Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Laserdisc PCM Bit Perfect Testkit
#1
I've been meaning to start on some tutorials for capturing and syncing laserdisc audio, but I figured this is a good place to start since this test is essential to a capture setup. So what is it and how does it work? DTS encoded CDs allow 5.1 channel playback via a standard CD on supported equipment (if you listen to a DTS CD on a car CD player it will just sound like noise). The reason it is of interest to us is because it can only be decoded from a capture that is bit perfect. So the basic idea is that you burn the DTS test CD, capture it through your laserdisc player, decode back to a DTS stream, and then cross your fingers it sounds like music and not garbled noise Smile This testkit is the custom DTS test CD I made, DTS Parser v2 (included for posterity since his website has been down for a long time), and the directions below.

1. Burn bin/cue to CD-R. ImgBurn is suggested with a Taiyo Yuden CD-R (I had issues with cheap CD-Rs during testing)
2. Capture and export as 16-bit, 44.1 kHz, Stereo PCM (WAV)
3. Run DTS Parser 2.0
4. Check "Rebuild Stream" checkbox
5. Click "Add DTS file..."
6. Select DTS WAV (.wav) from file type dropdown in bottom right
7. Select your captured WAV from Step 2
8. Click "Start Batch"

If everything works ok you should see the progress bar fill to about 75% and be left with a .dts file around 4:14 (37.5MB). If the progress bar doesn't fill, the file is empty, or you hear noise instead of smooth elevator grooves then something went wrong.

DOWNLOAD
https://mega.nz/#!KnRlAKYC!OMflJHdwkLf_P...MYuNVcmHpM
Reply
#2
I'm going to be adding another test CD with a setup check I found on the end of a Laserdisc. It goes through all the channels with a voice and tones, so I figured it'd be useful to help make sure you have everything setup correctly (particularly for analog captures). If anyone has any other suggestions please let me know.
Reply
Thanks given by:
#3
Good approach and application wise easy to verify on the AVR/end-user's side, yes.

However, given a certain amount of paranoia, one should bear in mind that "DTS disguised as 16 bit PCM audio" usually comes in a 14-bit mapping, so only 14 out of 16 bit are actually used with the two most significant bits set to zero. Reason for the latter apparently was to prevent white noise to be played back at full scale, but instead at less harming ~ -12dBFS.

I don't clearly remember the concrete scenario, but I think to remember having had a case where the DTS was still okay, but still some bits being altered.

In order to make sure that really the entire bitstream is correct, a technically even better approach (admittedly definitely less sexy and a bit more geeky) is to create source data containing a test pattern or archive data.

For example, with my little test-CD with leading zeroes followed by a "ABCDEF" pattern (for offset compensation and synchronization) and a RAR header in text form, I instantly can see whether it is "bit perfect" or not right after recording by checking the data in any hex editor:

[Image: spdif-wave.png]

[Image: spdif-hex.png]

When cutting the "intro stuffing" away, I can also save the rest of the data beginning with the "Rar!"-header and check the archive against its embedded CRC32 in no time.
Reply
Thanks given by: bronan , pipefan413
#4
That's great, you should share it! Thinking about it more, I guess it should be possible to even write a little app to parse the data for you and check for the header..
Reply
Thanks given by:
#5
(2020-05-15, 06:53 PM)little-endian Wrote: Good approach and application wise easy to verify on the AVR/end-user's side, yes.

However, given a certain amount of paranoia, one should bear in mind that "DTS disguised as 16 bit PCM audio" usually comes in a 14-bit mapping, so only 14 out of 16 bit are actually used with the two most significant bits set to zero. Reason for the latter apparently was to prevent white noise to be played back at full scale, but instead at less harming ~ -12dBFS.

I don't clearly remember the concrete scenario, but I think to remember having had a case where the DTS was still okay, but still some bits being altered.

In order to make sure that really the entire bitstream is correct, a technically even better approach (admittedly definitely less sexy and a bit more geeky) is to create source data containing a test pattern or archive data.

For example, with my little test-CD with leading zeroes followed by a "ABCDEF" pattern (for offset compensation and synchronization) and a RAR header in text form, I instantly can see whether it is "bit perfect" or not right after recording by checking the data in any hex editor:

[Image: spdif-wave.png]

[Image: spdif-hex.png]

When cutting the "intro stuffing" away, I can also save the rest of the data beginning with the "Rar!"-header and check the archive against its embedded CRC32 in no time.

Having issues with the DTS Parser / dts2wav route so tried this approach (which I rather like). However, the file I tried to use is basically just a maxed out waveform so clips and because it clips, the data is changed. Yours looks like it isn't clipping when it's pretending to be audio. Was this just experimentation with different files to see which ones looked plausible as "audio" in a waveform?

