[QuadList] RCA Hetrodyne Process and Early Color

[Chill315 at aol.com]

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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