[time-nuts] How do US TV stations disseminate time to DTV converter boxes?

[David I. Emery]

On Thu, Oct 01, 2009 at 08:11:42PM -0600, Greg Burnett wrote:
> 
> How do US TV stations disseminate time to DTV converter boxes? ...And is 
> this time derived from GPS data, or???

	There is a time of date message stream (TDT) defined for MPEG
transport streams and that and a slightly different version of in the
ATSC variant of MPEG used by US TV stations transmitting digital signals
(now essentially all of them except LPTVs).

	Where the data in this comes from depends on the station. Mostly
it originates either in a transport stream multiplexer that feeds the
ATSC modulator or in a separate system that generates PSIP and other
information (a PC perhaps or a dedicated box)... 

	Both the mux and the PSIP generator likely have the usual
battery backed up clock calendar chip as their default source of time of
day but can be configured to lock this to other time sources within the
station.

	Depending on how careful the station is about setting this up it
can of course get unlocked and drift or even set to the wrong time zone
offset or daylight savings versus standard time.

	Stations may have their entire plant time and frequency locked
to a GPSDO or Rb or Cesium, but there is no FCC requirement for anything
remotely close to that level of accuracy so many don't.   And stations
may lock time codes and timing of  genlock black burst references to GPS
or may not... and may lock the mux time of day to those or not... 

	I think the likely explanation of the recent time problem with
converter boxes in the midwest is some common piece of software and/or
hardware used by a number of stations that has a plain old bug with its
timekeeping... 

	It is true that one network (Fox) supplies more of the over the
air ATSC transport stream than the others (the rest simply send MPEG2
video and audio signals which are decompressed by the satellite IRD and
then re-compressed for the local air by local station equipment - Fox
supplies more or less the exact compressed audio and video the station
is required to transmit on its main channel and controls the device that
makes alterations to this (like inserting a local station logo) from its
master control center.   It is remotely possible that this means that
some of the time of day info for Fox stations originates with the
network or the network controlled splicer device and the network screwed
up and caused a whole region of Fox stations to transmit the wrong time
of day.

	Older analog NTSC stuff had an optional vertical blanking
interval messaging system that was used by PBS stations to transmit time
of day and program guide information and many VCRs and DVRs  and some
TVs can look for and find this signal in the vertical blanking interval
of a PBS signal and use it to set time of day.   PBS stations have a box
that generates this data stream and insert it... and depending on
funding and how careful they are it may or may not be locked carefully
to an accurate time source.

	I am not sure if it is currently done, but it is possible to
configure MPEG encoders and mux gear to pass these vertical blanking
interval lines in a digital signal including HD versions and some PBS
stations (or even most all) may carry a digitized version of the same
old VBI time data, as they definitely do carry closed caption data in the
VBI for older TVs...

	

-- 
  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."