On Jan 15, 2013, at 11:46 AM, Joe Subich, W4TV wrote:
> It is just as important with the built-in sound cards as
> with the external interfaces to use good proactive to keep drive
> levels to the proper level and avoid external sources of common mode
> RF/hum/etc.
Joe,
I think Andy's point (assuming I am interpreting him correctly) is that you
cannot overdrive a sound card past its full scale digital numbers (+,- 32767 if
16 bits converter). The full scale drives the codec to a known audio level,
often to 0 dBu or +6 dBu range (I have see high end codecs go a bit higher).
Since the rig manufacturers know this value, they can design their analog chain
so that the full scale digital value can never clip their other stuff.
All the junk we see in opposite sidebands and such are probably coming from way
overdriven balanced modulators, or there is a DC component in the AF going into
the balanced modulator. If the audio sine wave is not symmetric between
positive and negative voltages (which I can see happen when you are not careful
with your audio chain), you can indeed produce a small DC component.
With a built-in sound card, with the manufacturer doing the right things, the
only way you can overdrive or create a DC component, is that if the AFSK
generator of the software modem is horribly ill designed. I guess it can
happen by accident, but any error can also be corrected very quickly by the
developer.
As to hum and all that, the consumer simply cannot produce it with a built in
sound card short of opening up the radio and doing malicious things to the
hardware. If the radio manufacturer did the right things, their noise floor
can be as low as any external sound card, and probably better -- just look at
the codec in the Flex-5000; the noise floor is better than -120 dB from full
scale. Find me a ham quality digital interface sound card that can get within
20 dB of that!
73
Chen, W7AY
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