We study the Mediastreamer2 VoIP engine. Part 4

Article material taken from my zen channel .

Create a signal level meter

In a previous article, we clarified the correct termination of programs using a media streamer.

In this article we will assemble the circuit of a signal level meter and learn how to read the measurement result from a filter. Estimate the accuracy of the measurement.

In the set of filters provided by the media streamer there is a filter, MS_VOLUME, which is able to measure the rms level of the signal passing through it, attenuate the signal and perform a lot of useful and unexpected functions. We’ll cover the whole article with this filter later. But now we will use it as a meter.

As a signal source we will use a tone generator, the signal from which will be sent to the MS_VOLUME filter, to the output of which a sound card is connected.

In this example, we will use the generator filter in a slightly different mode - it will generate a single-tone signal for us, i.e. a signal containing only one sinusoidal oscillation.

, , , MS_VOLUME . MSDtmfGenCustomTone:

struct _MSDtmfGenCustomTone{
    char tone_name[8];     /*     8 .*/
    int duration;          /*    .*/
    int frequencies[2];    /*        . */
    float amplitude;       /*  , 1.0   0      600 .*/
    int interval;          /*        .*/
    int repeat_count;      /*  .*/
};
typedef struct _MSDtmfGenCustomTone MSDtmfGenCustomTone;

, MS_DTMF_GEN_PLAY_CUSTOM.

:

, .

/*  mstest3.c */

#include <mediastreamer2/msfilter.h>
#include <mediastreamer2/msticker.h>
#include <mediastreamer2/dtmfgen.h>
#include <mediastreamer2/mssndcard.h>
#include <mediastreamer2/msvolume.h>

int main()
{
    ms_init();
    /*   . */
    MSFilter  *voidsource=ms_filter_new(MS_VOID_SOURCE_ID);
    MSFilter  *dtmfgen=ms_filter_new(MS_DTMF_GEN_ID);
    MSFilter  *volume=ms_filter_new(MS_VOLUME_ID);
    MSSndCard *card_playback=ms_snd_card_manager_get_default_card(ms_snd_card_manager_get());
    MSFilter  *snd_card_write=ms_snd_card_create_writer(card_playback);

    /*  . */
    MSTicker *ticker=ms_ticker_new();

    /*    . */
    ms_filter_link(voidsource, 0, dtmfgen, 0);
    ms_filter_link(dtmfgen, 0, volume, 0);
    ms_filter_link(volume, 0, snd_card_write, 0);

    /*   . */
    ms_ticker_attach(ticker,voidsource);

    MSDtmfGenCustomTone dtmf_cfg;

   /*    ,   ,     
       ,    . */
    strncpy(dtmf_cfg.tone_name, "busy", sizeof(dtmf_cfg.tone_name));
    dtmf_cfg.duration=1000;
    dtmf_cfg.frequencies[0]=440; /*    ,      0.*/
    dtmf_cfg.frequencies[1]=0;
    dtmf_cfg.amplitude=1.0; /*        0.707.*/
    dtmf_cfg.interval=0.;
    dtmf_cfg.repeat_count=0.;

   /*   . */
   ms_filter_call_method(dtmfgen, MS_DTMF_GEN_PLAY_CUSTOM, (void*)&dtmf_cfg);

   /* ,   ,    . */
   ms_usleep(500000);

   /*   . */
  float level=0;
   ms_filter_call_method(volume, MS_VOLUME_GET_LINEAR,&level);
   printf("  %f      %f .\n", dtmf_cfg.amplitude, level);
}

, , mstest3. :

  1.000000      0.707733 .

, : sqr(2)/2=0,7071067811865475

The relative deviation of the result from the true value was 0.1%. We made an estimation of the measurement error at the maximum signal level. Accordingly, with a decrease in the level, the error should increase. I suggest you independently evaluate it for small signal levels.

In the next article, we will assemble a circuit that detects the presence of a given frequency input at the input of a tone using a tone detector. And also learn how to handle events generated by filters.