Prototype set is here

[miniman82]

Found a patent online which may be of some use to us:
http://www.freepatentsonline.com/2736859.pdf

It is a patent filed by Dalton H. Pritchard and Alfred C. Schroeder (assignors at RCA) on July 25, 1952, which details a 'Color Phase Alternation Control System'. It outlines a system whereby the phase of the red syncronous detector is flipped by 180 degrees at field rate by a transformer with windings arranged to accomodate this change in phase. According to the patent, this would eliminate any reflective errors induced in a system incorporating a double ended delay line, while at the same time reducing cost.

"Previously, color phase alternation has been brought about in the following manner. Each of the synchronous detectors in a receiver were coupled to a different tap point on a delay line having a total delay of 360 degrees of the color carrier frquency. During one field a wave of color carrier frequency was applied to one end of the delay line so that different phases of the wave were available at the different tap points. During the next field the wave of color carrier frequency was applied to the opposite end of the delay line. As is well known, the closer the tap point to the energized end of the delay line, the nearer it is to the phase of the wave applied to that end. Therefore by energizing opposite ends of the line during successive fields the phases of the wave at the different tap points are interchanged. Some difficulties have been experinced in that reflections from the non-energized ends of the line interfere with the apparent phase of the wave at the various tap points. In addition, the delay line attenuates the wave applied to it so that as the wave progresses from one of the line to the other its amplitude is reduced. Because the line is energized from one end during one field and from the other end during a succeeding field, the amplitude of the wave at any tap other than the one located at the exact center of the line changes from field to field. At the center the attenuation of the wave is the same irrespective of the end energized. Such an arrangement is useful where 2 or more synchronous detectors are employed.

In accordance with one of the objects of this invention, the necessity for using a delay line is completely eliminated and the attendant difficulties are entirely avoided in arrangements requiring only 2 different phases of the wave of color carrier frequency during any one field."


Sheet 2 of the patent outlines the actual phase alternating circuit; if Wayne's supposition that the 'CPA' transformer in this chassis has something to do with phase switching:

The air-core coil that Nick found looks suspiciously like it was built to reduce variations with temperature, spacing to the shield can, etc., etc., but we won't know until we discover whether there is a phase switching circuit.

Then I would assume that the 'CPA' transformer in this chassis is wired in much the same way as the patent outlines, we'll know for sure later on tonight when Tim and I trace the circuit out and compare it to the ones found here and in some of the other literature forwarded to me over the past few weeks. Additionally, the text of this patent may also explain why there is also a blank hole in the chassis marked 'delay'. It's possible that the 'delay' on this chassis has more to do with the type of delay described by the patent, and not any sort of later delay (such as the common luma delay lines) that might have been associated with later NTSC standards.

[Penthode]

I attached a Hazeltine article on Friday but upon looking for it this evening it is gone!? Don't know what happened but I will see if I can repost it tomorrow.

In the Hazeltine diagram, R-Y phase inversion is accomplished by simply using a field rate multivibrator (12AT7) locked to a field recognizer detector (another 12AT7) to the correct field. The CPA transformer secondary is center tapped so that either end (180 degrees apart) is selected field by field to supply R-Y or -(R-Y) as required.

Note that the Hazeltine implementation at that time did not include a crystal controlled subcarrier oscillator.

Did anyone of you have a chance to see the article/diagram I posted on Friday while it was there?

[John Folsom]

is this the one? I downloaded a copy... :-)

[jeyurkon]

If you're referring to post 177, it's still there.

[earlyfilm]

Originally Posted by Penthode

I attached a Hazeltine article on Friday but upon looking for it this evening it is gone!? Don't know what happened but I will see if I can repost it tomorrow. (snip)
Did anyone of you have a chance to see the article/diagram I posted on Friday while it was there?

You posted at 12:35 and I copied it at 1:55, but when I went to view it on my better monitor at home at 5:30PM it was gone. I assumed that you had canceled it to correct the upside down drawings.

One question.
The underside of this chassis does not show chassis mounting screws or mounting holes.

Is this common on prototypes?

Jas.

.

[Penthode]

Originally Posted by earlyfilm

You posted at 12:35 and I copied it at 1:55, but when I went to view it on my better monitor at home at 5:30PM it was gone. I assumed that you had canceled it to correct the upside down drawings.

If I've done this reply correctly, it is attached.

Jas.

That's the one, thanks. Yes, I did correct the upsidedown drawing and posted it. But later it was gone. Very odd.

I suspect their were many variations to the CPA demodulator. Note from the time this article was written, it appeared that CPA support was dying. I believe it was just too difficult to implement effectively in 1952.

Note the upside of IQ and a lower subcarrier frequency was elimination of the crosstalk and the bothersome flicker. The downside was reduced Q resolution and the lower subcarrier frequency reduced the luma bandwidth hence luma resolution.

Engineering is all about trade offs.

A sidenote: the BBC was planning to adopt a modified NTSC as late as 1966. The UK 625 line channel is 8MHz wide with the sound carrier 6.0MHz away from the video carrier. This means that the bandwidth was available for full double sideband chroma (4.43MHz +/- 1.4MHz). Hence IQ working in the UK would not have been necessary. The UK could have had wide matching bandwidth for B-Y and R-Y. The dropping of NTSC in favor of PAL in the UK was very late. Proof of this were the rubidium frequency standards used at Television Centre and Lime Grove in London: the standards were cut for NTSC and augmented equipment was required to provide the PAL subcarrier frequency (1/4 line offset plus 25Hz).

