[hpsdr] ADC Question

[Bill Tracey]

Alberto - there's nothing magic in 48/96/192, just happened to be 
what was there before our hardware.  In a purely technical sense, you 
can use whatever sampling rate you want.   The constraint is you need 
to have agreement on those sampling rates at your  interfaces.  In 
our case, PowerSDR  supported 48, 96, and 192 so that is what we went 
with at the time.   I could have rewritten the code to support 
arbitrary sampling rates, but I took the pragmatic approach and went 
with being compatible with what was at hand.   That also made life 
easier interfacing to VAC and others.  The problem I ran into in 
PowerSDR was the CW  generator converted timing data to sample counts 
assuming  48khz.

Does anyone actually know how well feeding non sound card based rates 
into VAC works?   Can Perseus's or QS1R's software feed to/read from VAC?

Larry -  what sampling rates are you looking for? Would 24/12/8/6 or 
384/576/768/1536 khz  - these are all integral decimations from 
122.88MHz so are not hard to do in the firmware.  Are you looking for 
absolutely arbitrary sampling rates, or would integral decimations 
from 122.88 MHz fit your needs?

As to where this is baked into the architecture - at the moment the 
transmit IQ samples are assumed to be 48 khz and the received IQ 
samples are allowed to be 48/96/192 - this is all the protocol 
between the PC and the device publishes.  The protocol can certainly 
be extended, just needs to be done with care as we'd like to avoid 
breakage to existing programs as much as possible

Regards,

Bill (kd5tfd)


At 04:50 PM 7/24/2012, Larry Gadallah wrote:
Is there really such a constraint ? Why the software running on the 
PC must be fed with
>48/96 or 192 kHz samples ? Aren't those rates an heritage from the 
>sound card era ?
>Take for example Perseus.  Its DDC sends to the PC rates of 125, 250, 500 kHz,
>1M or 2M samples/sec.  Both the original Perseus software and my 
>Winrad are perfectly
>capable to cope with those rates.  IMHO there is nothing magic with 
>48/96/192 kHz.
>
>--
>73 Alberto I2PHD
>
>
>
>I kind of agree with Alberto here: In addition to the Perseus, Phil 
>Covington's QS1R also has a wide variety of non-integral sampling 
>rates available. In my astonishingly small knowledge of DSP 
>techniques, I know that non-integral resampling is a difficult task, 
>but non insurmountable. Phil and Alex have been working recently on 
>improving the sampling rates, and this happens to be one of my 
>biggest wish list items, so it would be good if we could somehow 
>break away from the soundcard heritage and start to take advantage 
>of the true capabilities of the hardware (i.e. CW Skimmer Server and 
>5-band WSPR beacon).
>
>All of this kind of leads to a supplemental question: How much of 
>this "integral of 48 kHz" is baked right into the architecture?
>
>Cheers,
>--
>Larry Gadallah, VE6VQ/W7                          lgadallah AT gmail DOT com
>PGP Sig: B5F9 C4A8 8517 82AC 16B6  02B6 0645 69F0 1F29 A512
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