[time-nuts] TV Frequency control - slightly O/T.
kscally at bytecan.com.au
Mon Oct 8 19:28:55 EDT 2007
Just noting a few points made recently on this topic.
Do not confuse the Studio colour subcarrier frequency
(4.43 or 3.58 MHz) accuracy - or the digital equivalent
clock frequency - with the TV transmitters carrier
frequency of transmission. They are totally independent
of each other and may, or may not share a "common clock"
As others have noted, the TV carrier frequency need only
be held to kHz, but is (and has to be) held to much
tighter tolerances (Hz) for co-frequency (or "offset")
Geostationary satellite Doppler observed on downlink
signals is far from random. A vectorscope shows this
very dramatically. It is entirely predictable and
caused by diurnal changes in the satellite's position in
it's 3 dimensional "box" resulting in the spacecraft's
altitude (Z plane) varying +/- tens of kms about the
mean distance of 35,786 kms. This daily oscillation is
NOT corrected by the spacecraft's on-board thrusters,
neither is the associated "figure 8" wobble in the X-Y
plane. Gas-guzzling orbital corrections are used only
when necessary, normally monthly.
In the analogue PAL/NTSC world, received satellite
signals that are re-broadcast at a remote Tx location
have their video passed through a 'frame store' which
regenerates Fsc, line & frame sync from a
locally-generated crystal oscillator, so removing the
received incoming Doppler shift. (This has tricked many
a country T&F nut who thinks this source of 4.43 MHz is
derived from a Rb or Cs source !)
Similar issues exist today in a digital TV & analogue
transmission chain where the incoming SMPTE multiplex
from the studio (carrying an embedded master clock
signal from the studio) is decoded and a composite PAL
signal generated for transmission. In these instances,
the incoming clock may be recovered to lock/steer an
oscillator generating the required 4.43 MHz subcarrier
frequency. It would be a bold system engineer who relied
on the incoming digital multiplex's as his primary clock
source to keep his Tx on frequency! As Murray alluded
to earlier, GPS technology have made this task simpler
and considerably cheaper.
When the USA, Australian/NZ & UK analogue transmissions
are turned off in 5-10 years time (ha !), most of these
"problems" go away I guess.
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