Re: [Jack-Devel] raspberry pi simple in-out (raw)

On 04/24/2013 09:28 PM, Gene Anderson wrote:
> All:
>
> As I RTFM all that jackaudio has, my objective is to sample an A/D
> channel (10 bit) raw input

...and you're providing your own ADC, right?  (The RPi has none that I 
know of.)

 > and route each sample to the audio jack at an
> arbitrarily high rate, like, 50K samples per second; the period is never

...and you're providing your own DAC, right?  The RPi's audio outs are 
PWM outputs coming from the CPU... so the quality won't be so good and 
you'll probably be constrained on the sample rates you have available.

> precisely the same and I do not need fidelity or repeatability.  I'm a C
> coder searching for libraries to get me there from here.  I do not need
> to store any sampled data for later replay.

You'll need to do kernel driver work in order to get an ADC integrated 
into the RPi.

After that, using the alsa libs[1] to manually do the loop-back should 
be fine using a normal read/write API.  However, I find that I can get 
simple programs running faster using tinyalsa.[2]

> Has this wheel been invented?  I reward with beer, as I am creating labs
> for a new Raspberry Pi developers course at UCSD.

For an audio loopback experiment you'll want to add another board with 
an actual audio codec (sound chip) on it.  There might be add-on boards 
available for this.

Might also consider using a BeagleBone instead... I /know/ there are 
audio add-on boards you can get.

FYI, a lot of audio chips that do both capture and playback will also 
have loopback built-in.

-gabriel

[1] http://equalarea.com/paul/alsa-audio.html
[2] https://github.com/tinyalsa/tinyalsa

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