[time-nuts] Sound Cards for locking to GPSDO 10 MHz references

[Magnus Danielson]

Lux, James P skrev:
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com 
>> [mailto:time-nuts-bounces at febo.com] On Behalf Of Magnus Danielson
>> Sent: Tuesday, June 02, 2009 10:08 AM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] Sound Cards for locking to GPSDO 10 
>> MHz references
>>
>>>
>>> Some of the "pro" sound interfaces have a "word clock" input.  
>>>
>>> There are a variety of things that take a external input 
>> and generate a S/PDIF that's properly timed, as well. Lots of 
>> boxes will take a S/PDIF sync input (e.g. the Edirol FA-66 
>> which was used by lots of Flex-Radio folk), so maybe that's 
>> something you could easily generate from your 10MHz. 
>>> A chart at Cakewalk shows that MOTU has a USB interface (828MkII) 
>>> which has a word clock sync. It's going to be a pricey 
>> beast though, 
>>> with 8in/8out ($800?)
>>>
>>> Even if you have a word clock input, you're going to have to 
>>> synthesize that from the 10 MHz.  Maybe it's easier to just make a 
>>> S/PDIF which is a MUCH more common sync signal. ( I think S/PDIF is 
>>> something like 3 MHz)
>> S/P-DIF [iec60958-3] has a baudrate which is 128 x sample 
>> rate and a bit rate which is 64 x sample rate, which is 
>> inherited properties from AES/EBU [aes3] [tech3250] [iec60958-4].
>>
>> Locking up a S/P-DIF (128 x sample rate) is about the same 
>> job as locking up a superclock (256 x sample rate) or 
>> wordclock (1 x sample rate).
> 
> However, if you're buying an off the shelf audio interface, you're
> stuck with whatever the mfr is providing for a sync input, and a
> (very) casual inspection of what's available these days
> (particularly at low cost) shows that S/PDIF seems to be the most common.

Do they really lock up to the S/P-DIF input? I doubt it for the cheap 
boards. Rather, they decode the S/P-DIF signal and ship the samples into 
the DSP. The DSP tends to make very rought sample-rate conversions like 
dropping samples etc.

A lockable board isn't that expensive. You can get them off ebay for 
instance.

Cheers,
Magnus