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Are post CTC4 RCA color TVs really narrow band color
Are post CTC4 RCA color TVs really narrow band color?
I simulated the color amplifiers of the CTC7 through CTC12 and find that they are wideband, going out all the way to 1.5 MHz. They are 3 dB down at about 0.5 to 0.6 MHZ and then drop quite a bit more slowly, with reasonable guesses as to stray capacitance. If they are narrow band this would have to be done in the bandpass amp. Does anybody know. This would be easy to see looking at stray color from a continuous video sweep from a test DVD. Doug McDonald |
#2
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The 4 itself was a narrow band (high-level) demod scheme, truely one of a kind when it came to engineering.
When you say 'simulated' I assume you mean on a computer, right? Those programs can be wildly inaccurate, especially when it comes to tuned circuits. As you said, stray cap/ind can have a profund impact on high frequency circuits- I'm not sure I would trust a computer to be able to figure it all out. That said, it is generally accepted that all chassis beyond the CTC-2(B) are narrowband. I have heard there was a version of CTC-5 with wideband demod, not that it matters since the insufficient HV production of that chassis would tend to make it impossible to enjoy anyway. Not quite sure why people make a huge deal about it to begin with, the human eye is largely insensitive to color information but very sensitive to changes in brightness- a fact the NTSC took full advantage of. The main reason people associate wide band color demod with good color reproduction is its simultaneous marraige with the 15GP22 CRT, I strongly suspect that you wouldn't be able to tell the difference between a 15G driven by a CTC-2 chassis or a CTC-9. If you really want to know, you would have to sweep the color sections of the individual chassis. But looking at the schematics (which didn't change a whole lot after about 1956/1957 or so), it becomes aparant that RCA had it figured out narrow band style by then anyway- it was a cost thing more than it was a color quality thing, because like I said people didn't really notice a difference.
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#3
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Yes, the circuit simulators can be wildly inaccurate ... but I have the experience
(quite a lot) of measuring the various stages of my CT-100 with various peaking coils and playing with the simulator until it matched the data. No reasonable stray capacitance and peaking coil (there is only one) resonance frequency will make the CTC 7-12 amplifiers truly narrow band. Everyone call the later circuits narrow band, but all we know for sure without tests is that they are equal bandwidth in all three colors. The real bandwidth is likely determined by the bandpass transformer, which very well may be truly narrow band. Edit: Today I got the manual for the Heathkit tube color TV I had in the late 60s. It has a slightly different circuit with a small amount of negative feedback that broadens the color bandwidth even more. And they give the response of the color amp and IF. The result is, more or less, flat to 0.5 Mhz with a very slow drop off to zero at 1.5 MHz. This is not narrow band like Q in a CT-100, but not as wide as the I in a CT-100. It did have a very nice color picture, but I never saw real color bars on it that I remember. Last edited by dtvmcdonald; 09-18-2014 at 02:56 PM. |
#4
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The sets you are studying, like all later sets, were equi-band. That is the one thing you can state clearly. The overall chroma frequency response was less sharply defined as the circuits were simplified, so whether you call them wideband or narrowband is simply a matter of definition. Bandwidth is often stated as the 3 dB point. In later sets, some color modulation is usually visible beyond 0.5 MHz, but it is full of quadrature distortion and for that reason hopefully low level so it doesn't mess up color edges. One way of judging this is the transition between the green and magenta color bars. This may be smeared and look dark, but ideally goes through a neutral point and doesn't look like any particular strong wrong color at the edge.
So, rather than calling these sets wideband, I would call them sloppy band. |
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I've ALWAYS heard one of the supposed "Superior" things about CT-100s & the early color sets was that they demodulated the ENTIRE NTSC spectrum, whereas the "Later" sets, they found out they could "Cheat" a bit, & they DIDN'T show the entire palette, but our eyes "Blended" it all together, so that made little difference. Any truth to this, or just Stuff 'n' Nonsense ?!?
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#6
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Quote:
The 15GP22 and 21AXP22 had NTSC green, giving more capability for saturated greens and cyans. Later tubes went to a yellower sulfide green that improved brightness, but cut the saturation on the cyan side. Color detail is what all the discussion of bandwidth is about. |
#7
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A factory rep once discussed the reduced chroma definition very simplistically this way (paraphrasing)..
