Has anyone found a demodulator for CD4 (Quadraphonic) Vinyl

[TimDog73]

So I have some CD4 vinyl that I would like to convert in to multi channel audio files.

I propose to play them using a Kenwood TRIO KP-5022 turntable and capture it using my sound card.

What I am looking for is a CD4 demodulator plug-in for Audacity to enable me to save all four channels.

I wanted to avoid the purchase of hardware CD4 demodulator as my sound card only has stereo inputs ans it is an add expense.

Has anyone attempted this before and does such a plug-in exist for audacity?

Many thanks in advance,

Tim

[steve]

I've not seen a suitable plug-in, but it could be an interesting project to make one using Nyquist.

According to this page the carrier signal was in the range of 30 to 50kHz (30,000Hz to 50,000 Hz), so for this to work your audio equipment must all be capable of handling these very high frequencies. In particular, your sound card must be capable of recording with a sample rate of 192 kHz and must not filter out frequencies between 30 and 50 kHz.

I've not found much on the CD-4 specification, but there may be enough information here to work out how to do it: http://en.wikipedia.org/wiki/Quadraphon ... Quadradisc (though it would be better if we had the full and accurate technical specification)

A good place to start would be to record a short sample recording (WAV format) using a sample rate of 192 kHz so that we can examine it. See here for how to post recordings: http://forum.audacityteam.org/viewtopic ... 49&t=64936

[armyjazzer]

I would love to have this also!! I think there is a script for Adobe Audition, but it would be great to have an option for Audacity!

[steve]

I would love to have this also!! I think there is a script for Adobe Audition, but it would be great to have an option for Audacity!

I'd be happy to help develop a Nyquist plug-in for this, but I don't have CD4 capable equipment.
I wrote previously: "A good place to start would be to record a short sample recording (WAV format) using a sample rate of 192 kHz so that we can examine it". I'm still waiting.

[TimDog73]

Sorry for the delay Steve.

I have attached a short sample in FLAC format as the WAV file was a bit too short, unfortunately its only a 96kHz I hope that will do. I try and get a 192kHz sample and as soon as I do I'll post it.

Many thanks for your help,
Tim

[TimDog73]

Steve,

I've been searching around for some documentation on CD-4 (Quadradisc) and turned up a PDF of an original document that was pa-portably written by JVC entitled "CD-4 AN INTRODUCTION".

It goes in to detail about the modulation methods used; specifically that both frequency and phase modulation techniques are utilised. In addition there it goes in to detail about the "Automatic Noise Reduction System" (ANRS) it utilises to prevent interference between the audio signal and the modulated difference signal.

You can download it from here:

http://www.quadraphonic.info/manuals/JV ... UCTION.pdf.

I have also analysed the file I sent you and the is a clear peak around 30kHz which is what you expect based on the information we have about the CD-4 format. See attachment showing the graph

Regards,
Tim

[TimDog73]

OK so I have done some more digging and found some more documents:

http://www.quadraphonic.info/manuals/Fo ... f_CD-4.pdf
http://www.quadraphonic.info/manuals/Al ... System.pdf
http://www.quadraphonic.info/manuals/JV ... SYSTEM.pdf

All of these go in to further varying levels of detail that may be found to be useful.

[steve]

Hi TimDog73, Just to say I'm not ignoring you - I've just got back from vacation/holiday and I'm working my way through a mountain of e-mail. It'll be a while 'till I get to reading through the documentation that you posted but I look forward to doing so as soon as possible.

[TimDog73]

Hi Steve,

No problem, hope you had a good break!

I've used this time to try and understand the demodulation steps we have to follow to recreate the four channels and produced the attached diagram (with thanks to my friend Steve for peer reviewing the diagram).

Best regards,
Tim

[TimDog73]

Steve,

I've attached a flow diagram for the demodulator with the ANRS expander circuit split in to its filtering components and expansion components. There is also more explanation in this one too.

Regards,
Tim