[time-nuts] Measuring TV delays

Magnus Danielson magnus at rubidium.dyndns.org
Fri Jan 3 21:11:58 EST 2014


On 03/01/14 20:01, Glenn Little wrote:
> At the station that I worked at,  time code was recorded in the digital
> video stream in the camera, but, was only used for editing.
> The editors would use this time code to mark the segment end points.
> There was no correlation to the camera time code and actual time.
> When we taped a show from a feed, we used internally generated time code.
> This was used for the automation equipment.
> If a recording had time code of 2359 - 0000 in the show, the show had to
> be redubbed with time code that did not cross the day boundary.
> Our automation equipment did not understand day nor did it understand
> negative time.
> I would guess that if the station were 100% automated, everything that
> was broadcast would have valid time code.
> We broadcast Network, prerecorded syndicated shows, commercials, promos
> and locally produced programming.
>
> Because of the in field switching between cameras and graphic sources
> for a live broadcast camera time code was not important as the switching
> was done manually (you cannot automate a live event).
> There was probably continuos time code inserted at the output of the
> production switcher, but,this time code was not used at our station as
> we had to manually switch to a local source for promos, commercial, and
> locally prepared programming.
>
> I do not think that there would be a reason to preserve the camera time
> code from camera to air.
> I cannot see where it would be of much value as the camera would free
> run time code.
> It could be preset, but, there was nothing to guarantee frame accuracy
> for the length of a live event that was shot with numerous field cameras.
> The cameras were gen locked to allowing glitch free switching between
> sources at the production board, but, there is no way, that I know of,
> to sync time code.
>
> This is how the station that I worked at operated.
> Other stations my have operated differently.
>
> The bottom line is I know of no way to derive accurate time from a
> present day TV signal without the station inserting time information in
> the stream.
> This is how the VCR time auto set worked.
> But the uncertainties in the coding and decoding times would make this
> less accurate than time derived from other available sources.

Thanks for your story. The original quest was for delay, so accurate 
time at the venue is only to measure delay until decoded, so all that 
delay can be estimated.

Cheers,
Magnus



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