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spiderfood
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Post subject: How were LaserDiscs authored? Do files have headers? TOC? Posted: 01 Feb 2023, 20:41 |
Shows curiousity |
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Joined: 09 Dec 2022, 14:42 Posts: 19 Location: Afghanistan Has thanked: 0 time Been thanked: 2 times
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I've noticed that the silence at the beginning of discs, ie at the FBI piracy warning, that have a CD audio track have some form of silence that resembles a keep alive of sorts. It's not pure 0's but it looks like it has been crafted to be silent without being 0. It looks like random 8bit values > 0 < 6
It is digital and repeatable, thus technically part of the digital track but, my player never starts outputting over optical SP/DIF at the same sample. I can press stop and play 10 times, recording 10 different files and they will all be identical after a certain amount of samples but, each file might have a few more or less in front.
I used to use something called "Scenarist" for DVD authoring, many, many years ago and in a learning document it mentioned that DVD will read the headers of all files and start outputting from frame/sample 0. That doesn't appear to be the case with LaserDisc so I'm wondering how it knows when and where to start outputting sound at any point. For instance, if you skip 15 minutes of side A of the LaserDisc how does it know where to start reading again? Does a player ever drop 2 bytes and accidentally swap channels?
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admin
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Post subject: Re: How were LaserDiscs authored? Do files have headers? TOC Posted: 02 Feb 2023, 02:10 |
Site Admin |
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Joined: 07 Aug 2002, 23:37 Posts: 4549 Location: Tokyo Has thanked: 294 times Been thanked: 1144 times
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Yes they do have a TOC, telling about the number of sides, if CX decoding should be enforced, etc. Spec is https://webstore.iec.ch/publication/3720 but I have never found a copy online. Julien
_________________ HARDWARE DATABASE HLD-X0/9 LD-S9 OPPO 105/205 SL-1200G LDD-1 MSC-4000 R2144 PONTUS II C45 MC257
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atsampson
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Post subject: Re: How were LaserDiscs authored? Do files have headers? TOC Posted: 03 Feb 2023, 01:54 |
Genuinely interested |
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Joined: 08 Feb 2007, 17:24 Posts: 35 Location: Scotland Has thanked: 8 times Been thanked: 5 times
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spiderfood wrote: I'm wondering how it knows when and where to start outputting sound at any point. For instance, if you skip 15 minutes of side A of the LaserDisc how does it know where to start reading again? Does a player ever drop 2 bytes and accidentally swap channels? Each video field on a PAL or NTSC LaserDisc has several lines of digital information in the vertical blanking interval. There are codes for CLV/CAV positions, CAV stop frames, chapter numbers, safe fields to pause on, information about what audio channels are in use, and so on - see vbidecoder.h from ld-decode. So after moving the pickup, a player can tell where it is on the disc within a couple of fields. To find a given location, you make a guess about how far to seek, then see what location you ended up at - if it's not where you wanted to be, refine the guess and try again. The digital audio track is separate from this - it's encoded pretty much exactly like a CD, with audio broken into blocks, then interleaved into a different block structure for error correction with multiple channels of extra metadata being woven in (see Simon's code in ld-process-efm for exactly how this works). The metadata includes a table of contents in the lead-in, a timestamp on each block, and other details like whether preemphasis is in use. LD-G subtitles are also encoded as a metadata channel in the digital audio track, as with CD+G. So you've actually got two sources of timestamps while playing - the VBI lines in the video and the digital audio. in theory, at least - there are quite a lot of discs that are missing the digital audio timestamps for some reason, contrary to the spec, and real players don't seem to care. And many NTSC discs have two copies of the VBI data since there are both "MCA" and "Philips" formats on different lines. And some of the details of how exactly digital audio timestamps are meant to work aren't defined very well by the spec... Because the digital audio track is decoded in blocks, it's not possible for it to just drop 2 bytes - you either get a complete block of stereo audio samples at the end of the decoding process (a "frame"), or you know it's invalid.
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spiderfood
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Post subject: Re: How were LaserDiscs authored? Do files have headers? TOC Posted: 03 Feb 2023, 14:10 |
Shows curiousity |
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Joined: 09 Dec 2022, 14:42 Posts: 19 Location: Afghanistan Has thanked: 0 time Been thanked: 2 times
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atsampson wrote: ...you either get a complete block of stereo audio samples at the end of the decoding process (a "frame"), or you know it's invalid. After looking through some of the ld-decode files, particularly https://github.com/happycube/ld-decode/blob/master/tools/ld-process-efm/Datatypes/sector.cpp, it appears that the digital audio is as you say, pretty much exactly like a CD. Since my player starts outputting a variable amount of samples much less than the block/frame size to the SP/DIF, it seems impossible to collect/read all samples without a hardware modification. Thank you for the links, this information clears up my interest in the matter Out of curiosity, how is the RS correction data still present in the source file? What is used to read the file?
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spiderfood
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Post subject: Re: How were LaserDiscs authored? Do files have headers? TOC Posted: 04 Feb 2023, 17:04 |
Shows curiousity |
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Joined: 09 Dec 2022, 14:42 Posts: 19 Location: Afghanistan Has thanked: 0 time Been thanked: 2 times
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Thank you, I didn't understand that the "Doomsday Duplicator" was the source via USB 3.0. For me, that's a _LOT_ of money for those last few samples of silence . Although I guess you have to use that if you want some form of data other than music.
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fullspectrum2000
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Post subject: Re: How were LaserDiscs authored? Do files have headers? TOC Posted: 17 Feb 2023, 11:47 |
Knows how to post |
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Joined: 17 Feb 2023, 11:09 Posts: 9 Location: Australia Has thanked: 4 times Been thanked: 0 time
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spiderfood wrote: Well, What's the LD scene like in Afghanistan?
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blam1
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Post subject: Re: How were LaserDiscs authored? Do files have headers? TOC Posted: 02 Apr 2023, 18:53 |
Advanced fan |
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Joined: 21 Dec 2002, 18:44 Posts: 959 Has thanked: 0 time Been thanked: 122 times
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happycube wrote: While there can be a TOC in the CD/digital sound data (which is a side channel and will not show up in the audio), there's no TOC per se on the Laserdisc, chapter search on the player works by scanning the disk for the chapter flags in the vertical interval. Chapter, time and frame number is encoded in the vertical interleave. True TOC was added to discs in the late 80s, embedded into the PCM track. This include total run time, total chapter count and location of the chapters, and LD+G subtitles. If you do a chapter search on a disc without TOC, the player has to keep reading the RF signal to track the location. With TOC, the player knows exactly where the chapter marker is and search speeds are improved.
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