Exploring the Mediastreamer2 VoIP engine. Part 4

The material of the article is taken from my zen channel.

Create a signal level meter

In the past article we have clarified the correct termination of programs using the media streamer.

In this article, we will assemble a signal level meter circuit and learn how to read the measurement result from the filter. Let us estimate the measurement accuracy.

The set of filters provided by the media streamer includes 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. Later we will devote an entire article to this filter. But now we will use it as a meter.

As a signal source, we will use a tone generator, the signal from which we will send to the MS_VOLUME filter, to the output of which the 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 sine wave.

In addition to the frequency and amplitude, we need to set the time during which the signal will be generated, it must be sufficient for enough samples to pass through the MS_VOLUME filter for measurement. The MSDtmfGenCustomTone structure is used to pass the settings generator:

struct _MSDtmfGenCustomTone{
    char tone_name[8];     /* Текстовое название сигнала из 8 букв.*/
    int duration;          /* Длительность сигнала в миллисекундах.*/
    int frequencies[2];    /* Пара частот из которых должен состоять выходной сигнал. */
    float amplitude;       /* Амплитуда тонов, 1.0 соответствует уровню 0 дБ от милливатта на нагрузке 600 Ом.*/
    int interval;          /* Пауза в миллисекундах перед началом повторного проигрывания сигнала.*/
    int repeat_count;      /* Количество повторов.*/
};
typedef struct _MSDtmfGenCustomTone MSDtmfGenCustomTone;

To start the generator, we will use its MS_DTMF_GEN_PLAY_CUSTOM method.

Block diagram of signal processing:

The program code that implements this scheme is shown below.

/* Файл 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();
    /* Создаем экземпляры фильтров. */
    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);

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

    /* Соединяем фильтры в цепочку. */
    ms_filter_link(voidsource, 0, dtmfgen, 0);
    ms_filter_link(dtmfgen, 0, volume, 0);
    ms_filter_link(volume, 0, snd_card_write, 0);

    /* Подключаем источник тактов. */
    ms_ticker_attach(ticker,voidsource);

    MSDtmfGenCustomTone dtmf_cfg;

   /* Устанавливаем имя нашего сигнала, помня о том, что в массиве мы должны
    оставить место для нуля, который обозначает конец строки. */
    strncpy(dtmf_cfg.tone_name, "busy", sizeof(dtmf_cfg.tone_name));
    dtmf_cfg.duration=1000;
    dtmf_cfg.frequencies[0]=440; /* Будем генерировать один тон, частоту второго тона установим в 0.*/
    dtmf_cfg.frequencies[1]=0;
    dtmf_cfg.amplitude=1.0; /* Такой амплитуде синуса должен соответствовать результат измерения 0.707.*/
    dtmf_cfg.interval=0.;
    dtmf_cfg.repeat_count=0.;

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

   /* Даем, время половину секунды, чтобы измеритель накопил данные. */
   ms_usleep(500000);

   /* Читаем результат измерения. */
  float level=0;
   ms_filter_call_method(volume, MS_VOLUME_GET_LINEAR,&level);
   printf("Амплитуде синуса %f вольт  соответствует среднеквадратическое значение %f вольт.n", dtmf_cfg.amplitude, level);
}

We compile our example, just like we did before, only using the file name mstest3. Run for execution and get the result:

Амплитуде синуса 1.000000 вольт  соответствует среднеквадратическое значение 0.707733 вольт.

As you can see, the measurement result coincided up to the third decimal place with the theoretical value equal to the square root of two divided in half: sqr(2)/2=0,7071067811865475

The relative deviation of the result from the true value was 0.1%. We made an estimate of the measurement error at the maximum signal level. Accordingly, as the level decreases, the error should increase. I suggest you evaluate it yourself for low signal levels.

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

Source: habr.com