View Single Post
  #19  
Old 10-09-2017, 08:18 PM
benman94's Avatar
benman94 benman94 is offline
Resident Lunatic
 
Join Date: Jun 2011
Location: Detroit, MI
Posts: 1,190
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.
Reply With Quote