[ntp:questions] Using NTP to calibrate sound app

Rob nomail at example.com
Mon Jan 28 09:57:00 UTC 2013


Jeroen Mostert <jmostert at xs4all.nl> wrote:
> Even if you consider accurate detection of 1 cent to be good enough for tuning 
> purposes, your measuring equipment still needs to be an order of magnitude 
> better. A 0.1 cent difference is 57.8 ppm. 12 ppm is 0.02 cent, which isn't 
> excessive if you're going for 0.2 cent accuracy.

Ok but when you want to measure to 12 ppm and you assume an average
network jitter of 100 ms (which is better than reality when you are on
UMTS) you will need two samples that are at least 2.5 hours apart when
doing a calibration using SNTP.

There will be the extra complication that a UMTS connection goes into
a "sleep mode" when there is little or no traffic.  To "wake it up"
you need to send about 4 packets/second for some time.  This is a
very typical pattern on a UMTS connection:

ping -i1 172.31.32.18
PING (172.31.32.18) 56(84) bytes of data.
64 bytes from (172.31.32.18): icmp_seq=1 ttl=127 time=461 ms
64 bytes from (172.31.32.18): icmp_seq=2 ttl=127 time=377 ms
64 bytes from (172.31.32.18): icmp_seq=3 ttl=127 time=390 ms
64 bytes from (172.31.32.18): icmp_seq=4 ttl=127 time=369 ms
64 bytes from (172.31.32.18): icmp_seq=5 ttl=127 time=446 ms
64 bytes from (172.31.32.18): icmp_seq=6 ttl=127 time=370 ms
64 bytes from (172.31.32.18): icmp_seq=7 ttl=127 time=380 ms
64 bytes from (172.31.32.18): icmp_seq=8 ttl=127 time=385 ms
64 bytes from (172.31.32.18): icmp_seq=9 ttl=127 time=383 ms
64 bytes from (172.31.32.18): icmp_seq=10 ttl=127 time=425 ms
64 bytes from (172.31.32.18): icmp_seq=11 ttl=127 time=402 ms
64 bytes from (172.31.32.18): icmp_seq=12 ttl=127 time=496 ms
64 bytes from (172.31.32.18): icmp_seq=13 ttl=127 time=410 ms
64 bytes from (172.31.32.18): icmp_seq=14 ttl=127 time=391 ms
64 bytes from (172.31.32.18): icmp_seq=15 ttl=127 time=448 ms
64 bytes from (172.31.32.18): icmp_seq=16 ttl=127 time=457 ms

ping -i0.1 172.31.32.18
PING (172.31.32.18) 56(84) bytes of data.
64 bytes from (172.31.32.18): icmp_seq=1 ttl=127 time=552 ms
64 bytes from (172.31.32.18): icmp_seq=2 ttl=127 time=828 ms
64 bytes from (172.31.32.18): icmp_seq=3 ttl=127 time=809 ms
64 bytes from (172.31.32.18): icmp_seq=4 ttl=127 time=693 ms
64 bytes from (172.31.32.18): icmp_seq=5 ttl=127 time=593 ms
64 bytes from (172.31.32.18): icmp_seq=6 ttl=127 time=493 ms
64 bytes from (172.31.32.18): icmp_seq=7 ttl=127 time=448 ms
64 bytes from (172.31.32.18): icmp_seq=8 ttl=127 time=348 ms
64 bytes from (172.31.32.18): icmp_seq=9 ttl=127 time=246 ms
64 bytes from (172.31.32.18): icmp_seq=10 ttl=127 time=144 ms
64 bytes from (172.31.32.18): icmp_seq=11 ttl=127 time=50.8 ms
64 bytes from (172.31.32.18): icmp_seq=12 ttl=127 time=52.2 ms
64 bytes from (172.31.32.18): icmp_seq=13 ttl=127 time=102 ms
64 bytes from (172.31.32.18): icmp_seq=14 ttl=127 time=50.4 ms
64 bytes from (172.31.32.18): icmp_seq=15 ttl=127 time=51.4 ms
64 bytes from (172.31.32.18): icmp_seq=16 ttl=127 time=52.4 ms
64 bytes from (172.31.32.18): icmp_seq=17 ttl=127 time=47.6 ms
64 bytes from (172.31.32.18): icmp_seq=18 ttl=127 time=50.1 ms
64 bytes from (172.31.32.18): icmp_seq=19 ttl=127 time=51.4 ms
64 bytes from (172.31.32.18): icmp_seq=20 ttl=127 time=47.4 ms

As you can see, totally unsuitable for accurate time calibration.



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