CD-4 to DVD-A audio restoration / cartridges

[See Why Audio]

Hi all,

I've been pondering the best way to declick and restore audio to create a DVD-A from CD-4 vinyl.

My most pressing question is:

Is it possible to create a DVD-Audio disc, or take cleaned-up stereo analogue straight from sound card, with the clicks and glitches previously treated and replay this signal through a CD-4 demodulator to obtain a new clean 4 channel recording?

I've been restoring SQ records to DVD-A for a while now and I'm getting great results with script decoding in Adobe Audition by declicking before decoding but does this approach work with CD-4? I'm aware that I can always restore the decoded 4 X CD-4 channels but it's more than doubling my workload and besides my restoraton and declicking techniques work better in a stereo scenario.

I have an Ortofon MC25FL (the FL stands for 'fine line' apparently) with a stated frequency response of 20-22,000 Hz but I just made a test recording (in stereo at 192 Khz sampling) of a CD-4 LP and the recording shows a very strong signal up to and beyond 50Khz with the 30Khz modulation showing up as a well-defined line while slightly 'above' and 'below' 30K forms a kind of water-line reflection of the carrier. Just by looking at it I feel it MUST work with a decoder. (Err, but as yet I haven't obtained one)

As far as I can see, I can eliminate most clicks and glitches in the audio regions well below 30Khz while leaving the carrier almost entirely untouched. But I don't know if this is allowable! Anyone? Where does the 'audio' end and the 'carrier' begin?
What is the uppermost necessary frequency response for the demodulation?

Here's a picture:

Furthermore, can I get away with working at a stereo 96Khz sampling rate which tops out at mono=48Khz? (to save computer space etc.)

Anyone tried this?

I would be happy to work with someone who already has a set-up to record straight to 4 channel from the source, if I provide the source as a cleaned up stereo DVD-A? But does a DVD-A player output the required frequencies? I can supply a good set of cleaned up WAVs instead...?

Would any of this work?

So many questions in one thread... excuse me

Colin

PS I need to hear from a couple of you regarding previous projects...

 

[ArmyOfQuad]

If your pondering the best way to create a dvd-a from cd-4 vinyl, unfortunately I think the best way is going to be recording in 4 channels once, and cleaning it after decoding. The main issue is the fact that demodulators have a built in pre-amp, so the line level of the stereo dvd-a wouldn't work with a demodulator. Also, that would mean converting from analog to digital, back to analog, and back to digital again. I would much rather convert only once when trying to make the best possible conversion. I've had much luck doing cd-4 to dvd-a conversions by recording in the 4 channels and doing my noise reduction and declicking on the 4 channels, so it can be done, and can produce excellent results.

 

[winopener]

There are a couple of problem to work out:
- into the 0-48000 Hz the two audio bands, 0-17000 and 17000-45000 are treated differently: 0-17K pass thru a RIAA eq, 17-45K don't.
- level and impedance for input/output

The second is easily solvable for stereo or matrix LP because once you got the files into the PC you work out there and you don't need to go back to the analog world; with CD4 you must go back to analog for the decoding, so a proper linear amplification at first, then attenuation al the end, is mandatory. A cd4 demod accept phono-cart levels, not line levels!

Given all that, it may work.

One thing i would do is:
after the EQ/Cleaning process has been done with a Sample Rate of 96KHz (to have 0-48 bandwidth, needed for proper cd4 sampling), upsample the resulting 96K stereo file to 192K stereo then burn it as a dvda on a rw. It will fit in a dvda anyway and you can make the digital filter on the dvda player work better in the real audio band (0-48KHz), thus coming out with a better decoding.

 

[The Quadfather]

The lower frequencies below 15khz are unmodulated front plus back information (music). The 30khz carrier is FM modulated with the difference signal, that is front minus back. The modulation will swing the 30khz carrier from down to near 15khz and up to 45khz. A demodulator is actually two demodulators, one for left and the other for right channels. It is imperative that any attempt to record the undemodulated CD-4 signal must use equipment that can record and playback faithfully a frequency range of 20hz to 45khz in order to capture the audio and carrier intact. As far as the effect of noise reduction on this undemodulated signal goes, I would have no idea how well it would work. However, any process that leaves the original signal intact should work. However, I would suspect that the results would be much more controllable with signals in the demodulated state. As far as feeding the demod with signals from a computer card goes, a simple resistor attenuator ought to do the trick and make it work. The hard part would be to get the signal preamplified with a wide band flat amplifier to feed to the card. the RIAA processing would have to take place upon demodulation. If you try this, I would be curious as to how well it works out.

The Quadfather

 

[Bob Squires Jr]

Strange question:

With the advent of those turntables that have USB outputs that go directly to the computer,
could something like that be used / modified to send the entire CD-4 signal (audio + carrier)
to hard disk if the turntable could be fitted with the appropriate cartridge
and the machine with a good USB input device?
The resulting signal could then be played back through a CD-4 demod unit
thus preserving the disc.

(Or a PCI CD-4 demodulator card??? Now that would be novel!)

I'm sure someone must have tried it by now... if so, what was the outcome
or what would it be if it were given a go?

Just curious...

-Bob

 

[BananaSlug]

Well, the USB turntable must be doing the D/A conversion, if it's 96k it should work. I'd guess that it sends the signal at 44.1k though and the high frequency carrier would be lost.

 

[ClarkNovak]

The preamps in those USB turntables are crap. Don't use one of those things for anything important or archival.

The company I work for makes radio station gear that will allow you to take the 4 channels output from the CD-4 decoder directly into your computer via your Ethernet NIC, bypassing the need for recording through the sound card entirely. (Actually you could do 16 channels at once, but I digress.) I've used this system to master 5.1 interleaved WAV files from DVD-A and SACDs for use on computer playout systems. It works beautifully and the audio is very clean since there's no soundcard A/D in the audio chain; the catch is that the setup will run you around $3,500 list.

If that figure doesn't scare you, message me and I'll get you more info.