[WriteLog] Hum in Audio

Robert McGwier rwmcgwier at comcast.net
Mon Apr 5 17:31:44 EDT 2004

Furthermore, and in addition to Tom's statement, you do NOT want to
fix this by simply turning down the pot if what this does is causes
the level being sent to the D/A to be reduced digitally.  What this
gobbledygook means is the number of bits is reduced and the noise floor
goes up with respect to the signal, if your card does not have
an analog mixer following the D/A.  The sliders "Master Volume" and
PCM usually correspond to analog and digital respectively.  If you just
must do it by using the mixer, prefer the master volume.  An attenuator,
followed by keeping both Master and PCM well up (not max), is a better
solution as Tom states.  The TAPR circuit is ideal.


-----Original Message-----
From: writelog-bounces at contesting.com
[mailto:writelog-bounces at contesting.com]On Behalf Of
tmcdermott at chiaro.com
Sent: Monday, April 05, 2004 12:29 PM
To: writelog at contesting.com
Subject: Re: [WriteLog] Hum in Audio

Most soundcards put out more than 1 volt of audio pk-pk. The microphone
input on most rigs
is setup for about 10 millivolts pk-pk. Thus, the soundcard will overdrive
the mic input by 40 dB !
This will saturate the audio stages in front of the microphone gain control,
so even if you turn down
the mic gain, the audio has already been severely distorted. Sometimes its
difficult to turn down the
soundcard output by so many dB and have it still work well. An attenuator
helps a lot. Two resistors
in the cable from the soundcard to the mic input, or you can buy an
attenuating audio patch cable
at Radio Shack PN 42-2152, very inexpensively. See
for more details.

	-- Tom, N5EG

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