Dear LF Group,
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.
As far as dynamic range goes, with the current receiver/sound card
combinations used, it seems dynamic range is almost always limited by the
RX IF/audio stages rather than the sound card. In conventional use of the
RX, all unwanted signals are removed by the IF filters, so designing the
IF/Audio section to have high dynamic range would be redundant. However, in
our narrow band LF applications, unwanted signal rejection is performed in
the PC, and IF/AF non-linearity will have a major effect. Effectively we
are putting a high-gain, not particularly linear amplifier in front of the
main selectivity of the receiver. There would be a lot of benefit in taking
IF output from the receiver directly from the filter outputs, and mixing it
down to audio with minimal additional gain sufficient to get the band noise
above the noise floor of the sound card. I expect this would give a much
more radical improvement than adding more resolution bits to the sound card
ADC.
Cheers, Jim Moritz
73 de M0BMU
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