I'm guessing process here is basically:

1. Take an archive (.rar, .zip, whatever) and import it into something like Audacity but telling Audacity that it's 16-bit signed PCM at 44.1 kHz (which just makes it construct a .wav header)
2. Export as .wav (I verified this was the same as the source file but with a header stuck on it)
3. Burn .wav to CD as audio (I haven't actually done this since the 90s I think so I used Windows Media Player which may be a mistake)
4. Record playback from LD player through TOSLINK and export back out and inspect

I suspect something is definitely wrong with my capture chain, probably the hardware or the drivers, but it's not even just that that's going wrong for me (stuff that has nothing to do with it, for instance, like dts2wav.exe refusing to work at all regardless of what I feed to it). Also, I tried ripping the burned audio CD again and then inspecting it to take the capture thing out of the equation but even that wasn't a bit perfect match to source (which could just be Windows Media Player being a dick, idk).
Reply
Thanks given by:
#6
What kind of CD-Rs are you using? You should also try burning as slow as your burner will allow. You'd be surprised how easy it is for burned CDs to be incorrect..
Reply
Thanks given by:
#7
(2020-09-14, 02:09 PM)bronan Wrote: What kind of CD-Rs are you using? You should also try burning as slow as your burner will allow. You'd be surprised how easy it is for burned CDs to be incorrect..

The CD-Rs are Verbatim ones, which I've been using for years and don't actually remember ever having a single dud. But it's possible, I'm sure. The thing is, I always verify against the image and I've burned it twice, both are supposedly bit perfect burns. I'm concerned that it might just be that I can't get bit perfect caps with this sound card. Which would suck because it cost me a fair bit and I'm skint.

I'm wondering if I should just bite the bullet and grab something like an ESI Maya44 EX (which I am guessing will probably do bit perfect and also has 4 analogue inputs, which is useful for something else I want to do) but I'm also trying to figure out a capture solution for video and I only have so many PCIe slots, hahah. I could import the ESI U24 XL but it'd be even more money because of location and it would only solve 1/2 problems (bit perfect capture but not 4 inputs) so that's probably not ideal; I guess the issue would be if I do buy the Maya44 EX and it turns out to NOT be bit-perfect.

EDIT: Just thought of something that could either be obvious or obviously stupid. This won't confirm that the LD player is bit-perfect (although it should really be) though I guess it rules out a problem with the CD burning and all that. But the Creative card has TOSLINK *output* as well as input. So... surely to test that it at least can capture bit-perfectly, I could just run digital audio out of the TOSLINK output and then record it coming back to the DBPro TOSLINK input, then compare the binary of the recorded file, right? I suppose if I can get to that point, then I know the audio hardware's configured properly, which means it is at least *capable* of bit-perfect capture. The problem then would be confirming that the LD player is. But... theoretically, it should be anyway, right?

I'm wondering if it's something to do with the CD burning thing, even though ImgBurn insists what's on the disc is identical to what's in the cue file. I guess I could try burning it with a different drive but that other one is a Blu-ray drive so might be even worse than a DVD one for a CD-R. But the thing is, if I burn the test audio (.cue) to disc, then *rip* it back off the disc, I can't decode *that* file with DTS Parser either. Even though it hasn't gone through any sort of resampling / sound card stuff at all. Buuuut... the rip may not be bit-perfect even if it's burned bit-perfect, I guess, so that doesn't *necessarily* mean it's the sound card.

I don't suppose you might be able to help me rule something out by linking / sending me the DTS in .wav form (as in, one that is already ready to run through DTS Parser)? I could then run it through DTS Parser immediately to check that works (in case DTS Parser is the issue somehow). If that works, I'll record it after sending out and back in (the sound card, without involving the LD player or CD-R). If *that* works, then I know it's either the disc burning bit, or the LD player itself, somehow. Make sense?
Reply
Thanks given by:


Possibly Related Threads…
Thread Author Replies Views Last Post
  Laserdisc PCM Capture Guide bronan 80 10,250 2020-11-19, 12:09 PM
Last Post: zoidberg
  Bit-perfect LD-capture - What's necessary? Dr. Cooper 29 5,334 2020-09-12, 03:31 AM
Last Post: pipefan413
  Laserdisc capture - general thread willie1959 49 17,874 2020-07-30, 09:54 AM
Last Post: spoRv
  Locations List of people willing to assist with bit-perfect LD audio capture jerryshadoe 36 18,214 2020-07-01, 04:26 AM
Last Post: BDgeek
  hardware for capturing Laserdisc video markosjal 10 2,006 2020-04-17, 08:42 PM
Last Post: bendermac
  Laserdisc Audio - Normalize or Gain? Colek 2 2,913 2019-02-15, 08:59 PM
Last Post: Falcon
  Bit perfect or bust? zoidberg 12 5,515 2017-01-24, 01:34 AM
Last Post: spoRv
  Demodulating AC3 RF From Laserdisc zoidberg 4 3,507 2017-01-07, 03:45 PM
Last Post: zoidberg
  Laserdisc's resolution Colek 1 2,075 2016-07-26, 01:14 PM
Last Post: jerryshadoe
  Laserdisc Noise PDB 20 13,617 2016-05-27, 09:06 AM
Last Post: Colek

Forum Jump:


Users browsing this thread: 1 Guest(s)