[time-nuts] Using cheap sound cards for measurements

[Don Latham]

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.lux at 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
>>>
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>>>
>>
>>
>>
>
>
>
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-- 
Dr. Don Latham AJ7LL
Six Mile Systems LLP
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