On Tue, 19 Nov 2002, Bruce Douglas wrote:

*>The best links I know of on the web are:
*>
*>http://www.philrees.co.uk/articles/timecode.htm
*>http://www.inforamp.net/~poynton/notes/video/Timecode/index.html
*>
*>I made an LTC reader using a 628.  The approach I used was to rotate
*>the input bits into ten registers until the 16 last bits agreed with
*>the SMPTE identification bits and then decode the other registers to
*>HH:MM:SS:FF.
*>Since both up and down transitions are significant I used the interrupt
*>on state to read the signal.  I don't know if this is the best way but
*>it worked for me.
*>
*>I was toying with the idea of making a LTC generator but was put off by
*>the fact that this would require a precision of at least 1 frame (1/30
*>second) in 8 hours (1 part in 864000) and I think this is beyond the
*>precision of even temperature controlled oscillators.  If I'm wrong
*>about this I'd be grateful if you could let me know.

You are not wrong about this but if the LTC signal does not live by
itself but is connected to some video source then you can resync on the
video source. The LTC is not meant to be a real time clock that excceeds
the precision of the video source, the latter being +/-10ppm in the best
of cases. There is also the problem of interleaving LTC with frame
boundaries correctly, and that requires that the LTC follow the video
frames or vice versa.

Also if anyone told you that LTC from studio equipment that is not locked
to a rubidium clock is accurate to 1.6ppm, then imho they lied ;-)

Peter

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