Why Differeny Demodulators/
 

I have noticed that the CT-100 uses I and Q demodulators. The 21CT55 uses I and R-Y demodulators. I thought the CT-100 and the 21CT55 were almost identical in the color section.

Does anyone know the reason for the different modulators? Patents?

The best reason I've heard to date is that there was a legal issue, probably with Philco, who came up with the quadrature modulation method of stuffing two chrome signals into the existing NTSC monochrome standard. By shifting the I signal to the R-Y axis, poof! no RCA $ going to Philco.

Pete

Also likely the reason that Zenith used the "sheet beam" demodulators when they developed their debut color chassis, the 29JC20. No stinkin' patent money going to RCA, although they did have to use an RCA picture tube and deflection components in that set. I'll bet that Sarnoff laughed, all the way to the bank!

I think Zenith used those demods till they replaced them with SS components.

Zenith may have had to buy those parts, but RCA had to pay Zenith for their little old inventions of Electromagnetic convergence and FM MPX stereo so I imagine it was a case of 'you begrudgingly scratch my back and I'll begrudgingly scratch yours'.

The one I like best is that RCA had to pay royalties to CBS for the color CRT patents CBS developed to improve the tubes. I'll bet Sarnoff wasn't pleased.

For companies that developed lots of patents, a patent suit negotiation often went by both parties bringing their stacks of patents, and then "I'll drop one if you'll drop one" - and onward until the company with the fewest had none to trade - and RCA always had the biggest stack.

The other aspect of RCA vs. everyone else in color TV was years of court battles until RCA was forced to unbundle their patents. Before that happened, it didn't matter if you had technology that evaded RCA's patents, because they would only license them as a bundle. So, this was another reason (besides the slow public uptake) that many companies stayed out of color until the early-mid 60s. Some companies (like Zenith) were then able to be more selective in the licenses they took.

Zenith's sheet beam demodulators developed a strong enough signal that a stage of amplification between the demodulators and the CRT wasn't needed.

The modern beam deflection tube was developed by General Electric, not Zenith, so I doubt very much that patent licensing was the reason Zenith used the sheet beam tubes in their chroma demod circuits. Unless of course GE was charging less than the other companies for use of the sheet beam tubes.

The first sheet beam tube specifically designed for chroma demod was the General Electric 6AR8 tube from 1954-1955, though somewhat ironically, I don't think GE ever actually used their own invention. The principles at play had been worked out earlier in a more elaborate tube, the GE "Phasitron" used in early post-war FM exciters.

RCA engineers couldn't see the forest for the trees so to speak. The 15HP22 and 19VP22 were a huge leap forward, and it would have been insane for RCA to release what essentially amounted to a 19 inch 15GP22 with the suspended dot plate. The 15GP22 never should have been released in production sets. The CBS 19VP22 was about the first truly practical, more modern color CRT, and even that was a bit of a dog due to very poor quality control. The 21AXP22 was better yet.

This GE datasheet from 1954 gives a fairly good explanation of the sheet beam demodulator:

http://www.r-type.org/pdfs/6ar8.pdf

Another GE tube that they didn't embrace quite as much as competitors was the 6BN6 FM detector, which seemed to enjoy the most use from Westinghouse and Zenith.

The Phasitron use for FM is attributed to Robert Adler of Zenith:
http://www.w9gr.com/adler.pdf

Correction - earlier reference to Phasitron for FM:
http://www.w9gr.com/2h21.html

I'll be damned. I had always seen it attributed to an engineer at General Electric. You learn something new every day.

Perhaps Zenith did hold the relevant patents for sheet beam tubes.

Now that I think of it, I read an article somewhere attributing magnetic convergence to both Zenith and CBS-Hytron as a joint venture, though for obvious reasons the credit is usually just given to CBS.

Zenith never built their own signal tubes, and ISTR a Zenith film where they touted that demodulator circuit as being their own design. It is quite possible that GE color demod tube existed because Zenith had GE make it for them.