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[hfsig] Performance of 24/192 soundcards? USB Soundcards

Thomas Sailer t.sailer at alumni.ethz.ch
Mon Sep 27 10:32:32 UTC 2004


On Fri, 2004-09-24 at 13:51, Anthony.N.Martin at marconiselenia.com wrote:

> I have been asked several times whether I would design a packet
> modem for standard FM V/UHF radios for >9k6 using a soundcard and
> the standard mic and spkr connections. They always suggest QAM,
> but FM radio BW varies so much I think of some kind of rate-adaptive
> OFDM where we add or drop carriers accordingly and negotiate with
> the other station to optimise throughput.

Bad idea (to use the standard mic connector). Been there, done that. The
problems are:

* most mic connectors are followed by an AGC/limiting circuit, which
introduces nonlinear distortion

* some (most?) handhelds do not disconnect the internal microphone when
you connect an external one, so any ambient noise gets transmitted along
your modulation signal

The best I could get was about 4.8kBit/s, and that was pretty fragile.

Tom





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