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  #16  
Old 09-20-2014, 11:23 AM
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dtvmcdonald dtvmcdonald is offline
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I finally found alignment instructions for the CTC7, which clearly show
that it indeed is narrowband, roughly flat to 0.5 MHz (both sides
of 3.58 MHz).
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  #17  
Old 09-21-2014, 05:27 PM
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I believe the only set which facilitated wideband color demodulation after the CTC2 was the CTC5 Deluxe. However the bandpass was vestigial sideband on both X and Z demodulation axes. This implies that there would be quadrature crosstalk. I haven't gotten around to checking or realigning the chroma channel on my CTC5. But I do detect a dark stripe between the green and magenta color bars which I assumed to be quadrature crosstalk.

It would be interesting to know the rationale behind RCA designers providing the asymmetrical +0.5/-1.5MHz chroma bandpass in the CTC5 deluxe. Curiously RCA stuck with +/- 0.5MHz on all subsequent designs up past the CTC20.
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  #18  
Old 09-21-2014, 06:28 PM
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Quote:
Originally Posted by Penthode View Post
I believe the only set which facilitated wideband color demodulation after the CTC2 was the CTC5 Deluxe. However the bandpass was vestigial sideband on both X and Z demodulation axes. This implies that there would be quadrature crosstalk. I haven't gotten around to checking or realigning the chroma channel on my CTC5. But I do detect a dark stripe between the green and magenta color bars which I assumed to be quadrature crosstalk.

It would be interesting to know the rationale behind RCA designers providing the asymmetrical +0.5/-1.5MHz chroma bandpass in the CTC5 deluxe. Curiously RCA stuck with +/- 0.5MHz on all subsequent designs up past the CTC20.
Can you post the chroma bandpass alignment procedure/waveform for the deluxe chassis? Do they show the final response for R-Y and B-Y? It might help make a guess. - can only find the super alignment online.

A dark stripe does not indicate quadrature distortion, only reduced bandwidth. Quadrature distortion would show as a residual color during the transition, with the edges of the green and magenta bars discolored one way or another.

[Edit: this also depends on whether the source is narrowband, true I/Q, or equiband wider than 0.5 MHz. If the source is real I/Q, and the receiver really has wideband demodulation on two axes, you will see more distortion on transitions that involve strong I signals. Since the green/magenta transition is mostly Q, you won't see quadrature there with a proper I/Q source, unless the receiver has phase distortions below +/- 0.5 MHz (which can happen with poor alignment or misadjusted fine tuning).]

Last edited by old_tv_nut; 09-21-2014 at 06:36 PM.
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  #19  
Old 09-21-2014, 08:39 PM
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Quote:
Originally Posted by old_tv_nut View Post
Can you post the chroma bandpass alignment procedure/waveform for the deluxe chassis? Do they show the final response for R-Y and B-Y? It might help make a guess. - can only find the super alignment online.

A dark stripe does not indicate quadrature distortion, only reduced bandwidth. Quadrature distortion would show as a residual color during the transition, with the edges of the green and magenta bars discolored one way or another.
I haven't a scanner, but looking at the RCA T-6 Service Notes for the CTC5 Deluxe, the chroma bandpass is from 2.28MHz to 4.1MHz. This means that the lower vestige is 3.58-2.28=1.3MHz. The CTC5 Deluxe employs two stages of bandpass amplification which I would imagine furnishes the extra gain for the wider bandpass. CTC7 onward had only one bandpass amplifier stage.

And I stand corrected. The band between the green and magenta was not black but an orangey hue if I recall. But I agree: it would not be black.

I'll post the CTC5 notes tomorrow.
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  #20  
Old 09-22-2014, 08:11 PM
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Forgot to scan the manual today. So I took a photo of it. I trust you can read it.

The final overall bandpass response is down at 50% (6dB) at 2.28MHz. But the response suggests that at 1 MHz below the subcarrier the amplitude is up.

I will let you be the judge.
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  #21  
Old 09-22-2014, 09:08 PM
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My only guess is that they felt that some color in the details was useful and maybe they also felt that the quadrature distortion would not be so visible at normal viewing distance (ten feet) as to make it objectionable. In my experiments at Zenith, viewing was in the usual engineer's mode - from two feet or closer.

As was stated above, the bandpass was narrowed to be equiband in later chassis.

Edit: it occurs to me that the luminance delay should be shorter in the Deluxe chassis than in the super, if the chroma rise time in the Deluxe was faster.

Last edited by old_tv_nut; 09-22-2014 at 09:11 PM.
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