On Mon, Apr 17, 2006 at 12:28:46PM -0400, Lee Revell wrote: > On Mon, 2006-04-17 at 09:14 -0700, Bill Unruh wrote: > > On Mon, 17 Apr 2006, Lee Revell wrote: > > > > > > AC97 is the codec that delivers line and mic signals to the card. > > > > I am a bit confused with this description. I would think that the plugs on > > the card deliver line and mic signals to the card. What does it mean to say > > that the codec delivers line and mic signals to the card? > > > > It's the bit of hardware that turns the analog signals from line and mic > into digital audio.
Lee's probably been at this too long and doesn't quite understand the difference between his vast knowledge and what a newbie knows. His description above could apply equally to a simple ADC (Analog to Digital Converter). :-) (PLEASE don't take offense at that, Lee!) [ Sorry to suck up all this bandwidth, but I don't have time right now to edit this down to a reasonable length. ] I'm just getting into this audio stuff, so I'm maybe in a better position to recognize what I know now that I didn't a few months ago. (Then again, remember I'm new, too; consider that if you're going to TRUST the following info!) One thing that always frustrates me with these PC sound cards is trying to understand the signal routing. Semi-pro mixers almost always have a signal routing block diagram in the manual. Maybe there's one burried in the documentation of some sound cards, but I haven't seen one yet. Google to the rescue! 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. --> Steve Wahl -- Steve Wahl [EMAIL PROTECTED] If it's not stuck and it's supposed to be, duct tape it. If it's stuck and it's not supposed to be, WD-40 it. If it's not broken--keep me away from it! -- eldavojohn on Slashdot ------------------------------------------------------- This SF.Net email is sponsored by xPML, a groundbreaking scripting language that extends applications into web and mobile media. Attend the live webcast and join the prime developer group breaking into this new coding territory! http://sel.as-us.falkag.net/sel?cmd=lnk&kid=110944&bid=241720&dat=121642 _______________________________________________ Alsa-user mailing list Alsa-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/alsa-user
