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From: Moe Trin on 31 Jan 2008 14:56
On Wed, 30 Jan 2008, in the Usenet newsgroup comp.os.linux.networking, in
article <8c693970-601c-4635-8276-29561b5db4cf(a)s8g2000prg.googlegroups.com>,
Giff wrote:

NOTE: Posting from groups.google.com (or some web-forums) dramatically
reduces the chance of your post being seen. Find a real news server.

>ibupro...(a)painkiller.example.tld (Moe Trin) wrote:

>> Depends on how busy the network and both computers are - PROBABLY
>> not a problem, but hard to say without details.
>
>Well the computers will only be recording, which is actually a bit
>heavy for the one recording video. The network will not be busy, as I
>could actually use a crossed ethernet cable and connect the two pc's
>directly.

If the systems are otherwise idle, then the starting delay is going to
be a function of interrupt latency, and the amount of time the
recording programs need to pull their finger out. As the NTP programs
seem to be able to get time data to sub-millisecond accuracy, this
is not likely to be a significant problem.

>I should need to know the delay it takes from the trigger to the
>actual recording starts... Do you have any suggestions on what to use?

I'd probably use a time code generator, or the output of a radio
clock. Use the PPS (Pulse Per Second) output to trigger the start of
the recording. An IRIG-B time stream is a 1000 Hertz audio tone that
is modulated to show time of year. Merely count the number of cycles
between the start of the recording and the next time mark. The audio
output of a DCF77, HBG, or GBR receiver would also work.

>So, let me see if I have understood you. If T1 and T2 represent the
>time needed by the two applications to actually start recording (I
>should then measure these values), and T1>T2, I let machine 2 send
>the packet to machine 1 and wait T1-T2 before issuing the start
>recording command. This does not count the network delay, but I
>measure that too (approximately) and add it to T1-T2.
>Am I completely wrong?

That's the general idea. The accuracy you need depends on what you
are recording, but stuff like broadcast television, or movies
_PROBABLY_ don't need to be sync'ed tighter than 1/25th of a second
(which is, after all, merely two frames of video).

Old guy
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