[alsa-devel] The users view of an audio card

[Takashi Iwai]

At Thu, 19 Jul 2007 18:12:54 +0100,
David Webb wrote:
> 
> I've been editing a couple of the unofficial wiki pages and a number of issues 
> have arisen.  My own system has the NVidia/Realtek motherboard chips so I may 
> be in the dark about what is available on other chips.  Some of the questions 
> are background to ensure that what I write later in the wiki is correct.  
> Takashi Iwai suggested I raise the issues here:
> 
> 1.  A large number of users seem to be reporting problems with Nvidia and Via 
> soundcards.  As far as I can tell Nvidia and Via do not produce soundcards 
> but they do produce chipsets with southbridges which can connect to a 
> motherboard audio chip.  The when alsamixer (or Kmix) runs it reports 
> something like:
>   Card:  NVidia CK804
>   Chip:  Realtek AL650 rev 0
> Presumably because the southbridge is the primary PCI device.
> 
> So:
> 
> 1.1  Is the above always correct (so I can add it to the wiki)?

I don't know of problems.  Pointers?

> 1.2   Could alsamixer etc be modified to give a more sensible answer for 
> non-experts.  
> 
> Something like:
>   Card:  Motherboard via NVidia CK804
>   Chip:  Realtek AL650 rev 0
> might work.  I presume a look-up table is used to convert the PCI 2-byte 
> integers to the above names so presumably the extra information needed could 
> be obtained at this level.

Why the word "Motherboard" is so important?  Also, it's not too easy
to know whether it's a motherboard or not.  You'd need a large
database listing each device on the world.  The same controller chip
can be used for a desktop mobo, a laptop, an ATM, or any formfactor.

> 2.  On different motherboards a Realtek 650 may be connected using a NVidia 
> CK804 with the intel8x0 alsa driver or using a VIA VT8233 with the vt82xx 
> driver.
> 
> 2.1  Given that the audio chip registers are the same in both cases why are 
> two different drivers needed?  (Are they?)

Remember that you have both the audio controller and the audio codec
chips.  Intel (and compatible) and VIA have the completely different
controller design.  Thus different drivers are needed.

> 2.2  If it is something to do with the southbridges, where may the manuals 
> that describe the differences be found?  

The manuals don't tell you the difference usually.  It has no
comparison with other chipset.

> 2.3  What would happen if the motherboard manufacturers kept the same 
> southbridges but started using an audio chip with a different set of 
> registers?

Then it would be a pity :)

Usually, when the incompatibility is introduced to the controller, a
different PCI ID is assigned.  So, such a problem shouldn't occur in
practice.

> (2.2a  Where can an AD'97 manual be found? - I could not find one on the intel 
> site)

You mean AC97?  Look for ALSA ftp directory.

> 3.  The Realtek 650 datasheet implies that independently of the mixing in the 
> main input -> line-out pathway, the input to the ADC (i.e. on capture) may 
> include more than one source (i.e. both audio and microphone).  However 
> alsamixer (and kmix) only allows me one input stream.  So:
> 
> 3.1  Is the manual misleading or have I misread it?

You misread.  The driver gives the mixer from analog inputs to the
output as loopback, such as "Mic Playback Volume" and "Line Playback
Volume".  The recording source is exclusive.  Thus it's implemented as
an enum.

> 3.2  Is the manual correct but the driver is based on an earlier intel chip 
> which did not have this facility?

No.

> Related to this the manual seems to show a number of options which I cannot 
> identify in alsamixer.  For example the manual shows two surroundsound volume 
> stages between which there are switched links, one to and two from the main 
> stero path.  Alsamixer shows a single amplifier.  It has one link 'from' the 
> main path but I could not find a link 'to' the main path. 

Hum?  The driver gives stereo volumes.

> 3.3  Is the driver assuming a old layout which fits most of the new audio chip 
> layout but not all?  (This is really a question about what sort of 
> warnings/advice should my write up include).

Be more specific.  What is old and what is new?
The design of AC97 is already pretty old, as its name points :)

> 3.4  At which stage does alsa relate the registers to the descriptions, 
> aux-volume, aux-mute etc?  Again does it assume a layout which fits most 
> aspects of the chips listed?

I don't understand the question.  The aux mixer is always given as
long as the driver detects it.


HTH,

Takashi