DISTORTION ON 4 CH R2R TAPEs REAR TRACKS? SOURCED FROM CD-4 SYSTEM

[westgate]

hi folks!
for the first time in at least 30+ years i've gotten around to playing some of my home recorded quad tapes copied onto a sony tc-277-4 r2r quad deck from quad sources back in '73 and '74. those that were tape to tape direct copies have no problems.

two tapes were copied off of a friends cd-4 system-(panasonic 700 tt is what i had written on the tape box).
last night when i played these two tapes the first 8 or 9 songs were fine. i'm impressed with the cd-4 sound, seperation, etc. but towards the end a good bit of distortion crept into the rear channels of the last three songs on both tapes (jose feliciano maybe, on first tape, i think. i failed to write down the artists name here; no idea of artist on 2nd tape either) making the last three songs on each tape pretty much unlistenable.:howl

i played the same sections of both tapes on 2 other quad decks and the same distortion was there. so it's not my current decks. and other quad tapes made on the old sony quad deck are clean.

so, i guess my question is-as distortion increases on a regular stereo LP as the stylus moves towards the inside of the disc, would that scenario be even more pronounced on a cd-4 disc/system? enough to cause audible distortion when recorded onto said rear channels?

it's possible that maybe the cd-4 tt and decoder may not have been as finely tuned as they could have been. don't really know that tho.

(obviously, i must not have been monitoring the recording as it was being made (and the old sony was only a 2 head deck)-and haven't really played them all the way thru til now)

i should mention, i never owned a cd-4 setup myself and know less than nothing about them; the one i copied from was a friends setup.

thanks!

 

[JonUrban]

Sounds like exactly what my first thoughts were. Either a cartridge alignment issue on the play through, or maybe the LP had issues.

Anyone else?

 

[Doug G.]

I agree with Jon. There was something amiss in the CD-4 setup because, on a correctly operating CD-4 system, there should be no more distortion at the end of a record than at the beginning. In other words, no more than with a regular stereo system.

Most likely, there was a tracking issue all those years ago.

Doug

 

[segacd]

That happens often when I play CD-4 records on my Pioneer PL-50 turntable with a Grado cartridge. Although tracking, azimuth, and general cartridge problems are possible factors in creating distortion on CD-4 records, there is also a major reason distortion is heard on the inner tracks of a CD-4 record.

As you get towards the center of an LP record, as would be in any circular disk, linear velocity decreases. As a result, the data stored on the disk is much more dense at the center than on the outside of the disk. This is similar to decreasing the bitrate of an MP3. When you play a low bitrate MP3, you lose much of the high frequencies and also get artifacts since a lot of the data is lost during compression. On the LP, as you get towards the center, it becomes much more difficult to pack such high frequency waves, as required by the CD-4 system, at such an extremely high density and dirt/scratches become more pronounced. Since these dense high frequencies are very "sharp" and very fragile, they tend to wear down faster than the high frequency information outside of the record (where the linear velocity is greater, more area is covered per unit of time, the density of the high frequency information decreases, and, as a result, becomes much easier to preserve.) Another example of this phenomena is measuring the time taken to wear down the rough edges of a rock by a certain amount (AKA high density and/or high frequencies) vs. a smooth edge by the same amount (AKA low density and/or low frequencies).
The wearing down of the high frequency information is heard in the form of "sandpaper quad" or the distortion of which I'm guessing you're referring to.
~Later CD-4 albums used a stronger vinyl (JVC SuperVinyl) which attempted to prevent the wearing down of the high frequencies...~

I hope that made sense.

EDIT: Ah. I see you don't know much about CD-4 systems. Well, to put it simply, CD-4 records store the rear frequencies at frequencies WAY above the normal hearing level (The rear channels are stored at above 30kHz...or 10 THOUSAND Hz above even what the the best human ear can hear.) As a result, the rear channel frequencies are way above the frequencies stored on a normal LP. So if high frequencies wear down fast....the rear channels will be the first to go.
I hope this helped!

 

[westgate]

segacd,
actually, and iirc, i had read and understood much of what you wrote in your post back in the day when quad was 'hot'. but as i never aquired a cd-4 system, i pretty much forgot about it.

'sandpaper' quad was pretty much what i was hearing. at least it was only the last few songs on both tapes. the rest are quite nice sounding.

re the ultra hi freqs where the rear channels were 'stored', weren't they called 'carrier' freqs or something akin to that?

thanks! and welcome to QQ!

and thanks to jon and doug!

 

[segacd]

Basically. The carrier frequency is what the decoder looks for to automatically enable CD-4. So that is how the decoder knows if you're playing a stereo record or if its a quadraphonic record. The carrier is @ 30kHz. The actual rear channel information is stored ABOVE that! Since the range of human hearing is about 20kHz, as I had stated, that means to have good quadraphonic sound, you needed a needle that could pick up frequencies at ~50kHz or 2.5 times higher than the standard needle! No wonder why the stuff was crazy expensive/sensitive!

And thanks! I was actually a part of QQ before a few years back, but since then I forgot my (obnoxiously long) username/password combo so I decided to just make a new account. Haha.

