[ntp:hackers] Fwd: Video Frame Rate Ref Clock

Bart Schade onepv1d at gmail.com
Fri Dec 14 16:04:59 UTC 2012


Guys,

Thanks for the active thread.

The frame rate (or derivative) would impact NTP as a 1PPS (or FR) signal.
 The proposed RefClock is NOT intended to take time from embedded TimeCode,
 The time would be from another NTP server.

I like what Greg and Mark have described.

After some research into existing pulse RefClocks, I hatched the idea of
using a FR pulse from I/O card video signal.   While not 1PPS, I suspect
that so long as the signal is periodic and precise the conversion should
not >>too<< bad.

Bart


On Fri, Dec 14, 2012 at 5:23 AM, Mark Martinec <Mark.Martinec at ijs.si> wrote:

> > > I expect that it would be a SMPTE timecode.  I found this mentioned on
> > > Wikipedia.  Maybe Bart will describe the video standard being used and
> > > we can check what it supports.
> > >
> > > I would be worried that the time code might have the time from when the
> > > video was created rather than the current time.
> >
> > It generally does. And there's a few different time code fields and
> > standards to worry about as well (time within the picture and time filmed
> > at least). There's another field for the current time, but I don't know
> > how extensively it is deployed outside of terrestrial analog video
> > broadcasts.
>
> Don't mix SMPTE and other timecode that is used internally in video
> and film production, with what reaches the receivers. There is
> no longer any timecode there. The only useful data there is
> is the horizontal line rate, or the frame rate, which according
> to Bart they have it stabilized with GPS. Possibly a colour
> burst frequency (if stabilized too) can be used for the same
> purpose.
>
> Timestamps will need to come from some other source, perhaps from
> another NTP server.
>
> So, the most straightforward approach would be to derive a
> pulse-per-second signal from the video signal, as Greg Dowd suggests,
> and use that as a PPS source to stabilize the ntpd frequency.
>
> Don't know how complicated it would be to adapt NTP to accept
> some other low frequency pulses (not exactly 1 Hz) to stabilize
> the clock. For example the NTSC frame rate of 29.97 Hz is
> somewhat awkward to convert to 1 Hz.
>
>   Mark
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