$70 Decoder for "New" CX Records, Roberts, January 1982

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mediatechnology
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$70 Decoder for "New" CX Records, Roberts, January 1982

Post by mediatechnology »

"$70 Decoder for New CX Records," John Roberts, Popular Electronics, January 1982.

While looking for the Holman article I found this 1982 Popular Electronics Article by JR: https://proaudiodesignforum.com/images/ ... y_1982.pdf

It made the front cover.
Even though records aren't CX encoded these days it still seems like it could be a useful gizmo.
Hirsch-Houck Labs liked it.

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"$70 Decoder for New CX Records," John Roberts, Popular Electronics, January 1982 Cover.

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"$70 Decoder for New CX Records," John Roberts, Popular Electronics, January 1982.
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JR.
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Re: $70 Decoder for "New" CX Records, Roberts, January 1982

Post by JR. »

mediatechnology wrote:"$70 Decoder for New CX Records," John Roberts, Popular Electronics, January 1982.

While looking for the Holman article I found this 1982 Popular Electronics Article by JR:

It made the front cover.
Even though records aren't CX encoded these days it still seems like it could be a useful gizmo.
Hirsch-Houck Labs liked it.
Deja VU man... Yup getting the cover was how Popular Electronics sold me on doing it. The Jan cover date is actually the on the newsstands at Christmas issue so is supposed to be the "money" cover. It wasn't my idea, I suspect CBS approached the magazine trying to gin up some interest and the editor of poptronics called me.

They made me a CX licensee for free and gave me the full licensee design package including some recommended decoder circuits and even a copy of the professional encoder schematic, designed by URIE.

I decided to use a better OTA than their recommendations, and added a few JR touches. One of the difficulties for accurate CX decoding is that it requires a calibrated playback level for the below threshold decoder to be accurate. I designed a slick level meter based on Q5, Q6 and a bi-color LED. The playback level was calibrated when both the red LED and Green LED were the same brightness. More red is too hot, more green is too low, and when playing music it makes a nice bi color light show.

A few subtle design points, the URIE encoder used a tantalum cap for the major time constant C, so I used a tantalum cap for my decoder (C5) so they would both exhibit similar dielectric absorption tracking errors. In prudent design you don't use tantalum for time constants (or sample and holds) but that's what URIE used for the encoder so I tried to track them with similar DA for playback.

I've told this story before, but it never gets old (to me). While looking at the recommended playback decoder schematic I noticed an error where their attack time was something like 10% off. IIRC they left the release resistor in parallel with the attack resistor shifting the math slightly. The encoder was correct and agreed with the published spec so i made my decoder agree with the spec and the encoder, and sent it along to the magazine to prep for publication.

I wrote a letter to CBS and advised them about the error I found in their documentation package recommended circuit, and then forgot about it. As I recall this article was cranked out on a pretty short time line to hit the jan cover date so i was busy.

That November I went to the AES show in NYC and while walking about I introduced myself to the guys in the URIE booth. Apparently they already knew who i was, I was that guy who found the CX mistake. :shock: :shock: It turns out 2 or 3 other licensees did not catch the mistake and had already manufactured some tens of thousands of decoders, all wrong. :oops: CBS decided it would be more expedient to just change the encoder and official time constant to agree with the decoder mistakes, but they didn't bother to tell me. :evil: Now my CX decoder, the only correct one was the odd man out since they changed the spec. :roll: It was then only weeks before the magazine printed, so i frantically called the editors and tweaked one resistor value to once again agree with the rest of the CBS world. I got it fixed in time for the magazine print date but it was pure chance that I happened to meet the right guy in the URIE booth to find out what was really going on.

In hindsight i didn't need to bother. I sold enough kits to justify my effort but it was not a winner, and CX records pretty much tanked in the hifi market. Based on my experience with CBS's lack of engineering rigor I am not very surprised.

I still have a bunch of CX albums that CBS gave me. I doubt i still have a working decoder around...

JR

PS That was near the end of my kit business days... by 1985 I was working at Peavey.
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Re: $70 Decoder for "New" CX Records, Roberts, January 1982

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John, are the listening tests you or HH or PE?
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Re: $70 Decoder for "New" CX Records, Roberts, January 1982

Post by mediatechnology »

The kit article was later cited by Dolby in a patent.

