[time-nuts] disciplining sound card

Demian Martin demianm_1 at yahoo.com
Sun Jul 8 05:34:59 UTC 2012


We need a better idea what the goal is. If it's to sample and digitize data
at a specific time you may need to "roll your own" but if it's to figure out
the time of an event or to look at spectral info a mid price premium
soundcard like the Juli@ should be more than adequate. Knowing when in a
sample cycle the actual sample was taken may not have a lot of meaning since
the incoming info is bandlimited. The standard sample rates, 44.1, 48, 88.2,
96, 176.4 and 192 KHz have roots in the video world and are not nice numbers
per 10 MHz references.  Audio cards use either a single 24.576 MHz crystal
and PLL to generate the other frequencies or both a 24.576 MHz and a 22.5792
MHz crystal. The ones with separate crystals do have less jitter.

Word clock can only be used with difficulty if the card was not designed for
it. You cannot force word clock and have the rest of the I2C buss work
right. You can lock to an external SPDIF or AES signal (they are virtually
the same except for some info bits and the signal levels. AES is 4V P-P
differential into 110 Ohms. SPDIF is 1V P-P into 75 Ohms (an AES variant is
the same). The same receivers are used for both.

If absolute timing is important you can easily use an external capture
device (TI, ADI and AKM all have very good demo boards) that you can clock.
Clocking at 10 MHz will work in some systems on the external spdif input but
many will reject it since it's too far from an accepted frequency. The high
end cards that might will resample everything messing with your carefully
captured data. If you do get the data in the existing software will give you
confusing results. There may be a simple way to add a SMPTE time code to the
data as its captured. It's done in the video industry.

If you don't want a delta sigma ADC you can substitute a different kind but
there will be tradeouts. Usually bit depth vs sample rate vs. accuracy.

A simple way to discipline a 22.5792 and a 24.576 VCXO to a 10 MHz reference
would be very interesting. 

A good way to verify the performance and any issues with a capture system
would be to make a count down from the 10 MHz to an audio frequency and
capture it. Do a really deep fft and look for stuff that should not be there
(anything but the countdown).
           Demian Martin





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