[ntp:questions] Accuracy of audio tones via VOIP
spam at goes.nowhere.com
Tue Jul 16 12:33:20 UTC 2013
In article <51e48ee8.37077834 at news.eternal-september.org>, no-
one at notreal.invalid says...
> It is my experience working with audio applications that the only
> oscillator that affects reproduced audio frequency is the one in the
> soundcard. The software merely builds blocks of data samples and
> delivers them to the soundcard drivers which play them out at a rate
> determined by the soundcard clock oscillator. The "system" clock has
> no affect on this process.
Bear in mind however...
That Skype, along with a lot of other VoIP systems, AFIK use a "Psycho
Accoustical Model" codec (much like MP3) to compress the audio, so the
contents of the data packets are not just sampled audio data, there is
much less data transfered than that, and it make ones eyes water
thinking of just how it somehow recreates (suprisingly good) legeble
speech, and in any language.
Tones, and music do propagate, but do tend to get mangled, badly at
times, and not only caused by "lost packets" on the wire. The OS/CPU
might get bogged down doing something else it thinks is important for a
few milisec's at times, causing stutters etc, plus the codec software
may decide that something does not need to be reproduced for
intelegability needs at the recieving end. Or, encoded and sent from
the sending end for that matter.
For those reasons alone, I wouldnt trust any common or garden VoIP
system to transfer "Frequency" (or "Time") Standard signals in any way
shape or form. They might to the untrained ear sound OK, but in
reality, all bets would be off, to say the least, unless as someone else
said, +- a minute was good enough <<G>>
The PSTN PCM systems (at least when I was working for them in the UK,
way back when) use special AD/DA systems with fixed clocks (locked to a
local reference, that in turn was compared to a national reference) from
vague memory, so the difference between them and VoIP is like chalk and
cheese. (Look's the same at a distance, but, you wouldnt want to eat
(I have no idea at all, what they use now for digital PSTN codecs, my
guess is like everything else, technology has moved on somewhat.)
If you *NEED* a local high accuracy frequency standard, then get
something like a Thundabolt GPSDO (maybe put it in an airconditioned
room too?) Then divide down from that to get whatever audio reference
tone you need.. (And 1PPS pulses!)
If they are good enough for global cellular system base stations (among
others) that are very critical on timing, they should be good enough for
the rest of us.
Dave B. (Back under my rock.)
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