Decoding SQ with a JVC 4DD5 demodulator

[sspsandy]

I usually run everything other than CD-4 through a Fosgate Tate II surround processor that was completely restored by Bob Popham. However, I played the SQ version of Atom Heart Mother through the JVC 4DD5 demodulator with the receiver in the discrete mode. It decoded beautifully, near CD-4 like separation. I'm just curious to know whether the JVC has a matrix decoder circuit.

 

[JonUrban]

H-m-m-m. I wonder what you heard? That unit is strictly a CD-4 demodulator, not a decoder of any type.....

:-jon

 

[Cai Campbell]

You should have been hearing "dual stereo".

 

[sspsandy]

It could be the receiver I have, it's a Technics sa 6700X which has a width and depth adjustment on the decoder. I was running the demodulator through the discrete mode however, which has two different settings, one with AFD on and one with AFD off. The matrix decoder has two settings also, 0 degrees and 90 degrees, and the decoder circuit is controlled by two sliders marked width and depth referred to as Acoustic Field Dimension. A brochure I have says about it: " By incorporating two continuously variable controls, one for width coefficient and one for depth, Technics AFD ( Acoustic Field Dimension ) Control System makes possible precise adjustment to any matrix system in use today, as well as any that may possibly be devised in the future". Now I was running the discrete mode with the AFD off, but perhaps this amp applies some decoding to the CD-4. When playing Quadradiscs I get very sharp separation. But I definately was hearing 4 channel information through the demodualtor from both the SQ version of AHM as well as DSOTM. I even put on a mono copy of Jefferson Airplane "Bathing at Baxters" and it separated it into 4 channels, and the Tate doesn't even do that, it gives me a mono mix coming primarily from the front channels. I suspect the decoder scheme in this amp is similar to the Audionics Space and Image composer. This amp also allows you to run the demodulator and set it to the matrix position, and you get a smoothed out 4 channel mix from a Quadradisc. Since the receiver has three 4 channel tape monitors, it runs the information from the rear channels of the demodulator through the decoder.

 

[sspsandy]

I went back and gave a critical listen, comparing the demodulator to the Tate, and have decided that I'm just hearing double stereo. The demodulator pumps up the volume and enhances the sound, to my ear, and it does make mono recordings sound really good. I just really like CD-4, I guess, and want to find more uses for the demodulator. The AFD on the discrete mode is interesting, being able to adjust the width and depth of Quadradiscs is a cool feature.

 

[The Quadfather]

Dear Sandy:
I believe you are making a fundamental mistake. A CD-4 demodulator derives it's separation information from subcarriers on the record. this it mixes with the main audio to derive the original four channels. It does not try to derive four channels from two which is what a matrix decoder does. Even a quad encoded record in SQ or QS only has two channels in actuality. The original four channels can be approximated, but never fully realized, even with a Tate. A CD-4 record actually has four channels on it, two of main audio, and two subcarriers. What you get when you play a stereo record through a CD-4 demodulator is double stereo. left and right in front and the same left and right in the rears. If you play a mono record, you will get the same information (mono) in all of the speakers. It will in fact act like a stereo preamp. The subcarrier demodulators in the demod will be muted. Double stereo can fool you if you are not paying attention, because stereo separation is usually very good. The fact that mono only shows up in the front speakers of a Tate indicates that the Tate is doing it's job well, but has nothing to work with. I do in fact use my CD-4 demodulator (I have a JVC 4DD5 also) as a preamp when listening to SQ or stereo records through my Tate. I put it in 2 channel mode, and that simply turns off the demodulators, so they don't trigger and unmute on spurious high frequencies from the music. This is not necessary on most music, but occaisionally you will run across a record that will do this. Elton John's "Goodbye Yellow Brick Road" comes to mind. It does this.

The Quadfather