[time-nuts] Sound cards

[J.D. Bakker]

At 10:30 +1300 14-01-2009, Bruce Griffiths wrote:
>Magnus Danielson wrote:
>  > J.D. Bakker skrev:
>  >>> [F]or best noise/jitter-performance an external ADC should be used,
>  >>> connected through a digital link to a PC sound card.[...]
>  >>>      
>>>  The digital link in question is S/PDIF; with the current popularity
>>>  of Home Theater systems cheap cards with digital I/O have become
>>>  quite prevalent. As an added bonus, S/PDIF can be run over both
>>>  coaxial and optical media, the latter being attractive in further
>  >> isolating PC noise from any measurement setup.[...]
>  >
>  > The optical link commonly being used for S/P-DIF is TosLink and it seems
>  > like it can be the cause of many problems.[...]
>
>Relatively high jitter being one problem.
>Limited sampling rate being another.

Both true; IME practical limits are ~7m for 24bit/96ksps/stereo and 
~3m for 24bit/192ksps/stereo, depending on TX and RX. My best 
experiences are with Toshiba TX/RX and pre-made cable listed as 'ADAT 
Lightpipe' rather than 'TOSLINK'. Both are the same on a mechanical 
level; the former is a pro audio de facto standard used to transfer 
multichannel audio around in studios, and its customers tend to be a 
bit more picky wrt out-of-spec cables.

Jitter is not a problem as long as (a) the converter is the timing 
master and (b) the PC end doesn't try to do resampling or other 
'clever' tricks (I know of no current mainstream chipsets that do).

>If one has a cheap 16 bit sound card what will it do with 24 bit data
>from an external ADC?

It truncates it to its 16 MSBs. Note that you'll have a very hard 
time buying a new internal 16 bit sound card these days, never mind 
one with S/PDIF. The only 16 bit S/PDIF interfaces you could get are 
USB ones designed around TI's PCM29xx USB codecs (and not much else). 
As these top out at 48ksps, they are of limited use anyway.

Many of the cheapest Home Theater PCI Sound Cards are built around 
the CMI8738 chip or one of its derivatives. This chip offers 
no-frills S/PDIF I/O up to 96ksps for a very low price, and as pretty 
much all boards that I have seen are a clone of the manufacturer's 
evaluation board, the S/PDIF signal is almost always easily available 
on a .1in pitch pin header (which may or may not be populated). I use 
it for quite a few test setups, but I do have a more expensive card 
(RME 9652) if I need more channels or a higher sampling rate.

JDB.
[disclaimer: the drivers are also an issue, naturally. I do most of 
my signal processing under Linux, which has good drivers for the 
CMI8738. On Windows and/or OSX YMMV, but I would be surprised if it 
did not work at all]
-- 
LART. 250 MIPS under one Watt. Free hardware design files.
http://www.lartmaker.nl/