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 5y ago
    thank you for sharing your knowledge
  - gmueckl 5y 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.
davidferguson 5y 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`
gwbas1c 5y 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 https://en.wikipedia.org/wiki/Dolby_Digital#Cinema A constant bit rate of 320 kbit/s is used.
- Melatonic 5y 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-differences/ https://ottverse.com/mp3-aac-ac3-wav-wma-opus-audio-codecs-d...
  - dylan604 5y 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.
gwbas1c 5y ago
From https://en.wikipedia.org/wiki/Dolby_Digital#Cinema 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.
- davidferguson 5y 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.
  - gwbas1c 5y ago
    You can edit the Wikipedia article.
    - wolverine876 5y ago
      What makes you believe the GP is correct and whoever wrote that in the article is wrong, or that either are correct?
      - davidferguson 5y ago
        My knowledge comes from someone who helped develop Dolby Digital on film so I'm pretty sure it's correct.
        wolverine876 5y ago
        Sorry, I didn't mean to comment on your expertise, but rather the epistemology of who we believe. > My knowledge comes from someone who helped develop Dolby Digital on film so I'm pretty sure it's correct. That is certainly better information than anyone else here has, and you seem to actually understand the technology. I wish HN was 90% comments like yours and almost none of the rest, especially Wikipedia! But while we're talking about epistemology! Decades-old memories go wrong, and when vested interests are involved the memories can drift to desired beliefs or familiar narratives. That doesn't make the comment a bad one - it's very valuable, actual knowledge. But it's not sure to be correct either, and/or not precise.
toast0 5y 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 5y 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 5y 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.
  - davidferguson 5y 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.
jackvalentine 5y 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.
- LeoPanthera 5y 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.
  - davidferguson 5y 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.
  - jackvalentine 5y 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 5y 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.

110 points by zdw 5y 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 5y 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.
davidferguson 5y ago
Although interestingly, Dolby Digital on film isn't exactly AC3 - it's an earlier version that's incompatible.
lukeh 5y 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.

globular-toast 5y ago

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.

ghaff 5y 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.)
- davidferguson 5y 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.
  - ghaff 5y ago
    I think I've seen 2001, maybe Lawrence of Arabia, and perhaps one or two of the other big epic movies of that era on 35mm with mag sound. Even by the late 70s, decent prints were getting hard to find though. I would have thought Wikipedia would have a list but it's not obvious if they do.
yuubi 5y 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.
- lflux 5y 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.
  - davidferguson 5y ago
    If you try hard enough you can fit more! I've seen a projector with all of 35mm optical, 35mm mag, DTS, Dolby Digital and SDDS. Was a sight to behold!

woodruffw 5y 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 5y 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.
- gwbas1c 5y 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.
  - aardvark179 5y ago
    That sounds like you might be thinking of the earliest form of surround sound, as was used for 2001 and some other films. This was developed to become Dolby Stereo in the late 70s, and Dolby Pro Logic for home cinema in the 80s.
    - gwbas1c 5y ago
      You're confusing magnetic tracks and optical tracks. See https://en.wikipedia.org/wiki/Dolby_Stereo https://en.wikipedia.org/wiki/Dolby_Stereo
  - dylan604 5y ago
    There was also stereo optical recordings but it was typically mono. You could hold the film up to a light source and see the inverse of the waveform. There was one place in town that I was aware of that had a stereo optical on their telecine unit. It wasn't that hi-fi though.

evan_ 5y 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 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 5y ago
You could replace the THX sound with "never gonna give you up".
davidferguson 5y 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 5y ago
  thank you for sharing your knowledge
- gmueckl 5y 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.

davidferguson 5y 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`

gwbas1c 5y 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 https://en.wikipedia.org/wiki/Dolby_Digital#Cinema A constant bit rate of 320 kbit/s is used.

Melatonic 5y 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-differences/ https://ottverse.com/mp3-aac-ac3-wav-wma-opus-audio-codecs-d...
- dylan604 5y 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.

gwbas1c 5y ago

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

davidferguson 5y 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.
- gwbas1c 5y ago
  You can edit the Wikipedia article.
  - wolverine876 5y ago
    What makes you believe the GP is correct and whoever wrote that in the article is wrong, or that either are correct?
    - davidferguson 5y ago
      My knowledge comes from someone who helped develop Dolby Digital on film so I'm pretty sure it's correct.
      - wolverine876 5y ago
        Sorry, I didn't mean to comment on your expertise, but rather the epistemology of who we believe. > My knowledge comes from someone who helped develop Dolby Digital on film so I'm pretty sure it's correct. That is certainly better information than anyone else here has, and you seem to actually understand the technology. I wish HN was 90% comments like yours and almost none of the rest, especially Wikipedia! But while we're talking about epistemology! Decades-old memories go wrong, and when vested interests are involved the memories can drift to desired beliefs or familiar narratives. That doesn't make the comment a bad one - it's very valuable, actual knowledge. But it's not sure to be correct either, and/or not precise.

toast0 5y 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 5y 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 5y 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.
- davidferguson 5y 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.

jackvalentine 5y 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.

LeoPanthera 5y 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.
- davidferguson 5y 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.
- jackvalentine 5y 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 5y 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.