Using ALSA for Custom Real Time Audio Effects

I haven't played with the Cirrus Logic card on the Pi, but was curious, typically in *nix operating systems, everything to the os looks like a file. So the os doesn't make a distinction between a file on disk, whether it is a text file, binary file, or a device. The normal method in C would be to do an open() or fopen() on the device file, which in the case of audio might be /dev/snd/pcmC0D0c which would be a sound capture device, and use read() to read bytes. Have you tried this? Also, the device file I listed is what appears on my desktop, the actual devices on the Pi with the Cirrus Logic card may be named differently, but the general idea would be the same.

Just trying to get a starting point to enable further discussion.

Mike

as an example something like

int main(){
     FILE *fd;
     int c;
     setupAudiocard(); // do init here, setup sample rate here
     fd = fopen("/dev/snd/pcmC0D0c", "r");
     if(fd == NULL)
          printf("Error couldn't open file"); //error message
    
    while ((c = fgetc(fd)) != EOF)
     audioProcessingFunction(c); // your processing function that does reverb and outputs
}

This is just a general idea, I haven't tried compiling this

Mike

Exactly right although you may need to put in some IOCTL commands to setup the Sampling speed etc in the setupaudiocard section.

Of course setting up the reverb isn't  a total give a way either !

Yeah, like I said, I'm not really familiar with the Cirrus card, so I didn't know the IOCTLs to use, I figured either that or there was something in the card API to set that stuff up, he says he's already got the reverb/output part working, just needed help reading the samples, this seemed like the most unixy way to do it.

Mike

Oh, one other thing to play the sound back you would open

/dev/snd/pcmC0D0p

the playback device

and write the bytes out

Mike

Oh, one other thing to play the sound back you would open

/dev/snd/pcmC0D0p

the playback device

and write the bytes out

Mike

Okay, I tried opening those files in my C code and was successful, but when reading them it just returns ones for every bit (so it looks like ffffffff when I print it out in hex). What would a standard setupAudiocard function look like? I tried looking through this for help but couldn't really wrap my head around it. Would my setup have to be this long and complex? https://github.com/certik/record/blob/master/arecord.c

Will those files automatically always be interacting with the designated input and output ports? So for example if I write something to pcmC0D0p it will automatically output it? And does that mean that the files are constantly changing?

Thanks for the help,

Ryan