[ntp:questions] Can NTP sync within 1ms

William Unruh unruh at invalid.ca
Tue Apr 29 15:45:48 UTC 2014

On 2014-04-29, Rob <nomail at example.com> wrote:
> David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
>>> Unfortunately that is not the same as performing some task like outputting
>>> audio at an accuracy of 12us, but that is the next challenge :-)
>> Indeed, and you will likely want to look at very matched systems (PCs 
>> and transmitters - the whole chain), and having the minimum latency in 
>> the OS.  I'm not familiar enough with Linux to know which variant to 
>> recommend, or whether FreeBSD or some other OS might be better.
> I already learned how this can be achieved in Alsa and it looks good.
>> When comparing, be aware that there can be significant delays in things 
>> like RS-232 level converters etc., especially if they have rise-time 
>> limiting.  Not to mention any delays in the COM port control lines!
> I first was afraid of that, and I wondered if the 10-30us pulse
> typically output by a GPSDO could be interfaced using an RS232 line.
> I considered it better to use a parallel port for this.
> However, parallel ports are not present on modern systems, USB replacements
> are no good for this purpose, and adding a parallel port requires
> a slot, also something usually on short supply.
> However, I now realize that 10us is equivalent to 100.000 bps, and
> all serial ports operate to 115.200 bps some even to 921.600 bps.

And many serial ports will handle TTL level just fine, without any
serial level converters. (On RPi, there is a limit of something line 3V
on the input pins, so putting out the 5-20V of a serial level converter
could be very unhealthy.)

> So this probably will not be an issue after all.

The problem is not the electronics, it is the response of the system to
the interrupt. That interrupt processing time is in the usec range.

>> It sounds an interesting project.
> Yes, we sometimes need to try something new (at least to us) or else
> ham radio becomes boring pretty quickly.

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