[ntp:questions] Can NTP sync within 1ms

Jochen Bern Jochen.Bern at LINworks.de
Sun Apr 27 22:27:51 UTC 2014

On -10.01.-28163 20:59, Rob wrote:
> Jochen Bern <Jochen.Bern at LINworks.de> wrote:
>> I'm afraid I don't get it yet. You're trying to sync waveforms on the HF
>> side (~100 MHz?) instead of switching between two transmitters on the AF
>> side (couple kHz, with that much more leeway for the sync), a la
>> perfectly normal car radio with RDS AF and dual tuners, because ... ?
> It is not an FM broadcast system, it is an amateur radio repeater system
> with wide area coverage.

Alright, I think I'm closing in on some basic concepts now. :-} You have
existing mobile FM transceivers and existing individual repeaters
(including the non-equidistant rather far in-between sites they're
installed at) and you want to add sort of a central control to the
repeaters so that they act in unison, relaying one input to the *entire*
coverage area, and in such a way that the mobile transceivers do not
have to care (read: QSY) which single repeater they're currently covered
by - RX as well as TX.

First good news, you won't have much of a problem if a participant
getting handed over from one repeater to another experiences a "time
jump" (relative to the master audio stream) even of several tenths of a

Second good news, the audio stream will likely experience pauses
(everytime one OM hands the mic to another) more often than the mobile
devices will need to switch over to a new repeater. With tight enough a
"squelch", your repeaters can actually drop output power during that
pause, which might help the mobiles switch over even in the presence of
capture effect etc. during the power-up phases.

On the not-so-good side, once a random participant finally got the PTT
squished, your repeaters will have to have a quorum decision which
repeater's *input* to use for the master audio stream of the entire
system. (And you might want to modulate the repective repeater's ID over
it in CW, or else crocodile hunting will get pretty tedious. ;-)

However, that *does* leave me wondering where the 12 us figure comes
into play. With the typical distances between 2m and even 70cm
repeaters, the mobile transceivers will see shifts *far* beyond that
between different repeaters' signals. FWIW, will you have the audio
cards output AM (that will then get modulated onto an otherwise
unsynchronized HF), or do you plan to have the card generate HF directly
into the PA?

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