> > Wouldn't the cards need to be synchronized, though? Essentially, > > you're still comparing the two signals with each other, just > > doing it in software, rather than in hardware, in the classical > > time interval counter scheme counting 1Hz (or 123Hz). Syncing > > inexpensive cards is a real chore (and the only reason to be > > thinking about using this in the first place is to keep the cost > > to a minimum, otherwise, you might as well build a special > > purpose little box with counters & A/Ds, and an interface) > > I'm not sure it's that important (or helpful) for the ADCs to share a common > clock. Presumably the ~100 Hz beatnotes being digitized are on the order of > 1/100000 of the frequencies being measured. That means that a microsecond > of synchronization error between the ADCs would have an effect similar to a > picosecond-scale error on the DUT/reference sides of the mixers. > > Getting microsecond precision out of an audio ADC is going to require > processing multiple successive samples, and IMHO it will also require some > kind of auto-calibration scheme since sound-card clocks probably drift more > than 1 ppm per minute or so anyway.
They're not that bad. Fairly high aging, fairly substantial variation with temperature, but that actually stays pretty constant. Given the need for autocalibration -- > probably through a high-frequency sidetone sent to both channels in phase -- > the difference in complexity between supporting two ADC clock domains and > one is probably not a deal-killer. Yes.. but if you start feeding multiple signals into the ADC (e.g. a calibration pilot tone), then you start running into intermod effects from the inevitable ADC nonlinearities. I don't have a good intuitive feel for just how good the digitizing needs to be for this approach; I guess if I want to go further, I need to sit down and do the math. > > Most installations would probably need to use a beatnote frequency > closer to 1 Hz, so that would take a lot of pressure off the ADC clocks. It > *might* be enough to get you out of the autocalibration business, but my > guess > is that matching the phase tempco of the (AC-coupled) sound card inputs > might still be necessary for good long-term results. But at 1Hz, you're down in the LF rolloff of the ADC. They probably roll off around 10-20 Hz, and none too predictably (e.g. they just slap a suitable cheap ceramic capacitor in series with the audio as a DC block) But that DOES bring to mind an even cheaper approach.. the DATAQ $25 data acquisition unit. 4 10 bit ADCs at 1kHz or so _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
