Getting volume value from pulseaudio - c

Question

I've written this code by looking at various examples: Python pulseaudio monitor, Pavumeter source, async playback example, and Pacat source.
I have successfully connected to a sink and am able to record it, but my problem is, I'm stuck at getting the volume value out. If I try printing value from the read function, I just get a bunch of random numbers at a second's interval.
Now I'm not asking for someone to finish writing the code for me, I'd just like some tips, help so that I could head towards the right direction. How do I retrieve the volume value?
#include <stdio.h>
#include <string.h>
#include <math.h>
#include <pulse/pulseaudio.h>
static int latency = 20000; // start latency in micro seconds
static int sampleoffs = 0;
static short sampledata[300000];
static pa_buffer_attr bufattr;
static int underflows = 0;
static pa_sample_spec ss;
// This callback gets called when our context changes state. We really only
// care about when it's ready or if it has failed
void pa_state_cb(pa_context *c, void *userdata) {
pa_context_state_t state;
int *pa_ready = userdata;
state = pa_context_get_state(c);
switch (state) {
// These are just here for reference
case PA_CONTEXT_UNCONNECTED:
case PA_CONTEXT_CONNECTING:
case PA_CONTEXT_AUTHORIZING:
case PA_CONTEXT_SETTING_NAME:
default:
break;
case PA_CONTEXT_FAILED:
case PA_CONTEXT_TERMINATED:
*pa_ready = 2;
break;
case PA_CONTEXT_READY:
*pa_ready = 1;
break;
}
}
static void stream_read_cb(pa_stream *s, size_t length, void *userdata) {
const void *data;
pa_stream_peek(s, &data, &length);
data = (const unsigned char*) data;
printf("%u", data);
pa_stream_drop(s);
}
int main(int argc, char *argv[]) {
pa_mainloop *pa_ml;
pa_mainloop_api *pa_mlapi;
pa_context *pa_ctx;
pa_stream *recordstream;
int r;
int pa_ready = 0;
int retval = 0;
unsigned int a;
double amp;
int test = 0;
// Create a mainloop API and connection to the default server
pa_ml = pa_mainloop_new();
pa_mlapi = pa_mainloop_get_api(pa_ml);
pa_ctx = pa_context_new(pa_mlapi, "Simple PA test application");
pa_context_connect(pa_ctx, NULL, 0, NULL);
// This function defines a callback so the server will tell us it's state.
// Our callback will wait for the state to be ready. The callback will
// modify the variable to 1 so we know when we have a connection and it's
// ready.
// If there's an error, the callback will set pa_ready to 2
pa_context_set_state_callback(pa_ctx, pa_state_cb, &pa_ready);
// We can't do anything until PA is ready, so just iterate the mainloop
// and continue
while (pa_ready == 0) {
pa_mainloop_iterate(pa_ml, 1, NULL);
}
if (pa_ready == 2) {
retval = -1;
goto exit;
}
ss.rate = 44100;
ss.channels = 2;
ss.format = PA_SAMPLE_U8;
recordstream = pa_stream_new(pa_ctx, "Record", &ss, NULL);
if (!recordstream) {
printf("pa_stream_new failed\n");
}
pa_stream_set_read_callback(recordstream, stream_read_cb, NULL);
r = pa_stream_connect_record(recordstream, NULL, NULL, PA_STREAM_PEAK_DETECT);
if (r < 0) {
printf("pa_stream_connect_playback failed\n");
retval = -1;
goto exit;
}
// Run the mainloop until pa_mainloop_quit() is called
// (this example never calls it, so the mainloop runs forever).
// printf("%s", "Running Loop");
pa_mainloop_run(pa_ml, NULL);
exit:
// clean up and disconnect
pa_context_disconnect(pa_ctx);
pa_context_unref(pa_ctx);
pa_mainloop_free(pa_ml);
return retval;
}

Answer 1

Looking at the original question from UNIX.StackExchange, it looks like you're trying to create a VU meter. It can be done using an envelope detector. You have to read the input values and then average their rectified value. A simple envelope detector can be done as an exponential moving average filter.
float level = 0; // Init time
const float alpha = COEFFICIENT; // See below
...
// Inside sample loop
float input_signal = fabsf(get_current_sample());
level = level + alpha * (input_signal - level);
Here, alpha is the filter coefficient, which can be calculated as:
const float alpha = 1.0 - expf( (-2.0 * M_PI) / (TC * SAMPLE_RATE) );
Where TC is known as the "time constant" parameter, measured in seconds, which defines how fast you want to "follow" the signal. Setting it too short makes the VU meter very "bumpy" and setting it too long will miss transients in the signal. 10 mS is a good value to start from.