[alsa-devel] redirecting audio to another desktop machine

[Hans-Kristian Arntzen]

On 06/25/2010 10:53 AM, Viral Mehta wrote:
> Hi,
>
>    
>> -----Original Message-----
>> From: alsa-devel-bounces at alsa-project.org [mailto:alsa-devel-
>> bounces at alsa-project.org] On Behalf Of Hans-Kristian Arntzen
>> Sent: Monday, June 21, 2010 7:21 PM
>> To: Viral Mehta
>> Cc: alsa-devel at alsa-project.org
>> Subject: Re: [alsa-devel] redirecting audio to another desktop machine
>>
>> On 06/21/2010 02:10 PM, Viral Mehta wrote:
>>      
>>>        
>>>> On 06/21/2010 12:00 PM, alsa-devel-request at alsa-project.org wrote:
>>>>
>>>>          
>>>>> Date: Mon, 21 Jun 2010 14:57:00 +0530
>>>>> From: Viral Mehta<Viral.Mehta at lntinfotech.com>
>>>>> Subject: [alsa-devel] redirecting audio to another desktop machine
>>>>> To: "alsa-devel at alsa-project.org"<alsa-devel at alsa-project.org>
>>>>> Message-ID:
>>>>>
>>>>>            
>> <70376CA23424B34D86F1C7DE6B9973430254343B3E at VSHINMSMBX01.vshodc.lntinfo
>> tech.com>
>>      
>>>>> Content-Type: text/plain; charset="us-ascii"
>>>>>
>>>>> Hi list,
>>>>>
>>>>> I have an embedded device running linux kernel and a minimal
>>>>>            
>> filesystem.
>>      
>>>>> I want to access this device remotely which means I should be able
>>>>>            
>> to connect to the device just like we do Remote Desktop over ethernet.
>>      
>>>>> I am able to do this. I am able to send key-event and framebuffer
>>>>>            
>> data whenever my desktop machine connects to the device.
>>      
>>>>> But, I am not able to send audio data on remote side.
>>>>> I am really not sure which device file (/dev/snd/ ???) I should
>>>>>            
>> read and capture the data and should send across....
>>      
>>>>> Or can I read the playback device ? I think NO.
>>>>> Do I need to create some kind of virtual sound device ?
>>>>>
>>>>> Any pointers/suggestions are appreciated.
>>>>>
>>>>> Thanks,
>>>>> Viral
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>            
>>>> If you want to reroute an ALSA application over the network, you can
>>>>          
>> use
>>      
>>>> output plugins. Afaik, both PulseAudio and RSound provides plugins
>>>>          
>> for
>>      
>>>> this. You generally want to set up a virtual device in .asoundrc or
>>>> similar.
>>>> -- Hans-Kristian Arntzen
>>>>
>>>>          
>>> Hi,
>>> thanks for the reply.
>>>
>>> Let me make sure I understood it in general..... Is it possible to
>>>        
>> use same plugins (or any other) if my embedded device is connected to
>> my  desktop machine via USB (assuming that I have all USB data transfer
>> mechanism/drivers underlying).
>>      
>>> If I setup a virtual device in .asoundrc, audio (or say aplay)
>>>        
>> applications will pass audio data to my virtual device ??
>>      
>>> Do I also need to create any such virtual interface from kernel code
>>>        
>> as well ?
>>      
>>>
>>>        
>> With output plugins for ALSA, you can in general do whatever you want.
>> The plugins will receive audio data, and they can do anything. When you
>> say you want to output to your device via USB cable, I'm not sure if
>> that is supplied ootb. I assumed you wanted to output via ethernet.
>> Pulse might do USB, but I'm not sure.
>>
>> In any case, your output plugin will probably look something like this:
>>
>> pcm.usb_device {
>>      type usb_output # there's no output plugin like this, just a random
>> name to illustrate
>>      device "your device"
>> }
>>
>> Then you could use aplay -Dusb_device.
>>      
> Thanks for the reply.
> I think neither Pulse nor Rsound supports USB.
>
> So, I believe I will have to write something mine.
> First thing as you explained, I would write one plugin.
>
> And then my ALSA user space application will throw data on that virtual
> Device; so I need to have one kernel space driver as well, right ?
> Is it in anycase relevant to Sound Dummy (/dev/null) device ?
>
> And in my Virtual Driver, I may do usb_register and then submit data through
> URBs. Am I on a right path ?
>
> Thanks,
> Viral
>
>
>
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> ______________________________________________________________________
>    
If you can communicate with your USB device using userspace libraries, I 
doubt you'd need to make a driver.
I don't think /dev/null is relevant here. I assume that usb_register 
opens up a USB device, so you can communicate with it, if so, then I 
think you're on the right path. You could probably have a look at the 
OSS plugin in the alsa-plugins package, and use it as a base to 
implement your usb output.