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Dick PA0SE,
It was not until recently that I found out about
the extent of "mixer control" placement(s) in a sound card. There are
analogue and digital mixers. The MIC-IN, LINE-IN and CD-IN have inputs
split to feed an A/D converter as well as an anlogue mixer going to the
master volume control and LINE-OUT (also an internal power amplifier and speaker
out on some sound cards, newer types have speaker matters external to the
PC).
The A/D converted data has a source selector switch
(A/D or WAVE data) before reaching the PC bus. On newer sound cards the
bus has an AC97 compliant interface. The DSP "number crunching" is done by
the PC CPU and processed data then comes back to the sound card (delayed a
little), where a digital mixer selects the source, it then goes through a stereo
D/A converter, and further to the same analogue mixer that handles the MIC-IN,
LINE-IN and CD-IN (the analogue input signals can bypass the digital
processing).
Sound card drivers do not seem to provide user
access to all mixers and switches on a card. Different application
software may include user input to selections of analogue and digital mixers, or
pre-programme selections. There is at least one FFT filter programme I
have used that does apply the digitally filtered sound to the analogue
output. On the other hand, there are other programmes that do just as you
describe, with only analogue going to audio out, presumably because the
digital output is intended to go to the screen.
Some sound cards also have access to tone controls
but I have yet to experience one. I'm not sure whether tone control is on
the input or output side, as I have seen some "spectrum displays" that seem to
have minor frequency tilt, which it would be nice to correct to "flat" with tone
controls.
I trust my text explanations are useful. I do
not have a block diagram at hand of a sound card, but if another reader does,
and can scan it, then I'm sure it would be a good way to add to the
information.
73, Bob ZL2CA
----- Original Message -----
Sent: Tuesday, December 24, 2002 2:03
AM
Subject: LF: Audio problem
To All from PA0SE
Some programs, like Spectran, have
the possibility to make the processed signal heard via the PC speakers.
But my problem is that the use of filters provided by the program
does not make any difference to the audio at all. The spectrum is nicely
displayed, so there is no problem on that side.
The same problem exists
with other programs that are supposed to provide audio filtering via the
computer. What comes out of the PC speakers is the signal at the input
of the sound card.
Jan Harte, PA0HRT, experiences the same
problem.
Whatever is done with the volume controls, nothing
helps.
Any suggestions?
73, Dick, PA0SE
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