[Musix-usuarios] ==¿Qué es el AC97 en el mixer de mi tarjeta de sonido?==

[Marcos Guglielmetti Gmail]

==¿Qué es el AC97 en el mixer de mi tarjeta de sonido?==

(Nota: traducir al inglés)

En las tarjetas de sonido Sound Blaster y (hasta dónde sabemos) en 
otras, AC97 es un componente de hardware que convierte la señal de 
sonido analógica desde las entradas de micrófono y línea en señales de 
audio digital.

Esto significa que cuando queremos realizar una grabación no sólo 
debemos asegurarnos de que hemos habilitado la entrada de línea o MIC 
en el mixer (gamix, alsamixergui, gnome-alsamixer) sino también que 
AC97 deberá estar habilitado y tener suficiente volúmen.

=== Descripción más extensa (en inglés) sobre el funcionamiento de una 
placa de sonido ===

(NOTA: TRADUCIR AL ESPAÑOL!)

Esta explicación fue tomada de la lista de correo: 
alsa-user en lists.sourceforge.net, en la cual surgió la duda al respecto 
de ¿qué es el AC97?

Steve Wahl <steve (@) pro-ns.net> contestó realizando una descripción 
basándose en un diagrama.

After understanding that a lot of sound cards
have an AC97 part: Google for AC97 datasheet, and examine a few of the
results.

Here's one I found with a nice block diagram at the bottom of the
front page:

http://www.national.com/ds/LM/LM4548.pdf

[ Something like that diagram should sneak into the ALSA documentation
somehow, IMHO. I've refrained from suggesting this because making a
suggestion like that is probably tantamount to volunteering to write
the documentation. :-) ]

What follows is my interpretation of that block diagram.  Hopefully
some will find it of value.  If you can interpret that diagram on your
own, you can probably stop reading this message right here...

JUST GOING FROM THAT BLOCK DIAGRAM (in other words, working knowledge
but by no means the last word on AC97!):

The AC97 part contains a number of analog audio inputs AND outputs, a
stereo pair of analog to digital convereters, and a stereo pair of
digital to analog converters.  And it has a mixer section on the
analog side; controls for this mixer are registers that your sound
card driver writes to. *Some* of alsa's mixer controls probably write
these registers directly -- other alsa mixer controls may deal with
other parts of your sound card, or maybe software/driver functions.

What you can see is (first following where the inputs turn from going
right to going down) that each input gets an adjustable gain level and
mute switch (boxes labeled GAM in the diagram) into a mixer that goes
through a bit of processing [Nataional 3D sound processing, probably
not a standard AC97 feature] and out to the line outputs.  Those gain
/ mute switch controls would be the playback volume controls from your
alsa mixer application.  The output of the Digital to analog
converters (aka DAC, greek sigma-greek delta D/A in the pictures) also
is one of the channels that gets mixed in here, that's how playing
mp3/wavs/etc gets to your line output.

Now, following the paths that kept going to the right, that big,
unlabeled, narrow, vertical D-shaped outline represents a switch (I
believe), so you can select one of the analog intputs to go into the
analog to digital converter (aka ADC, greek sigma-greek delta A/D in
the pictures), which is where your software records from.  Things I
note here:

1.  All (well, most) inputs to this switch are before any GAM (gain /
mute) boxes, so the playback level controls won't (or shouldn't)
affect the record level.

2.  The output of the mixer that feeds the line out jacks is also
routed up and to the right into this recording selection switch.
This must be the "record what you hear" option I've seen, and when
that input is used, the playback gain controls WOULD affect the
record level.

3.  There's a GAIN/MUTE box between the record select switch and the
ADC; this must be that mystery IGAIN control I've seen in the
mixers but never quite understood.   Obviously, it's your record
level.

Next two are quite minor:

4.  The Mic gain (0 or 20 dB) comes before the record select switch,
so it'd affect both the playback mix and the recording -- as it
should be.

5.  The PC_BEEP_IN connection does not go to the record select switch,
so it probably can't be recorded except by the "what you hear"
selection.

I think that's about it.  It'd help to view datasheets for a couple of
different parts that claim to be AC97 and compare them, but I think
this is a good start to understanding.

I believe I understood your typical sound card a lot better after
discovering this information, so I hope sharing it with the rest of
you will be of value to someone.



-- 
Marcos Guglielmetti  
* Director del desarrollo de Musix GNU+Linux, 100% Software Libre
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