[time-nuts] ADEV vs MDEV - using sound card

[Magnus Danielson]

Bob Camp wrote:
> Hi
> 
> My main concern with the low frequency pole in the sound card is the quality of the R/C used. You can certainly model what ever you have. If they used an aluminum electrolytic for the "C" it may not be the same next time you check it ....

Do consider to bypass it. This is routinely done both by audio folks and 
various other. The cap is there to remove DC offsets which can be 
problematic in audio editing. I am quite sure you feel at home with the 
soldering iron to do that upgrade.

> On a 10 Hz system, a 1 Hz pole is probably not an issue. It might get in the way with a 1 Hz beat note. 
> 
> Another thing I have only seen in passing: "Sigma Delta's have poor low frequency noise characteristics". I haven't dug into it to see if that's really true or not. If you buy your own ADC's, you certainly would not be restricted to a Sigma Delta.

Strange, most Sigma Delta's I have seen would have the opposite said 
about them. It's the upper end that is problematic.

> Even with a cheap pre-built FPGA board, you could look into higher sample rates than a conventional sound card. You would drop back to 16 bits, but it might be worth it. 

In fact, one of my FPGA demo-board does 3 MS at 14 bit. Crappy for 
audio, but maybe good enough for this application.

Cheers,
Magnus