[QuadList] RCA Hetrodyne Process and Early Color

[Randy Hall]

Howard Armstrong was probably pleased!
Randy
K7AGE

On Tue, Nov 10, 2009 at 1:09 PM, <Chill315 at aol.com> wrote:

>  There was a post some time ago regarding this issue.
>
> First there were different record standards to start.  What we know as Low
> Band Mono is 4.5 to 5.4.  The emphasis curve was different between the first
> RCA and Ampex machines.  This was later standardized to the Ampex method.
>
> Because RCA had an early curve that was non standard, there were for years
> a variable position on the Demodulator to allow an operator to correct for
> this.  Look at all the early Low Band machines and you will see this
> position.
>
> Third was Low Band Color.  It was used to prevent the moire that was
> generated between 3.58 and the FM record frequencies.
>
> All systems used direct record and playback.
>
> Now the tricky part.  The monochrome playback was good enough without any
> time base correction.  It met FCC standards and all the TVs could handle the
> time base errors.  Color was impossible because of the high degree of
> stability required to playback NTSC.  Even is you had an H Lock, it was
> still not stable enough.
>
> RCA came up with a method.  They put a color processor rack in the system.
> The signal was routed from the demodulator to this rack.
>
> The signal went to a burst oscillator that stripped the burst from the tape
> signal.  This would then generate a 3.58 signal know as signal sub carrier.
>
> The sub carrier converter chassis would then convert the house reference
> sub carrier to 19.8 MHz.  It also converted the Tape sub carrier to 19.8
> MHz.  We now have two signals.  One that is unstable and one that is stable.
>
> These signals were sent to the Chroma Processor chassis.  Here the path was
> as follows.
>
> First the video is sent through a low pass filter to remove the chroma.
> There is another path that is a high pass filter to provide chroma only.
> The Chroma is mixed with the reference 19.8 MHZ signal.  Buffered and then
> mixed with the tape 19.8 MHz signal.  And guess what, we now have stable
> chroma.  The chroma is now added back to the mono signal and we have full
> color.  BUT it is no longer true NTSC because the 3.58 and the H sync are
> not locked.
>
> A little bit more to be done.  The burst has to be regenerated.  That was
> the function of the burst processor chassis.
>
> Lastly, the signal was sent to the output proc amp. It had the final job of
> allowing for the adjustments required for proper playback.
>
> I have a complete set of schematics for the TRT-1 that include the color
> rack.  It is interesting to look at this and realize how well the pioneers
> did in their design of the quad.  Yes there were competing ways to
> accomplish the same thing but it still amazes me that the technology is
> still useable today.
>
> Chris Hill
>
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