Message-ID: <C2E27FA5F1D34564B0BE4A737654B667@HPt680a>
From: "VK2ZTO" <vk2zto@djirra.com>
To: <rsgb_lf_group@blacksheep.org>
References: <22E5FF108AB94135883D0A402EC9B85C@JimPC>
Date: Fri, 2 Oct 2009 09:08:05 +1000
MIME-Version: 1.0
Subject: LF: Re: RX and Sound Card
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G'day Jim,

Thanks for this.  It is a very thorough treatment of the method - including 
the limitations.   It will be one of my "print out and file" reference 
documents and I will alert the VK LF-ers to the link.

While in the general area of soundcard frequency accuracy - there was a time 
in the distant past when worrying about sampling frequency errors was less 
of a concern to me as all the cards I tested were well within 100ppm (< 
0.1Hz @ 1000Hz).  In that distant past soundcard sampling rates were 
44.1ksps, 22.05ksps, 11.025ksps and 5.512ksps (usually implemented as 
5.5125ksps).   That is, all sampling rates were "harmonically" related. 
Now we have the 48ksps, 96ksps (and higher?), which, while self related 
harmonically, are not harmonically related to the legacy sample rates.

The practical consequences of this caused much head scratching on my part, 
until the fog cleared, when I first dabbled in W*S*P*R ("*"'s added to avoid 
spambots...;-).   Everything was going swimmingly until I cross-checked the 
frequency of my transmission as reported by WSPR against ARGO.   There was a 
discrepancy of some ten's of Hz.   For someone who is a bit AR about 
frequency accuracy this caused mild apolexy.

To make an already long story not grow too much longer, I always use 
external USB soundcards (EDIROL UA-1eX) which have a physical hardware 
switch to select sampling rates (44.1ksps, 32ksps, 48ksps and 96ksps) built 
into them.    For some reason I had switched the sampling rate to 48ksps. 
This is the "native" rate for WSPR - so it was reasonably accurate.   When I 
booted up ARGO, the OS drivers had to somehow downsample the 48ksps hardware 
sampling rate in software (?) to the nominal 5.512ksps rate used by ARGO, a 
process it did with sufficient roughness to produce the very significant 
frequency error.   I proved this by hardware switching to 44.1ksps on the 
external soundcard, upon which the discrepancy from the 48ksps setting WSPR 
reading disappeared.   Note that now (when switched to 44.1ksps), the WSPR 
reading had moved a few ten's of Hz as now the OS drivers had to upsample 
from 44.1ksps to 48ksps.    Worryingly, at the same time, the decoding 
sensitivity on WSPR took a dive.

IMHO - bottom line to this is - if you have a newer PC you are likely to 
have a card based around 48ksps.  This means you are probably good to go 
with WSPR, but may have frequency errors in ARGO.    If you have an older PC 
and/or soundcard it may be based around 44.1ksps, in which case you are good 
to go with ARGO, but there may be frequency errors and poor decoding 
performance on WSPR.

How much of this effect you see will depend on the quality of the 
downsampling/upsampling/interpolating process in the drivers and/or OS. 
You may not see any discrepancy at all - but it is worth checking if you are 
having poor decoding on WSPR.

In my case I can avoid all problems by hardware switching to 44.1ksps when 
using ARGO and 48ksps when using WSPR.

Sorry for the long post.

73 Steve VK2XV

----- Original Message ----- 
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Friday, October 02, 2009 3:09 AM
Subject: LF: RX and Sound Card


> Dear LF Group,
>
> I have just uploaded "RX_Soundcard_cal_v1.pdf" to the files page at the 
> UK500kHz group web site (go to 
> http://groups.google.com/group/uk500khz/files - it is down at the bottom 
> of the list). This describes a way of calibrating receiver frequency and 
> sound card sampling rate using an off-air standard frequency such as MSF 
> or 198kHz R4, without requiring an accurate audio frequency to calibrate 
> the sound card. This is addressed primarily to WOLF mode transmission and 
> reception, where this calibration has to be done fairly accurately in 
> order for the mode to work properly, but could also be generally useful 
> where sound card sample rate calibration has to be done, and there is no 
> precision audio frequency source available in the shack.
>
> Any comments would be welcome,
>
> Cheers, Jim Moritz
> 73 de M0BMU