Working CD-4 (software) Demodulator!

[kfbkfb]

https://www.quadraphonicquad.com/forums/index.php?threads/cbs-sq-matrix-talking-points.26686/^^^
Indirectly mentions the CD-4 baseband level issue and
also has a paragraph about the RCA CD-4 discs same price
as Stereo discs business plan.


On a separate issue, is there any evidence that the ANRS
decoding curve change in the JVC CD4-50 is to deal with
the distortion caused by the unwanted/added angle
modulation of the FM carriers by high baseband levels?

Kirk Bayne

 

[Brice]

Comparing the CD4-50 with an early model like the Marantz CD-400 (JVC 4DD-5), the circuits look more different than they really are. (For example, discrete op-amps replace single-transistor common-emitter amplifiers in the early design.) But the circuits are fundamentaly similar.

I understand that it is widely regarded that this later decoder certainly behaves rather better when presented with "difficult" CD-4 material. It appears that the way that the ANRS is aligned is different in the CD4-50. It's quite possible that this alignment change was a reaction to poor quadraphonic performance as recording levels got driven upwards by commercial pressure. But I have no proof of that.

Richard

 

[kfbkfb]

2 suggestions (for the HQ/Classic CD-4 Decoder):
Check Box for: 12dB F/B separation mode
Check Box for: F/B Hi-Blend (fixed, not dynamic)

The idea is to use as little as possible of the F-B
difference signal, thus, if the difference signal is
somewhat distorted, the HQ decoder can still be
used, rather than using the soft fail decoder.

Kirk Bayne

 

[Brice]

Hi All,

Just to let you know that there is a new version of Stereo Lab available from today (3.1.14) which includes a quadraphonic decoder for BBC Matrix H (HJ) material. This will also work reasonably well for UMX, BMX, UD-4 discs.

Best,

Richard

 

[Mendemender]

Thanks for the info Richard. I'll install it in due course

 

[kfbkfb]

Mendemender - Have you tried ClickRepair on CD-4 discs?

From ClickRepair manual:
"...whilst the damage itself is replaced by an interpolation based on analysis of the surrounding sound"


Seems like this method would distort the CD-4 carrier(s).

Kirk Bayne

 

[Brice]

Stereo Lab has its own ClickRepair type algorithm (which is generally regarded to be very good). You can use this with CD-4 discs because we have placed it in the signal chain AFTER the low-pass filtering to create the sum channels.

Kirk is right that you cannot de-click the raw needle-drop without trashing the sub-carrier information.

Best,

Richard

 

[Mendemender]

I have tried ClickRepair on sandpapery demodulated signals (without any success) but not on the raw needle drops. One advantage of ClickRepair is that you can control the declicking whilst it is happening. Theoretically I guess you could experiment and set it up so that it always stops when it looks like there may be a click. Then you could adjust the section to be repaired manually. If there is sub-carrier there then you could leave it unchanged. If only damage is present then I guess ClickRepair would interpolate some 30kHz into it. Whether the results would be better than demodulating pure damage remains to be seen (another thing on my tuit pile). Also processing a whole record manually would take forever if the record isn't almost perfect. The best solution I have found is to buy a good copy of the record

 

[proufo]

Does anyone know what exactly the radar light in demodulators responds to?

Thanks in advance.

 

[Circular Vibes]

It tells you that there is a 30 kHz signal present, that is all. It is not a sign that you have a clean signal or anything else. If it flickers, your signal is breaking up.

 

[proufo]

It tells you that there is a 30 kHz signal present, that is all. It is not a sign that you have a clean signal or anything else. If it flickers, your signal is breaking up.

Is that as it ONLY measures the 30khz signal, so it has the inverse of a notch filter?

 

[Circular Vibes]

That I do not know. I just know it comes on with the presence of the carrier siganl and with worn mono and stereo records. I even triggered it with worn 78s. I have to figure it is the presence of a signal above a certain point but I have no way to measure the point it kicks in. If I slightly slow the platter with my finger, it loses its lock and then becomes audible.

