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  #16  
Old 10-09-2017, 12:23 PM
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Gents, is ths what we are talking about here?

https://www.amazon.com/Oudin-Coil-Te...rds=tesla+coil

Tis like the one I remember seeing at various CRT production/rebuildng facilities years ago.

not affiliated,
jr
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  #17  
Old 10-09-2017, 01:13 PM
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I have two, one similar to that and a smaller one that operates from a 9 volt battery. Both can fully illuminate a CFL, neither shows any gas on the CRT.
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  #18  
Old 10-09-2017, 08:58 PM
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Hopefully this isnt too off topic, but it has been intriguing me for awhile now. Is there a reason why the H840 uses RY GY demodulation and the CT-100 uses Q demodulation? Both sets are from the same year and I believe RY GY became standard soon after. Did RCA consider Q demod superior to RY GY?
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  #19  
Old 10-09-2017, 09:18 PM
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Quote:
Originally Posted by SwizzyMan View Post
Hopefully this isnt too off topic, but it has been intriguing me for awhile now. Is there a reason why the H840 uses RY GY demodulation and the CT-100 uses Q demodulation? Both sets are from the same year and I believe RY GY became standard soon after. Did RCA consider Q demod superior to RY GY?
I/Q demodulation is superior to difference demodulation, yes. (For the record, the Westinghouse is R-Y and B-Y, and it was this that briefly became the preferred demod axes). X and Z demodulation, as explained in Wayne Bretl's response to my query on difference demod in another thread, is essentially just a variant of R-Y B-Y demodulation.

I/Q is more expensive to implement. With R-Y B-Y, you have red and blue essentially ready to go right from the demodators. Just amplify it to drive the grids of the CRT and mix the luma in at the cathodes of the CRT. G-Y can be recovered from a simple network of passive components mixing R-Y and B-Y in the correct proportions. Then amplify it, send it to the grids, and ta-da, you have a color picture.

Note however, that this is not how the Westinghouse accomplished this. The Westy uses triodes to mix the difference signals and the luma, amplifies it, sends the result to a DC clamp, and then the grids are driven with actual R, G, and B.

The RCA method is more complicated. I and Q (or -I and -Q) are recovered from the demodulators. I and Q are then inverted. You now have I, -I, Q, and -Q available. These four signals are mixed in the correct proportions, along with the luma, so as to produce R , G, and B. These signals are then sent to a DC clamp, and the grids are driven with true R, G, and B.

More complicated = more costly. It isn't hard to see why RCA abandon this method of demod for the CTC-4 and all subsequent chassis (during the tube era at least). Difference demod is cheaper. If you want to sell more TV sets, you find a way to make them less expensive.

Now, would the difference between these demod methods actually have been visible on a Westy and a CT-100 both connected to an antenna circa 1954: not no, but hell no.

Watching a 15G is similar to watching TV through a screen door: the dot pattern is plainly visible and all fine detail is essentially obliterated.

On a 21 or 19 inch set, I think the difference may have been visible to very observant viewers under the right set of conditions. Whether those conditions would have existed "in the wild" with an antenna on the roof circa 1954 is again doubtful.
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  #20  
Old 10-09-2017, 09:27 PM
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Interesting, seems like Q demodulation was just another factor in the poor sales of the CT100.
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  #21  
Old 10-10-2017, 01:20 AM
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As I probably explained in another thread, the transmitter side Q filters were really not properly specified by the NTSC/FCC. To guarantee good results, Q baseband should have a notch at 900 kHz, so that in the receiver, when the upper sideband is killed by the receiver 4.5 MHz sound trap, the lower Q sideband also has no energy at 900 kHz below chroma and there would be no quadrature distortion, which was the aim of I/Q modulation in the first place. So, when I/Q demodulation was tried on some later sets with larger screens, the quadrature distortion could be quite visible, and results depended on the Q response of the particular color encoder in use at the studio. So, I/Q was a great idea, but suffered from under-specified filter standards. I believe none of the experiments that led to the choice of I/Q actually used an end-to-end system including the RF/IF parts. Later, when the whole system was being tested, there were a million problems to fix, and if the quadrature distortion was visible (say on a large-screen trinescope), it would have been easy to ignore as possibly a small receiver issue that could be fixed later.

Edit: Also, probably posted by me elsewhere, I did an extensive series of tests on I/Q demodulation when I worked for Zenith in the 80s, and even using baseband I/Q with no quadrature distortion, there were some effects that people weren't used to, like the vertical strokes of yellow movie titles turning orangish because the I signal was full amplitude for narrow objects while the Q signal was smeared and reduced in amplitude.
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  #22  
Old 10-10-2017, 02:54 AM
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The last three comments are way over my head.

