Hi Jim and all,
I suspect that the aliasing problem may not be in the sound card,
but rather in the driver's software sample rate conversion.
Most recent Windows systems are set up to always run the sound
card at 48 kHz, and to convert from/to other sample rates in
software, even if the sound card is capable of running at the
other rates! That's so that you can have multiple applications
playing sound at the same time, even though they are playing
data at different rates. Likewise, multiple apps can record the
same source simultaneously at different rates (in theory, this
is buggy on my machine).
Sample rate conversion is processor intensive, so some control
is provided, to allow the user to conserve compute power at
the expense of quality. It's possible that your sample rate
converter is not set to maximum quality, and this is resulting
in aliasing artifacts of greater amplitude.
The description below applies to my Win XP with SoundBlaster Live;
yours may differ somewhat. Open Control Panel, then Sounds and
Audio Devices. Select the Audio tab, and under Sound playback
click Advanced. Select the Performance tab, and under Sample
rate conversion quality, there is a slider. My system has
three settings called Standard, Improved, and Best. IMO,
they should be called Awful, Poor, and Mediocre.
Another possibility would be for Jason to be capable of
output at a higher sample rate. This would only slightly
increase the compute power needed, which would probably
still be less than what's required in receive mode. Sampling
at 48 kHz (or 44.1 kHz if it makes the computation easier),
you could put the output at 15 kHz or higher, which should
ease filtering the image frequency. If it generated two tones
in quadrature on the stereo outputs, then you could use an
image reject mixer in the Tx, and very little additional
filtering would be needed.
73,
Stewart KK7KA
Aliasing certainly is significant in (fairly recent) sound cards - I
designed my Jason TX implementation to take audio from the sound card at
5kHz, and mix/filter it up to 137kHz. When I tried this, I found a spurious
output on 6.025kHz only about 20dB down on the desired 5kHz output.
Changing the output frequency to 4.5kHz effectively eliminated the problem.
Evidently anti-aliasing filtering is there, but not very sharp cut-off. I
have observed significant aliasing effects also when using SpecLab in
"software receiver" mode.
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