 

[Doug G.]

Actually, the information that modulates the 30kHz carrier is information necessary to reconstitute all four channels.

The sum information of the front and back channels is engraved just like a regular stereo record so if the CD-4 record is played on a stereo system, all of the information for both front and back channels appears at the single speaker for each channel.

The information carried by the modulated 30kHz carriers is the difference information of front and back for each channel. The demodulator demodulates the carriers, thus retrieving the audio difference signals. The sum and difference signals are then added positively or negatively to create the four channels for a CD-4 discrete wonderful thing.

Of course, if the modulated information is not retrieved, there would be no back channels and the front channels would still output all of the information (front AND back) on each side. But what usually happens with most demodulators, if the 30kHz carrier is not detected, is it will switch out of quad mode and just output the total (sum) information to both front and back channels so you end up with four-speaker stereo. This is what happens when you play an ordinary stereo record through a demodulator.

All of this is similar to the standard FM system that has been in use since the early sixties. It's just double.

It was necessary to do it this way so the CD-4 system would be compatible with regular stereo.

Doug

 

[MidiMagic]

If you put the entire album on one tape, I would expect the noise occur twice: At the end of each side of the record.

If it happens at only the end of the tape, then there is trouble with the tape.

Also, the sandpaper quad effect should affect all channels, not just the back ones.

This sounds more like the tape deteriorated. But only the back channels?

Maybe a dirty control in the signal path made more noise some time after it was set.

 

[westgate]

re distortion happening twice, good point. but it doesn't. just on the last two or three songs of both tapes.
there may have been distortion in the front channels also but it was so strong in the rear channels i may not have noticed it in the front ones.
i need to listen to the tapes again to more accurately pinpoint what's going on. this was the first time the tapes had been played all the way thru, ever, iirc, since i made them in '73.

it's all kind of moot anyway, as all this was done 37 years ago.

 

[Q8]

I know this is drudging up a moldy old thread, but I have been playing with cd-4 this weekend and have been looking through threads on cd-4 distortion.

What I find interesting, is that it seems that end of side distortion tends to happen more on side B (actually on bot cd-4 and stereo records). It seems when end of side distortion is enough to drive you nuts, it seems like 99% of the time its on side B. Could this be something with the way these were pressed? Is side b usually the top stmper or the bottom stamper or is there usually no stndard to this? This could explain why the posters tapes have the issue at the end. Also it seems that Warner Bros CD-4s have this problem most often.

 

[JonUrban]

The only thing I remember is that Nick Perugini once told me that as the tonearm traveled towards the label, the chance for distortion increased. This was why original CD-4 LPs seemed to have "squished grooves", with a very large run-out area at the end of the audio portion of the LP.

He also said that the CD-4 engineers that did the final Arista CD-4 knew it would be a final product, so they cranked that LP to the max, so if your CD-4 demodulator could play "This One's For You" all the way through, then you had a bad ass demodulator.

Of course, over the years, I've forgotten more than I probably originally knew! @:

 

[Q8]

Thanks Jon! That is very good information. I'm starting to understnd why so many people dislike CD-4. I'm not a quitter though and CD-4 is great when working properly. Also while CD-4 is more discrete than matrix systems, discrete still seems to be a term used loosely. There is still a good amount of channel crosstalk, especially if you don't want as much sandpaper quad. But even at max seperation I wouldn't call it exactly discrete. Maybe that's just because I do the sepertion adjustment listening to the rear channels though headphones. I have more fiddling to do. I have left the carrier signal pot in the factory location cause the mnual says you do not have to do otherwise if you have radar. From posts I've read from the quadfather, it is possible to get less distortion by adjustment.

Also, the inner groove distortion seems to be there on discs with the issue even when played in stereo. This makes me think that possibly CD-4 records didn't press as well towards the inner groves sometimes Thdue to the carrier signal. That along with the geometry issues natural to records is probably why earlier records tried not to press close to the label. Also probably why they tried using better vinyl.

 

[Doug G.]

Keep at it, Q8. I have my system to the point that playing CD-4 records isn't really any different than playing regular two channel discs as far as distortion.

I have reached the conclusion that if I do have trouble with a particular disc, it is probably that disc's history at fault. Probably played too many times with a stylus which damaged the carriers. Discs I have acquired sealed all play perfectly.

I have also realized that the initial fear of using the normal disc area for CD-4 was unfounded as the ones I have using the full side work at least as well as the "skinny groove area" ones and have better fidelity. Gordon Lightfoot's "Sundown" for instance (OK, it's bit skinnied up but not to the extent as a lot of other CD-4 records). It just sounds wonderful. Some of us have even postulated that it deserves to be called the best CD-4 record ever.

I believe most problems with CD-4 are due more to tracking/signal transfer issues than the demodulator. Most demodulators were fairly competently designed and if they got good signals, they could separate the information into four discrete channels. Discrete, of course, is a relative term but with CD-4 you can go right up to any of the four speakers and still hear what's supposed to be coming out of that speaker instead of the quad soundfield becoming smeared or indistinct.

Doug