From a common man’s point of view (average but devoted), it seems to me from recollections and observations back in 1956 and subsequent up to approximately the late 70’s, the “state of the art” in television color broadcasting was incapable of exploiting the potential of the 50’s CRT color reproduction and resolution capabilities.
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Last edited by etype2; 10-10-2017 at 02:59 AM.
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  #23  
Old 10-10-2017, 08:43 AM
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The 15G actually has pretty poor resolution, owing to its diminutive size. Good color yes, but the 19 and 21” sets are where you could really start to get a good color picture at a size you didn’t have to squint at.
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  #24  
Old 10-10-2017, 02:23 PM
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Brought the set up on a Variac today. I figured if I could get the set to produce HV that would definitively say whether the 15G is gassy or not. No HV, and before I had a chance to poke around further a small electrolytic under the main chassis popped.
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  #25  
Old 10-10-2017, 02:54 PM
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If I'm not mistaken X Y color demodulation required limiting the bandwidth of the wider band signal in the transmitted I/Q signal. On something like fireworks where there are colored lights of diminishing size the difference between wide and narrow band color might be noticeable in a side by side comparison. Were there viewers that smart, signals that good, two such sets side by side....Perhaps at the FCC hearings prior to NTSC approval and in some TV labs, but unlikely in the real world.

Congrats on the set Ben.
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  #26  
Old 10-10-2017, 04:49 PM
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Quote:
Originally Posted by Electronic M View Post
If I'm not mistaken X Y color demodulation required limiting the bandwidth of the wider band signal in the transmitted I/Q signal. On something like fireworks where there are colored lights of diminishing size the difference between wide and narrow band color might be noticeable in a side by side comparison. Were there viewers that smart, signals that good, two such sets side by side....Perhaps at the FCC hearings prior to NTSC approval and in some TV labs, but unlikely in the real world.

Congrats on the set Ben.
Thanks Tom! :-)

After much thought and deliberation, I've decided the Westinghouse restoration is more than I want to tackle, and admittedly a bit more than I feel I am capable of.

So, Nick Williams will be restoring and aligning the chassis, a fine furniture restoration shop will be completely refinishing the cabinet, my friend Kevin Stankovic will be drawing up model of the mask and knobs in CAD and then reproducing them, and I will simply cooridnate all those efforts and do the final assembly and setup myself.
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  #27  
Old 10-10-2017, 09:01 PM
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I welcome the challenge.

I have a Halolite on the bench right now, that will be followed by another 621 and a pair of Curtis Mathis color chassis, then I’ll be ready for the westy.
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  #28  
Old 10-10-2017, 10:26 PM
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The big problem with I-Q demod is that to get it really right required two
delay lines, one for Y, one for I. And to get them right and the filters with good phase response is not easy ... without digital (or bucket brigades).

My personal opinion is that best is to use any two axes and get the response
quite flat to about 700-800 kHz then rolling off to zero at 1.5 MHz. on both
axes. Few folks notice the resulting bleedthrough from I into Q. Using
equal bandwidths the results are identical independent of the axes used.

However, other people may seriously object to the bleedthrough.
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  #29  
Old 10-10-2017, 10:47 PM
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Quote:
Originally Posted by dtvmcdonald View Post

My personal opinion is that best is to use any two axes and get the response
quite flat to about 700-800 kHz then rolling off to zero at 1.5 MHz. on both
axes. Few folks notice the resulting bleedthrough from I into Q. Using
equal bandwidths the results are identical independent of the axes used.

However, other people may seriously object to the bleedthrough.
Yep. It was less disconcerting to have the color saturation of details reduced than to have the wrong hue - and if the level of quadrature distortion was low enough, at normal viewing distance in the home it appeared to add some color detail without being obviously wrong. With the engineer's nose to picture tube, the quadrature was visible as a rainbow effect on small strongly colored objects, but it also tended to be hidden by the luminance overshoot due to video peaking.

Since later solid state sets usually simplified the chroma bandpass such that they had this gradual chroma cutoff rather than the classic double-tuned response, it meant that they could actually benefit from composite video input that had double sideband chroma, meaning that the color detail would be somewhat better with a digital source (DVD or digital broadcast converter) fed into the baseband composite input than was possible with RF feed.
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Last edited by old_tv_nut; 10-10-2017 at 10:54 PM.
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  #30  
Old 10-11-2017, 01:14 AM
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Quote:
Originally Posted by SwizzyMan View Post
Interesting, seems like Q demodulation was just another factor in the poor sales of the CT100.
Demodulation had nothing to do with poor sales.
The Consumer Price Index says $1,000 (price of CT-100) in March, 1954 is supposedly equivalent to $9,127 today.
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