[time-nuts] Measuring TV delays

Magnus Danielson magnus at rubidium.dyndns.org
Thu Jan 2 13:15:13 EST 2014


On 02/01/14 18:10, Gregory Muir wrote:
> Reading all of this brings back memories of a project I was involved in back in the early 70's in the Denver area.  NBS-Boulder was experimenting with injecting their time standard into the video of the analog signal before it hit the transmitter.  We installed a prototype unit that was slaved to Boulder and performed the function of adding the time signal into the video.  Part of it also provided station sync slaved to the unit as well.  NBS then measured the system prop delay and adjusted the timing accordingly to compensate for any latency before it went to RF.  The experiment lasted for a couple of months and then the equipment was removed.
>
> Alas... satellites and digital...  given buffering, processing and routing delays, you are sort of now on your own in this one-bit world.

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


More information about the time-nuts mailing list