[HN Gopher] 35mm cinema film and digital audio
___________________________________________________________________
35mm cinema film and digital audio
Author : zdw
Score : 70 points
Date : 2022-04-05 15:01 UTC (8 hours ago)
(HTM) web link (everydaything.substack.com)
(TXT) w3m dump (everydaything.substack.com)
| globular-toast wrote:
| 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.
| yuubi wrote:
| 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 wrote:
| 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.
| ghaff wrote:
| 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 wrote:
| 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 wrote:
| 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.
| evan_ wrote:
| 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.
| davidferguson wrote:
| 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 wrote:
| thank you for sharing your knowledge
| p1mrx wrote:
| You could replace the THX sound with "never gonna give you up".
| jackvalentine wrote:
| 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.
| davidferguson wrote:
| 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`
| toast0 wrote:
| 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.
| gwbas1c wrote:
| 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.
| Melatonic wrote:
| 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...
| klodolph wrote:
| 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.
| dylan604 wrote:
| 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.
| Melatonic wrote:
| I am mainly thinking about this for modern sources - I
| haven't run a DVD player in years and years. But I do have
| a Roku with a Dolby Digital hardware encoder built in that
| is encoding anything 5.1 from Netflix/HBOMax/Amazon etc etc
| to Toslink to send to my somewhat older receiver. It sounds
| pretty damn good so I am hoping it is defaulting to the
| highest bitrate it can but who really knows. Have my
| Chromecast Audio also using Toslink which I assume is doing
| stereo AC3
| klodolph wrote:
| TOSLINK is just an optical connector system, it's not a
| system for encoding. Just like "RCA" is a type of
| connector, and you can run whatever signal you want.
|
| Optical audio in consumer equipment is very limited. It
| seems like it should be high-bandwidth because it's
| optical, but it falls short. For consumer audio
| equipment, ordinary RCA cables and TOSLINK optical cables
| do the same job, and the optical cables don't have better
| bandwidth or let you do longer cable runs.
|
| You get two channels of PCM audio, or more channels of
| lossy audio. If you're running stereo or 2.1, I think it
| makes sense to configure your Roku to send a stereo
| S/PDIF signal over TOSLINK.
| Melatonic wrote:
| I understand how optical works - its just that my
| receiver only has optical or RCA. Early 2000s receiver.
| Roku only has HDMI out and Toslink. Trust me - definitely
| the best (and I believe ONLY way) to get 5.1 to this
| thing is using Toslink. No S/PDIF connections on either
| device.
| dylan604 wrote:
| I had to switch to a TOSLINK cable for my soundbar. My LG
| TV and LG Soundbar are really meant to connect via ARC
| using HDMI cable. However, mine had serious problems with
| going out of sync and playing distored audio for 10-15
| seconds before correcting itself. It was very very
| annoying. I switched to a TOSLINK cable and no longer
| suffer that issue.
| Melatonic wrote:
| Yea that is a somewhat common issue with HDMI and audio -
| if the device passing it through cant keep up then there
| can be sync issues. Or the source device can also be a
| problem (or an app on the source device).
|
| I have found toslink and SPDIF to be both very reliable
| in this though (probably since they are such mature
| technologies at this point).
|
| Make sure your TV is actually outputting the correct
| format over Toslink to your soundbar - many times they
| default to 2.1 instead of 5.1 or the wrong format
| (Assuming you have a soundbar with more than just 2.1). A
| lot of TV's also downmix to 2.1 audio if you pass through
| a source to them and then use the Toslink output of the
| TV to a receiver (like say if you did Apple TV --> TV -->
| Receiver)
| gwbas1c wrote:
| 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.
| lflux wrote:
| 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.
| gwbas1c wrote:
| 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.
| davidferguson wrote:
| 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 wrote:
| You can edit the Wikipedia article.
| babypuncher wrote:
| 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 wrote:
| 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 wrote:
| Although interestingly, Dolby Digital on film isn't exactly AC3
| - it's an earlier version that's incompatible.
| woodruffw wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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.
| dylan604 wrote:
| 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.
| gwbas1c wrote:
| See https://en.wikipedia.org/wiki/Dolby_Stereo
|
| > The optical soundtrack on a Dolby Stereo encoded 35 mm
| film carries not only left and right tracks for
| stereophonic sound, but also--through a matrix decoding
| system (Dolby Motion Picture matrix or Dolby MP[1])
| similar to that developed for "quadraphonic" or "quad"
| sound in the 1970s--a third center channel, and a fourth
| surround channel for speakers on the sides and rear of
| the theater for ambient sound and special effects. This
| yielded a total of four sound channels, as in the 4-track
| magnetic system, in the track space formerly allocated
| for one mono optical channel. Dolby also incorporated its
| A-Type noise reduction into the Dolby Stereo system.
| HPsquared wrote:
| 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.
___________________________________________________________________
(page generated 2022-04-05 23:01 UTC)