Was the subtractive VCA in the CBS design?
The simple yet elegant rectifier - in which the op amp never goes open loop - is nice too.

Now I know what project those bi-color LEDs I have came from...
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Re: $70 Decoder for "New" CX Records, Roberts, January 1982

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ricardo wrote:John, are the listening tests you or HH or PE?
It sure wasn't me.... Julian Hirsh lived in White Plains NY convenient to NYC where Popular Electronics was based so he could have easily auditioned the prototype I gave the magazine for photography. I wasn't personally involved and don't even remember that, I hope he was nice (he usually was). I wouldn't be surprised if CBS was behind pushing for that since HH and Popular Electronics are not normal bedfellows.
=====
I once carried Rudy Bozak down (from CT where Rudy and I were based) to Julian's house with a consumer surround sound delay line that I designed for Bozak, for Julian to review. It was nice to hang with the two of them for a few hours, and I listened more than I spoke. Julian had a lab and listening room in the finished basement of his house.

Julian was notorious for "never hearing a product that he didn't like" :lol: but if you read between the lines of his reviews you can sometimes parse out criticism, and IIRC he has declined to review some complete turd products. Julian seemed like a genuinely nice guy who enjoyed his position in the audio community. He seemed to have a more realistic sense of how typical audio products were more similar than different from each other. With no funny business, quid pro quos like from some of the small audio phool rags..

JR
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Re: $70 Decoder for "New" CX Records, Roberts, January 1982

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mediatechnology wrote:The kit article was later cited by Dolby in a patent.
I didn't know that either... Do you have the pat # handy?
Was the subtractive VCA in the CBS design?
Subtractive? Actually the R35 shunting around the OTA is same polarity so additive (I believe). I had forgotten details but it looks like that R sets a limit for the max amount of attenuation. Crude enough that it needed trim for unity gain and DC feedthrough (not my best work).
The simple yet elegant rectifier - in which the op amp never goes open loop - is nice too.
A lot of that side chain was a variant on CBS or URIE design, it doesn't look like my work since all the diodes and base-emitter junctions will vary with temperature so generally not be very precise. I did do the attack/release section myself (because pro forma circuit was wrong) and I scaled impedances up so I could use a smaller tantalum cap for time constant. (URIE probably used a 10uF but I used a 1UF to be cheaper. proforma design used aluminum electrolytic there).

[edit] fwiw I just found a bag of 1uF tantalum caps back in my lab... no doubt left over from this kit, since I never used tantalum caps on purpose in fresh designs. [/edit]
Now I know what project those bi-color LEDs I have came from...
YUP... nice blinky lights. Not very cheap back then.

JR
Last edited by JR. on Thu Jun 04, 2015 11:20 am, edited 1 time in total.
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Re: $70 Decoder for "New" CX Records, Roberts, January 1982

Post by mediatechnology »

The patent was US4922535 by Ray M. Dolby.

https://docs.google.com/viewer?url=pate ... 922535.pdf

I wasn't sure if the OTA was inverting or not so it could be additive or subtractive.

I think the review was HH.
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Re: $70 Decoder for "New" CX Records, Roberts, January 1982

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mediatechnology wrote:The patent was US4922535 by Ray M. Dolby.

https://docs.google.com/viewer?url=pate ... 922535.pdf

I wasn't sure if the OTA was inverting or not so it could be additive or subtractive.

I think the review was HH.
Thanks, that pat was 40 pages of mumbo jumbo to describe a speed up circuit.... The CBS CX design has a fast side chain path in parallel with the slow path driven from -6dB.. I guess my article was the only publication of the CBS CX design details.

If Dolby wanted to look at something clever they should have looked at my last tape NR design... that one had a cap coupled fast attack path that would respond quickly to transients but for longer term, since it was cap coupled would not introduce tracking errors due to phase shift that changes the shape (peak amplitude) of transients. Now that was clever... :D

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Re: $70 Decoder for "New" CX Records, Roberts, January 1982

Post by emrr »

Good stuff, thanks!
Best,

Doug Williams
Electromagnetic Radiation Recorders
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