I think NTSC for the wide channel UK 625 would have looked terrific and better than PAL.

Some may refer to NTSC as "Never Twice the Same Color"

But in a similar vein, PAL may be referred to as "Problems are Lurking"

[David Roper]

Nope. Color wheels work with sequential fields of color which is a different principle entirely.

[Penthode]

I have attached another version. The drawing is sideways: at least it is better than upsidedown.

It is interesting to put this paper on CPA in context with the development of the all-electronic compatible color. By mid 1952, much of the development of what would finally end up in the December 1953 standard was there. The constant luminance principle was there as was the chrominance quadrature modulated on a odd multiple of half line frequency to facilitate luma/chroma interleaving. But also the elements which would soon disappear including the burst pedestal as was assymetrical B-Y and R-Y bandwidths and the higher subcarrier frequency.

It is interesting that the original RCA RGB Dot Sequential sample frequency is fairly close to the later NTSC subcarrier of 3.58MHz. The original impetous for CPA was to extend the bandwidth of the chroma channel within the limited bandwidth of the US 6MHz wide channel. Hence the choice of 3.9MHz for the CPA subcarrier I suggest was for two reasons: to provide wider bandwith (vestigial sideband chroma) for both R-Y and B-Y and to extend the luma channel response. Remember that there is a luma 3.58MHz trap in early NTSC receivers which effectively cuts the high frequency luma response to 3MHz or less. A 3.9MHz chroma subcarrier would have extended the luma response a little bit.

CPA employed for PAF or PAL (Phase Alternate Field or Phase Alternate Line)was simply inverting the R-Y axis at either field rate or line rate. The reason for doing so was to eliminate differential phase errors which would result in hue errors and to cancel quadrature crosstalk between R-Y and B-Y at demodulation. Remember, crosstalk was eliminated with I and Q by ensuring both were double sideband to 500kHz and only I extended from 500kHz to 1.5MHz. CPA facilitated vestigial sideband R-Y and B-Y and the crosstalk was to be cancelled by averaging.

In 1952 only visual averaging of the PAF and PAL was available which led to the crosstalk and hue errors resulting in the annoying field flickering (PAF) and a vertical line crawl (PAL). The European engineers later termed the PAL line crawl as "Hannover Bars" and may be readily seen on what is referred to as "Simple PAL" decoders or PAL decoders without the electronic delay line averaging.

In 1952, the NTSC was exploring PAF as the field flickering was deemed less objectionable to the line crawl. I agree with the later NTSC assessment that moving away from the CPA to the I Q solution was one step beyond CPA in the development of compatible color TV. Successful deployment of CPA with PAL fifteen years later depended upon the one-line delay line which was not readily available in 1952.

We should look forward to finding out what remains of the CPA circuitry in this extremely unique prototype. I trust you will share with us the circuit as you trace it. I am particularly interested in the CPA transformer: the alternating R-Y phase I feel would have been derived from reversing at field rate either the chroma before demodulation or the regenerated subcarrier used for R-Y demodulation. I suspect the transformer is in the chroma signal path as per the diagram in the attached article?

[miniman82]

Originally Posted by Penthode

I suspect the transformer is in the chroma signal path as per the diagram in the attached article?

Actually it's closest to the subcarrier oscillator, so right now my assumption is if flips the reference signal 180 degrees on alternating fields for R-Y. Only time will tell for sure, this coming weekend we are hoping to delve into the demod sections in order to figure it out. Tim did spot an odd part that I hadn't previously seen: right near the CPA tranformer hidden under some variable capacitors there is a toroid with 2 or 3 windings on it. It's about an inch and a half in diameter, and looks to have something like #30 wire wrapped around it. We have absolutely no idea what its purpose could be.

[TVtommy]

Good gawd ya'll, it's beaen days now, the suspense is killn' me! Updates, puleaze!

[miniman82]

Patience, grasshopper. There is some discussion going on behind the scenes with the early color guys, I don't want to release any information until I am sure of what I'm looking at (to avoid confusion). Right now, I have most of the chassis schematic sketched out on paper. Once the rest of it is done it will be transferred to computer graphics so it's easy to read, and I'll post it up here for all to see. I still need a couple weeks to get it all finished up, only deflection and a few random pots remain...

[TVtommy]

I shall remain quietly in wait regards, tommy

[Mal Fuller]

A little confusion is a fundamental quality in a good Forum.

[TVtommy]

It's just that the only thing that to me that could be as interesting or as suspensfull (Did I spell that rite?) as this adventure would be some un-earthed Zenith color proto's or documentation showing the evolution of Robert Adler's beam deflection color demodulation circuitry!

[earlyfilm]

Originally Posted by TVtommy

It's just that the only thing that to me that could be as interesting or as suspenseful as this adventure would be some un-earthed Zenith color proto's or documentation showing the evolution of Robert Adler's beam deflection color demodulation circuitry!

Zenith was unique in that they first made FCC testing sets for both the CBS system and then later for the NTSC system. If this is not an RCA prototype, then it most probably is a Zenith. (While Westinghouse was in it early, I don't think they were in it this early.)

James