Re: 700mhz band?


"Don Klipstein" <don@xxxxxxxxxxxxxx> wrote in message
news:slrnfba9g9.dlm.don@xxxxxxxxxxxxxxxxx
(I am not sure what the horizontal resolution is supposed to be, though I
consider displaying 200 bright and 200 dark vertical lines barely a tall
order).

The system was designed to provide approximately "square"
(i.e., same horizontal and vertical) resolution capability. There
are about 484 active lines per frame, which, since it IS an
interlaced format and thus subject to a "Kell factor" loss of
vertical resolution, winds up delivering a vertical resolution of
about

0.7 (the assumed Kell factor) x 484 = ~340 lines

(This is an approximation, of course. The actual delivered
vertical resolution depends on a number of factors, but this is
what the system was designed to provide.)

Since TV uses a 4:3 aspect ratio, the luma (Y) channel
bandwidth was chosen to given a horizontal resolution
equal to about:

1.33 x 340 lines = 450 lines

....or, for the purposes of this discussion, 225 cycles
(line pairs) per horizontal scan line.

The horizontal blanking time in the U.S. scanning standard
leaves about 53 microseconds active time per line, so
the required bandwidth for 225 cycles/line is

225/(53 microseconds) = 4.245 MHz

....so, not surprisingly, we're right back to the 4+ MHz
range normally quoted for the Y signal bandwidth.

750 KHz of the 6 MHz channel is wasted in lower sideband of the video
signal, and I suspect that is because of limitations in practical SSB
technology when we first got broadcast TV.

Not really wasted; TV video is broadcast using vestigial-sideband, but
full-carrier, AM. Even today, suppressed-carrier AM (either DSB or
SSB) isn't a good choice for sending any sort of signal where you really
care about preserving the frequency/phase info in the original
signal. True SSBSC was certainly available when the TV system
was being designed - it just wasn't the right tool for the job.

Bob M.


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