On Mar 24, 2016, at 1:36 PM, Michael Haack via RTTY wrote:
> Is there a decided advantage to using a high powered external sound card vs
> the internal card for RTTY/MMTTY/2tone use?
I am not sure what you mean by a "high powered" sound card. Sound card works
with millivolts to hundreds of millivolts, and very high input impedances.
Very little power involved.
If you mean the dynamic range of the sound card, I think I have already
addressed that in an earlier posting.
You can measure your own sound card by using free spectrum analyzer programs
that exists in all the OS platforms that I know. (On Mac OS X, you can use my
Amici spectrum analyzer/tracking generator program.)
With the radio tuned to atmospheric noise (no signal), measure the difference
between the noise floor and the full scale (clipping level).
If you are running 48000 samples/second, there is another 27 dB or so of
available dynamic range from the processing gain from a good modem. If you use
24000, samples per second, you have 3 dB less (24 dB processing gain).
Add the processing gain to what you measured earlier, and you pretty much have
the dynamic range of your modem.
If this number is greater than your receiver's dynamic range, the sound card is
adequate. If this number is worse than your receiver, you will either need to
use a better sound card, or ride the gain of the sound card so that a signal is
always higher than the noise floor, but is not clipping.
If you are using a wideband system (e.g., for a skimmer or a wideband RTTY
demodulator), you will also need to measure the intermodulation distortion and
the harmonic distortion of the sound card.
73
Chen, W7AY
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