Laserdisc PCM Capture Guide

[bronan]

Yes the AC-3 RF output is the raw FM signal from the player which the demodulator filters and converts to a bitstream signal. I think that using the right channel analogue audio output (assuming it isn't muted) would result in the signal being FM demodulated which could corrupt the data, which is QPSK modulated, but couldn't say for sure. It's certainly an interesting development!

Ahh, good catch! I forgot the analog audio is FM encoded, so the player analog out will definitely have some processing going on. I've updated my initial post above to correct that.

[borisanddoris]

Borisanddoris, going back to your initial problem, could you elaborate? It sounds like you're saying when you run a LD through your demodulator into your A/V receiver it locks instantly whereas when running into a capture device there's a delay?

Correct. But not always.  I mean it's entirely possible that the Mac itself is having a hard time locking on but I don't think I've updated since my last batch of captures a couple years back since it's old as the hills and is no longer updated anyhow.  

I could move the Sony demodulator to the capture area but I prefer it in the main theater for its ability to switch inputs with the push of a button should I desire AC3 or LPCM/DTS.  

I think I can work around the issue fairly easily provided the side changes are well placed and have a little black lead time.  I don't have that much more to capture on the AC3 end honestly so for the off chance I need to still, this is entirely doable.

[zoidberg]

I guess it's possible the Sony is quicker to output a steady bitstream to which your capture can lock whereas the Yamaha takes a beat longer to get there, and your receiver is just very quick at identifying/decoding bitstream audio.

FWIW I've been using a Japanese Yamaha demodulator of late for captures as my Meridian 519 is having issues, yet it is fine for real-time playback of LDs into my receiver. With the Yamaha I can (if I want) leave recording running during the side change and the capture will continue flawlessly, with a clean gap of silence during the side break. I prefer to capture each side separately though as very occassionally a disc may be mastered with an abrupt auto-turn point and I'm paranoid about capturing every last frame of audio.

[Dr. Cooper]

But the AC3-RF-output has to be captured through Reaper just like in a bitperfect PCM-capture, correct? And instead of two channels you're just recording a Mono-WAV?

[zoidberg]

I'm not sure exactly, I don't know if you have to use the Domesday device or not

[Dr. Cooper]

I'd definitely like to try it out, just struggling with finding the right settings to get the "noise" from the RF-output. And how would the settings for Reaper be? Since it's connected to the U24XL, it assumes 2-channel-input.

[bronan]

According to the description on the page:

"Developed for LD-Decode, this project decodes AC3 Audio from an RF signal. The first executable, ld-ac3-demodulate, takes a stream of unsigned 8-bit samples at 46.08MHz to produce a stream of QPSK symbols, ready for the next executable. The second executable, ld-ac3-demodulate, decodes the stream of symbols into playable ac3 audio frames, while producing Reed-solomon, CRC and other statistics in the log file."

You'd just need to record the RF output from the player to a mono channel in Reaper. They are passing off the RF to sox as a regular 16-bit PCM file so I don't think there is anything more specific than that needed. I think it might be alright to record as 24-bit too, you'd just need to adjust the first -b 16 parameter to -b 24. You are basically just converting your recorded file to the bandpass filtered, unsigned 8-bit format that ld-ac3-demodulate needs with the first step.

[Dr. Cooper]

Doing some more testing, I think the problem is actually the LD-player switching to AC3. It's a Pioneer DVL 909. Unfortunately the manual doesn't tell much about how to get it to play AC3 from a Laserdisc, all it says is that the RF-output needs to be connected to a demodulator.

Anyone having ideas about how to get it to play AC3?

[BusterD]

Pretty sure the AC3 output will always be active while an AC3 disc is playing, like how the PCM will always be output on the digital outputs.

[Dr. Cooper]

It's really weird. The "Dolby Digital"-sign on the display is on, so the player recognizes the AC3-track, but when trying to capture whatever comes out of the RF-output it doesn't get anything. I assume it should actually be some weird noise like when DTS is captured?

Are there any options in the configuration of the U24XL that need to be adjusted to get the signal?