dsi@unccvax.UUCP (Dataspan Inc) (02/27/85)
While we are on the subject of NTSC (again) what are the theoretical
grounds for restricting the Q-channel bandwidth to 500 kc? I know all the classic
arguments from the 50's, but given the advances in electronics since sheet
beam balanced mixers were the "in" subcarrier generation device, what are
the problems in just making both I and Q channels identical. That way, you'd
not have to include the extra delay line to match I and Q in time for wideband
demodulation.
I didn't mean "identical" of course but identical in transmission
bandwidth.
dya
.karn@petrus.UUCP (03/05/85)
My understanding of the reason for the different I and Q channel bandwidths is that it was based on human visual perception. The human eye is much more sensitive to changes in intensity and hue for some colors (e.g., flesh tones) than it is in others. The I and Q axes were arranged so that the I channel carries the information requiring wide bandwidth (reddish-yellow flesh tones and their complements) while the Q channel carries blue-green signals and their complements. The eye is much less sensitive to bandwidth restrictions in the Q channel. Given its age, NTSC is still a remarkable tradeoff between human factors and engineering constraints, which is even more amazing when you consider that it was produced by committee. Phil
dsi@unccvax.UUCP (Dataspan Inc) (03/06/85)
a
Shortly after my inane question, the reason for the narrow Q-bandwidth
was privately made known. As the I-channel is vestigial sideband suppressed
carrier AM, the frequency components from 500 kc to 1.5 mc will appear in
phase quadrature (all together now!) in the Q channel. The choppage of one
of the sidebands causes phase modulation of the other..... Oh well.
Now, on the other hand, three RCA engineers described an adaptation of
NTSC that was fully compatible in the sense that their system could also receive
NTSC broadcasts. Both I and Q channels were extended to 1.8 mc bandwidth and
cancellation of the sound subcarrier (ala pilot cancellation in modern FM
receivers) was used. They didn't (dare ?) mention what the picture looked
like on a standard receiver although I suspect the narrow-chroma receiver
wouldn't even care in the least. It's in 1970-73 IEEE Transactions on Broadcast
Receivers (or something like that).
Back to my sickly-green lettuce commercials
dya
.