[time-nuts] Pulling a signal from DirecTV
David I. Emery
die at dieconsulting.com
Fri May 9 17:23:14 EDT 2008
On Fri, May 09, 2008 at 09:40:55AM -0600, Andrew Novick wrote:
> Does anyone have experience getting a measurable frequency (in phase with
> the broadcast FQ) from a consumer DirecTV box?
What do you mean by this ?
Traditional SD DirecTV boxes have NTSC composite and Y/C outputs
- AFAIK, the clock for these is derived from a 14 MHz crystal and has no
special timing properties any better than a garden 14 MHz crystal
oscillator. Thus the 3.58 MHz color-burst is not going to be
exceptionally accurate, nor is the sync and frame timing based on it...
HD DirecTV boxes also have component (Y, Pr, Pb) and and HDMI
outputs, I am not certain whether timing for these in current designs is
in any way locked to the incoming signal but I'd bet they also just use
some convenient crystal oscillator frequency.
The only actual fine grained timing references in the incoming
DirecTV satellite signal is the PCR in the transport streams and the
QPSK/8PSK symbol clock. Internally in DirecTV boxes there are hard and
soft PLLS that lock timing to the symbol clock in the demodulator and
the PCR time stamps in the transport stream for the audio/video
decoding, but whether this gets all the way to the output video isn't
It would be possible (the current digital video standards do
work this way) for all the timing and frequencies of the output of the
box to be locked to the PCR timing of the incoming transport stream
(though with what jitter and wander is uncertain). This does require
that the entire box timing (or at least all the video path timing) is
locked to a 27 MHz VCXO which is firmware PLL'd to track the incoming
PCR 27 MHz time stamps.
Locking video timing to this ensures that no frames are ever
dropped or repeated due to clock differences but does require that all
the necessary frequencies are synthesized from that one 27 MHz VCXO -
using separate cheap oscillators does result in occasional frame repeats
or drops but infrequent enough be imperceptible given reasonable
assumptions about frequency errors.
I do not know how accurate the Castle Rock uplink signal PCR is
or how accurate the symbol clock is either... both depend on
multiplexers at the uplink and could be just the timing of a random TCXO
somewhere or could be based on a highly accurate house frequency
And of course the satellites drift around in the sky
significantly resulting in substantial doppler errors, so the even if
the uplink reference is right dead to nuts on - the downlink at an
observer won't be except very long term.
Dave Emery N1PRE/AE, die at dieconsulting.com DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."
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