The detail and definition that the eye "sees" is supplied by the luma signal, while the color is simply washed in. Thus the average viewer never perceives that the color detail is fudged. Last edited by old_coot88; 09-18-2014 at 08:05 PM. Reason: Added the word 'average' |
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Fine Color Detail, CTC2B vs CTC4
In 1964 I questioned the reasons why my 1954 RCA 21CT55 with its first full NTSC spected CTC2B chassis and the last roundy 21FBP22A crt produced picture quality almost comparable with my solid-state, comb-filter 1984 and 1992 Sonys. I checked all the RCA schematics from the CTC4 up to the CTC38, tube types and also the all solid-state CTC40 of 1970. All of them have narrow bw chroma processing and hi level crt luminance/chroma combining. Did RCA ever go back to wb IQ video processing? The dramatic difference in picture quality between wb IQ and nb R-Y/B-Y was obvious to me in 1964 as related in my first AK message of March 28, 2008:
……..When I bought my 21CT55 for $50 in '64, I brought it into the house after I got it working and ran it along side my Dad's old CTC4 he left me. Both had 21FBP22s so it was a good comparison of narrow bw color R-Y B-Y vs. full bw color I Q. The extension of full color into fine detail on the CTC2B was startling! Color persisted in fireworks until extinction which I never saw before. The CTC4 fireworks turned into white long before extinction. The CTC2B carried full color way into the shadows, the CTC4 went gray to black……I get really awesome displays on the 21FBP22A with vibrant, accurate color shading. Having the CTC2B's super wide dynamic range due to full bandwidth I Q demodulators feeding low level I-Q-Y matrixing to the CRT grids only. I kept the CTC4 in the house for the kids but brought the CTC2B back to the garage workshop for further study……. |
#9
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Quote:
they are wide or narrow band? That's the point ... you can't. You can tell whether they are equal-band ... which they are in the presence of an equi-band signal (i.e. a modern one from a dvd that is wideband in all colors) ... but not the actual bandwidth. |
#10
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In the 80s, RCA produced a chassis with I/Q decoding. They developed a circuit that could drive a delay line and sense its output with one analog IC pin and one external transistor, one of my favorite patents of all time.
I had a project at Zenith to investigate possible wideband chroma, and unfortunately found that a combination of circumstances made it impractical (list below). Of course, we did a competitive analysis of the RCA, and found that the I high frequency response was rolled off severely, which I think was probably due to them discovering the same problems I did in my project: 1) The IF amp designs were "haystack" shaped rather than attempting the flat top response in very early color sets. This was for a combination of reasons; lower cost, better transient response when used with a simple analog sharpness control, less sensitive to fine tuning error. But it meant that chroma sidebands were on a slope that had to be compensated in the chroma take-off / bandpass circuits. 2) The original NTSC Q filter specs in the color encoder were not strictly enough specified. Many encoders had sufficient Q sidebands that non-symmetrical upper and lower sidebands would be present due to the sound carrier notch in the transmitter and especially in the receiver. This produced quadrature color distortion on edges - could be OK for some things, but when the edges of faces get wrong color, that's a deal breaker. If only the original NTSC gurus had thought to specify a 920 kHz null in the Q baseband filter, both Q sidebands would have been suppressed, and the I/Q unequal bandwidth would have been guaranteed to work as advertised. I went through a complete series of experiments, from full wideband RGB to composite baseband with wideband equiband chroma; to composite baseband with double-sideband wide I and narrow Q using a commercial encoder; to the same over RF (where it got bad); to an encoder with a redesigned Q filter over RF (edges fixed). When I say edges fixed, I mean edges of faces were OK; but we could find some objects that showed distortion. For example, a thin yellow box surrounding titles on one test slide turned orange on the vertical strokes, because only the I component had significant amplitude on the thin line. Yellow title letters were not so bad because they were usually thick enough to get some Q response; but you could see some effect. Speculation on why the original NTSC specs didn't have the notch specified: 1) effects weren't easily visible on small screen 2) analysis of some original schematics indicates there could be an inadvertent notch (or at least strong high frequency attenuation) in the Q path depending on the mutual coupling in the phase-equalized filter design; but this was never specifically required in the NTSC specs. |
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#11
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Quote:
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#12
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NTSC Video vs Equiband RCA Video
For sure all RCAs after the CTC2B up to the CTC20 had two identical chroma demodulators and two identical high level CRT grid drive circuits. The fact that the CTC4's fireworks lost all color and displayed white as the fireworks trails narrowed and extinguished suggests a limited bandwidth chroma video applied to the CRT grids compared to the full bandwidth luminance video applied to the CRT cathodes. I'm sure viewing a 20 inch screen at 20 feet, no loss of color detail would be noticed on either CTV.
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#13
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You keep mentioning the 2B like it was the first of it's kind...
There's a single resistor different from the CT-100 to your set, and the 21-CT-55 was NOT the first...
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#14
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I think he said that the 2B was the LAST of its kind, the kind being
intentionally split bandwidth and I-Q or close it it (i.e. R-Q). |
#15
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It was not the first, the CT-100 was the first. The 21-CT-55 is nearly identical, the deflection circuits and a minor change to the demodulator are the only difference.
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