[Q] Why is it needed to add an alsa module to v4l audio capture devices?

[Q] Why is it needed to add an alsa module to v4l audio capture devices?

[Ezequiel Garcia]

Hi all,

(I hope this is a genuine question, and I'm not avoiding my own homework here.)

I'm trying to support the audio part of the stk1160 usb bridge
(similar to em28xx).
Currently, the snd-usb-audio module is being loaded when I physically
plug my device,
but I can't seem to capture any sound with vlc.

I still have to research and work a lot to understand the connection
between my device
and alsa, and altough I could write a working module similar to
em28xx-alsa.ko, I still
can't figure out why do I need to write one in the first place.

Why is this module suffficient for gspca microphone devices (gspca, to name one)
and why is a new alsa driver needed for em28xx (or stk1160)?

Hope my question is clear enough,
Thanks in advance,
Ezequiel.

Re: [Q] Why is it needed to add an alsa module to v4l audio capture devices?

[Laurent Pinchart]

Hi Ezequiel,

On Thursday 07 June 2012 15:26:04 Ezequiel Garcia wrote:
> Hi all,
> 
> (I hope this is a genuine question, and I'm not avoiding my own homework
> here.)
> 
> I'm trying to support the audio part of the stk1160 usb bridge (similar to
> em28xx). Currently, the snd-usb-audio module is being loaded when I
> physically plug my device, but I can't seem to capture any sound with vlc.
> 
> I still have to research and work a lot to understand the connection between
> my device and alsa, and altough I could write a working module similar to
> em28xx-alsa.ko, I still can't figure out why do I need to write one in the
> first place.
> 
> Why is this module suffficient for gspca microphone devices (gspca, to name
> one) and why is a new alsa driver needed for em28xx (or stk1160)?

Some devices expose audio functionality by implementing the UAC (USB Audio 
Class) specification. The snd-usb-audio module is all you should need for 
those devices.

Other vendors support audio by using their own proprietary protocol. In those 
cases you will need a device-specific kernel module.

-- 
Regards,

Laurent Pinchart

Re: [Q] Why is it needed to add an alsa module to v4l audio capture devices?

[Mauro Carvalho Chehab]

Em 07-06-2012 15:26, Ezequiel Garcia escreveu:
> Hi all,
> 
> (I hope this is a genuine question, and I'm not avoiding my own homework here.)
> 
> I'm trying to support the audio part of the stk1160 usb bridge
> (similar to em28xx).
> Currently, the snd-usb-audio module is being loaded when I physically
> plug my device,
> but I can't seem to capture any sound with vlc.

Laurent already explained why snd-usb-audio is loaded.
> 
> I still have to research and work a lot to understand the connection
> between my device
> and alsa, and altough I could write a working module similar to
> em28xx-alsa.ko, I still
> can't figure out why do I need to write one in the first place.

On em28xx, some devices use UAC (USB Audio Class), while others use a proprietary
vendor class protocol. The em28xx-alsa module is used only for the devices with a
vendor class.
> 
> Why is this module suffficient for gspca microphone devices (gspca, to name one)
> and why is a new alsa driver needed for em28xx (or stk1160)?

On em28xx, even the ones that use UAC require some configuration, for audio to work.

some setup is required to configure the audio input associated with a video input,
and to enable clock for the audio sampler. Such setup is made when a video input is
selected. You likely need something similar for stk1160.

Regards,
Mauro

Re: [Q] Why is it needed to add an alsa module to v4l audio capture devices?

[Ezequiel Garcia]

Hi Laurent and Mauro,

On Mon, Jun 11, 2012 at 10:30 AM, Mauro Carvalho Chehab
<mchehab@redhat.com> wrote:
> Em 07-06-2012 15:26, Ezequiel Garcia escreveu:
>> Hi all,
>>
>> (I hope this is a genuine question, and I'm not avoiding my own homework here.)
>>
>> I'm trying to support the audio part of the stk1160 usb bridge
>> (similar to em28xx).
>> Currently, the snd-usb-audio module is being loaded when I physically
>> plug my device,
>> but I can't seem to capture any sound with vlc.
>
> Laurent already explained why snd-usb-audio is loaded.
>>
>> I still have to research and work a lot to understand the connection
>> between my device
>> and alsa, and altough I could write a working module similar to
>> em28xx-alsa.ko, I still
>> can't figure out why do I need to write one in the first place.
>
> On em28xx, some devices use UAC (USB Audio Class), while others use a proprietary
> vendor class protocol. The em28xx-alsa module is used only for the devices with a
> vendor class.
>>
>> Why is this module suffficient for gspca microphone devices (gspca, to name one)
>> and why is a new alsa driver needed for em28xx (or stk1160)?
>
> On em28xx, even the ones that use UAC require some configuration, for audio to work.
>
> some setup is required to configure the audio input associated with a video input,
> and to enable clock for the audio sampler. Such setup is made when a video input is
> selected. You likely need something similar for stk1160.
>

It is *much* clear now, thanks to both.

Ezequiel.