[time-nuts] Measuring TV delays

[Gregory Muir]

Not certain as to how NBS achieved the transfer of timing accuracy but it was not believed to be in the VITS interval.  If I can recall, there was a special waiver (FCC/NTIA) that was required in order to run the experiment.  And it required some esoteric hardware to decode the signal at the other end.  Mind you, this project was for local timing information dissemination only and didn't relate to programming aspects.  This project was above and beyond the use of the local transmitted television signal sync pulses that NBS was using to synchronize the Fort Collins atomic clocks with the Boulder ones.

I do agree, for those of us who tend to dabble in the sub-yoctosecond world, :) something like this is would be a rather coarse approach.

Greg


On Thu, 02 Jan 2014 19:15:13 +0100, Magnus Danielson wrote:

>Just to help confusing matters even more, SMPTE-12M time code (which is 
>what you can suspect to be in the TV-signal) and it's Drop-frame 
>algorithm causes a drift of time over the day, as the drop-frame 
>mechanism isn't perfectly aligning up to 30000/1001 frames per second 
>over the 86400s day. This requires the production-time to be jammed into 
>alignment regularly, such as every day (off-hour). With the evolving 
>standards, the halting mechanics of drop-frame correction is not 
>changed, but just standardized. The jamming mechanism is also used for 
>leap-seconds and DST change-overs.

>So, in the US and other 30000/1001 frames per second countries (formerly 
>NTSC), encoded time is not going to be useful for precision work. For us 
>in the 25 frames per second world, we only need to jam for leap-seconds 
>and DST change-overs, but that is enough of an upset, but can be more 
>easily predicted with only a few handful of bits extra information.

>I prefer using MLS measurement for audio delay measurement. If you do it 
>right, you get 20,833 us step resolution, as a result of the 48 kHz 
>sampliung clock. MLS delay measurement is trivial using the Analog 
>Precision test-set.

>Cheers,
>Magnus


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