I'll look for them! Don Bruce Griffiths > Don > > One potential problem with most USB sound cards is the preamp. > The gain of these is set by a front panel pot over a very wide range. > Consequently the preamp gain cannot be all that stable. > Unless these preamps can be bypassed they may limit the performance when > used for measurements. > > Bruce > > Don Latham wrote: >> Well, I've just ordered two $9.98 USB sound "cards" on good ol' ebay. It >> will take some time to get here from China. I'll however have no >> compunction about opening up and so forth. Maybe some secrets will spill >> out :-). Still will not beat the EMU 0202 probably but when wrecked by >> fiddling will not provoke as big a fit. I'll let you all know how it >> comes >> out... >> Don >> >> Magnus Danielson >> >>> Hal Murray wrote: >>> >>>> james.p....@jpl.nasa.gov said: >>>> [External clock at strange frequency.] >>>> >>>> >>>>> That's an interesting idea. I would imagine that the clock going >>>>> into >>>>> the chip is probably some multiple of the sample rate (e.g. >>>>> 48kHz*16*2 >>>>> = 1.536 MHz), so you could pick the closest 1/N from 10 MHz and pump >>>>> that in. >>>>> >>>>> However, what about the USB interface? These are inexpensive >>>>> devices, >>>>> and I'll bet all the rates are carefully chosen so that everything >>>>> shares one clock. >>>>> >>>> I guess somebody will have to take the lid off and look inside. >>>> >>>> Most USB gizmos that I've looked at have something like a 24 MHz >>>> crystal. I >>>> assume that is a sweet spot for cost. At the root hub, that turns >>>> into >>>> the >>>> clock/bit rate. At the device end, I think it's PLLed to the upstream >>>> clock. >>>> >>> 24,576 MHz is common, as it is 512 x 48 kHz. >>> >>> >>>> My guess is that any claimed-to-be-good audio gear would have it's own >>>> audio >>>> clock just to avoid the wander as the PLL follows its view of the >>>> upstream >>>> clock. >>>> >>> When you lock the clock you want it to follow the source, but you as >>> the >>> user needs to ensure the source is good. For professional systems, the >>> Audio Engineering Society (AES) have standardised this in the form of >>> AES-11 while the professional audio standard AES-3 do standardise the >>> jitter transfer between timing reference and output. >>> >>> >>>> I don't understand the audio numbers. Is there a crystal frequency >>>> that >>>> works well with all normal sampling frequencies? I don't see one if >>>> you >>>> want >>>> both 44.1 and 48 KHz. (There could easily be some sneaky scheme I >>>> don't >>>> know >>>> about.) >>>> >>> 44,1 kHz is a consumer number, as a result of Sony/Philips working on >>> how to shoe-horn into the limits of the CD. They wanted 72 min music >>> for >>> a suitable size (120 mm) and optical technology. >>> >>> The professional audio prefers 48 kHz (a simple x 6 from traditional 8 >>> kHz audio) and power of 2 multiples (24 kHz, 96 kHz, 192 kHz or 384 >>> kHz). An older standard is 32 kHz, which has a simple relationship to >>> the modern series (2:3). >>> >>> Even more hair-pulling is tossing in the 1000/1001 factor and its >>> inverse 1001/1000 for all places. Makes alot to cause troubles for >>> frequency syntesis. If we could do away with that, then I would be much >>> happier. >>> >>> The 48 kHz sampling rate has known and defined relationship to frame >>> rate to TV standards, as defined in AES 5 and AES 11. >>> >>> Professional rates when not infected by 1000/1001 factors makes sense, >>> is easy to correlate to frames, GPS and whatever is relevant for >>> production. >>> >>> Wordclock is very similar in behaviour, but has no real definition. >>> AES-11 has an informative annex covering it. >>> >>> The AES-2id and AES-12id would be recommended reading for someone >>> wanting to peak into the issue of jitter (and wander) from the audio >>> perspective. >>> >>> Unfortunatly the AES papers isn't free on the web. Other resources is >>> available. Julian Dunn have written several very good papers. He has >>> also written two of the Audio Precision (http://www.ap.com) application >>> notes, one on jitter and the other on AES/EBU digital audio interfaces. >>> >>> None of these uncover the mysterious sample rate numbers thought. >>> >>> Cheers, >>> Magnus >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >> >> >> > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >
-- Dr. Don Latham AJ7LL Six Mile Systems LLP 17850 Six Mile Road POB 134 Huson, MT, 59846 VOX 406-626-4304 www.lightningforensics.com www.sixmilesystems.com _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.