 

[proufo]

That I do not know. I just know it comes on with the presence of the carrier siganl and with worn mono and stereo records. I even triggered it with worn 78s. I have to figure it is the presence of a signal above a certain point but I have no way to measure the point it kicks in. If I slightly slow the platter with my finger, it loses its lock and then becomes audible.

Many thanks CV.

I have the notion that a demodulator with a CD-4 record could be used to set VTA, if the stylus is of the line-contact type (long contact area).

 

[Circular Vibes]

I have no idea how to do that. I have heard of using an oscilloscope, but do not know the process. I wonder if my Technics SH 3433 would do the job and what discs were used. A local stereo shop used to provide this service for over $125. I'd like to know how to do it myself.

 

[proufo]

I have no idea how to do that. I have heard of using an oscilloscope, but do not know the process. I wonder if my Technics SH 3433 would do the job and what discs were used. A local stereo shop used to provide this service for over $125. I'd like to know how to do it myself.

My guess is that any misalignment between the very-thin grooves and the LC stylus would cause a reduction in the amplitude of the ultrasonic frequencies (and perhaps distortion). This would be detected by the demodulator (set to a borderline carrier measurement level) by turning off the radar light.

 

[fredblue]

I have no idea how to do that. I have heard of using an oscilloscope, but do not know the process. I wonder if my Technics SH 3433 would do the job and what discs were used. A local stereo shop used to provide this service for over $125. I'd like to know how to do it myself.

i think maybe if you are already getting a beautiful steady clear 30kHz tone out of one of those CD-4 adjustment 45's with no distortion, no dropouts, no warbling/wobbling, just a lovely long "meeeeeep" all the way through you're on the right lines to getting things setup as best you can because as soon as i change the VTF, azimuth, VTA, anti-skate, etc on my tt that's the first track to go tt's up!

 

[proufo]

i think maybe if you are already getting a beautiful steady clear 30kHz tone out of one of those CD-4 adjustment 45's with no distortion, no dropouts, no warbling/wobbling, just a lovely long "meeeeeep" all the way through you're on the right lines to getting things setup as best you can because as soon as i change the VTF, azimuth, VTA, anti-skate, etc on my tt that's the first track to go tt's up!

Right, but it is possible that you can compensate for bad VTA alignment by increasing the 30kHz gain.

This is not related strictly to playback of CD-4 records, but to using a CD-4 record (any, not a hard-to-get test record) for setting up VTA, for all records.

 

[proufo]

i think maybe if you are already getting a beautiful steady clear 30kHz tone out of one of those CD-4 adjustment 45's

The grooves in a 45rpm record would not be as thin as the ones in a CD4 regular 33rpm Lp. It is likely a regular CD4 Lp is a bit more sensitive to bad VTA.

You don't hear a clear 30kHz tone, but a clear 400Hz tone. I guess the level of the carrier signal would be a combination of tracking accuracy and demodulator gain. I guess you can have bad VTA compensated by higher gain to get no playback distortion.

 

[fredblue]

Right, but it is possible that you can compensate for bad VTA alignment by increasing the 30kHz gain.

This is not related strictly to playback of CD-4 records, but to using a CD-4 record (any, not a hard-to-get test record) for setting up VTA, for all records.

well my last two demodulators are auto lock- on jobs with no adjustment so they either lock on to the 30kHz carrier or they go schizoquadrophonic!

 

[fredblue]

The grooves in a 45rpm record would not be as thin as the ones in a CD4 regular 33rpm Lp. It is likely a regular CD4 Lp is a bit more sensitive to bad VTA.

You don't hear a clear 30kHz tone, but a clear 400Hz tone. I guess the level of the carrier signal would be a combination of tracking accuracy and demodulator gain. I guess you can have bad VTA compensated by higher gain to get no playback distortion.

good point, i also have a 33rpm Project 3/Fisher CD-4 disc with all the setup and adjustment tones and it can get through that too no problem

sorry, my bad, of course you don't hear the 30kHz tone but you have the audible pilot tone for calibration purposes.
all i know is, when i get that 30kHz/400Hz tone to "beeeeeeeep" as smooth as silk on my setup, CD-4 discs demodulate the best i've been able to get them so far

edit: Project 3, not 4! haha!! damn that Amoretto..