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Back to the Future Trilogy 4K Laserdisc PCM Syncs |
Posted by: crampedmisfit1990 - 2020-10-24, 11:02 AM - Forum: Requests, proposals, help
- Replies (9)
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I was wondering if anyone has the 3 BTTF LD PCM (bit perfect) tracks at least synced to the older BDs? I was waiting for someone to sync these tracks to the new 4K releases but haven't heard anything as of yet. The new transfers look gorgeous and a hell of a step up from the BDs from years ago and I really wanted to watch the 4K versions with the original stereo tracks. Thanx.
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Weird analogue / digital audio alignment on an LD |
Posted by: pipefan413 - 2020-10-18, 01:03 PM - Forum: Capture and rip
- Replies (12)
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Currently having a bit of a fight with one particular LD and I'm not sure what's causing it, but maybe somebody who understands more about the LD format itself may be in a better position to follow what's happening.
My usual method for syncing the digital audio to the video is as follows:
1. Extract the in-sync 48 kHz analogue audio from raw LD video/audio capture (which I'll have made some effort to ensure actually is in sync, with AviSynth and VirtualDub2)
2. Resample that to 44.1 kHz and dither to 16-bit in RX to use as sync target for digital audio
3. Delete samples from start of digital capture to precisely sync with analogue audio, therefore exactly sync with video as well:
![[Image: digital-analogue-ld-sync.png]](https://i.postimg.cc/Fh8qWB3T/digital-analogue-ld-sync.png)
4. Verify no other changes to digital audio contents by checking in hex editor that it matches exactly apart from the missing zero bytes from start
Then of course I'd go on with the resync of the digital audio.
However, I've just hit a wall because I've found an LD this does not work on. It's NTSC like all the others I've captured thus far so it isn't a PAL thing. I'm using the exact same hardware for capture as well. But basically, I can align a point near the start fine, but by later on they've desynchronised again. With every other LD I've captured, sync remains rock solid to the sample from start to finish using this method, but with this disc, the analogue track drifts and gradually falls earlier than the digital one over time. It's admittedly a very small discrepancy at least, but it's consistent across sides 1 and 2 (which is the whole thing). By the end of side 2 for example, it's something like 86 samples out of sync (analogue ahead and/or digital behind).
What might be going on here?
One thing I'm wondering: if an LD has digital audio, is that just converted through an ADC and chucked out both the TOSLINK (as digital) and RCAs (as analogue), or is there a completely separate "legacy" analogue audio track also stored on the disc which is just played directly out of the RCA output? If so, I guess it could be that this disc has a slight discrepancy between the analogue and digital tracks, and it's just the first time I've found that on a disc. If the digital track is converted in real time and thrown out the analogue output though, something's going wrong somewhere in the chain and I have no idea why it's suddenly different. I guess my next move should be capturing another disc (ideally one I already captured, but that feels like potential time wasted) to see if the problem resurfaces without changing anything else. I already captured this one repeatedly with identical results on both side 1 and side 2.
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