Exploring the VoIP engine Mediastreamer2. Part 5

The article material is taken from my Zen channel.

Tone Signal Detector

Previously article We created a signal level meter. In this lesson, we will learn to detect a tone signal.

In the old days, when not every family had a television and half of them switched channels using pliers, an intriguing news item appeared in the foreign technical press: one of the television manufacturers equipped its devices with a wireless remote control. Details revealed that the remote worked without batteries thanks to an unusual approach—it was mechanical and represented a hybrid of a musical instrument, the metallophone, and a revolver. Inside the revolver's drum were metallic cylinders of varying lengths, and when the hammer struck one of them, the cylinder began to resonate at its own frequency. Presumably using ultrasound. The electronics in the television could hear this signal and, upon identifying its frequency, performed the corresponding action—switching the channel, changing the volume, or turning off the television.

Today, we will attempt to reconstruct this command transmission system using our knowledge from the media streamer.

To simulate the remote, we will use the text from our tone generator example. We will add control of the generator's frequency from keyboard presses and a receiver with a decoder that will output the received commands to the console. After the change, the generator should emit tone signals of 6 frequencies, which we will use to encode commands for increasing/decreasing the volume, changing the channel, and turning the television on/off. The detector is configured using the structure:

struct _MSToneDetectorDef{  
     char tone_name[8];     
     int frequency; 
     int min_duration; 
     float min_amplitude; 
};

typedef struct _MSToneDetectorDef MSToneDetectorDef;

You can pass 10 such structures to the detector, thereby allowing one detector to be set up to detect ten dual-tone signals. However, we will use only six single-tone signals. To pass settings to the detector, the method MS_TONE_DETECTOR_ADD_SCAN is used.

To enable the detector to notify us that a signal with the desired frequency components has been received at its input, we need to provide it with a callback function that it will execute in such a case. This is done using the function ms_filter_set_notify_callback(). It receives a pointer to the filter, a pointer to the callback function, and a pointer to the data that we would like to pass to the callback function (user data).

When the detector is triggered, the callback function will receive user data, a pointer to the detector filter, an event identifier, and a structure describing the event:

/** * Structure carried as argument of the MS_TONE_DETECTOR_EVENT**/
struct _MSToneDetectorEvent{ 
      char tone_name[8];       /* Имя тона которое мы ему назначили при настройке детектора. */
      uint64_t tone_start_time;   /* Время в миллисекундах, когда тон был обнаружен. */
};

typedef struct _MSToneDetectorEvent MSToneDetectorEvent;

The structural scheme for signal processing is shown in the header image.

Now, here’s the program code with comments.

/* Файл mstest4.c Имитатор пульта управления и приемника. */
#include <mediastreamer2/msfilter.h>
#include <mediastreamer2/msticker.h>
#include <mediastreamer2/dtmfgen.h>
#include <mediastreamer2/mssndcard.h>
#include <mediastreamer2/msvolume.h>
#include <mediastreamer2/mstonedetector.h>

/* Подключаем заголовочный файл с функциями управления событиями
 * медиастримера. */
#include <mediastreamer2/mseventqueue.h>

/* Функция обратного вызова, она будет вызвана фильтром, как только он
 * обнаружит совпадение характеристик входного сигнала с заданными. */
static void tone_detected_cb(void *data, MSFilter *f, unsigned int event_id,
        MSToneDetectorEvent *ev)
{
    printf("                      Принята команда: %sn", ev->tone_name);
}

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);
    MSFilter  *detector = ms_filter_new(MS_TONE_DETECTOR_ID);

    /* Очищаем массив находящийся внутри детектора тонов, он описывает
     * особые приметы разыскиваемых сигналов.*/
    ms_filter_call_method(detector, MS_TONE_DETECTOR_CLEAR_SCANS, 0);

    /* Создаем источник тактов - 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, detector, 0);
    ms_filter_link(detector, 0, snd_card_write, 0);

    /* Подключаем к фильтру функцию обратного вызова. */
    ms_filter_set_notify_callback(detector,
            (MSFilterNotifyFunc)tone_detected_cb, NULL);

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

    /* Создаем массив, каждый элемент которого описывает характеристику
     * одного из тонов, который требуется обнаруживать: Текстовое имя
     * данного элемента, частота в герцах, длительность в миллисекундах,
     * минимальный уровень относительно 0,775В. */  
    MSToneDetectorDef  scan[6]=
    {
        {"V+",  440, 100, 0.1}, /* Команда "Увеличить громкость". */
        {"V-",  540, 100, 0.1}, /* Команда "Уменьшить громкость". */
        {"C+",  640, 100, 0.1}, /* Команда "Увеличить номер канала". */
        {"C-",  740, 100, 0.1}, /* Команда "Уменьшить номер канала". */
        {"ON",  840, 100, 0.1}, /* Команда "Включить телевизор". */
        {"OFF", 940, 100, 0.1}  /* Команда "Выключить телевизор". */
    };

    /* Передаем в детектор тонов приметы сигналов. */
    int i;
    for (i = 0; i < 6; i++)
    {
        ms_filter_call_method(detector, MS_TONE_DETECTOR_ADD_SCAN,
                &scan[i]);
    }

    /* Настраиваем структуру, управляющую выходным сигналом генератора.*/
    MSDtmfGenCustomTone dtmf_cfg;
    dtmf_cfg.tone_name[0] = 0;
    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;
    dtmf_cfg.interval = 0.;
    dtmf_cfg.repeat_count = 0.;

    /* Организуем цикл сканирования нажатых клавиш. Ввод нуля завершает
     * цикл и работу программы. */
    char key='9';
    printf("Нажмите клавишу команды, затем ввод.n"
        "Для завершения программы введите 0.n");
    while(key != '0')
    {
        key = getchar();
        if ((key >= 49) && (key <= 54))
        {
                printf("Отправлена команда: %cn", key);
            /* Устанавливаем частоту генератора в соответствии с
             * кодом нажатой клавиши.*/
            dtmf_cfg.frequencies[0] = 440 + 100*(key-49);

            /* Включаем звуковой генератор c обновленной частотой. */
            ms_filter_call_method(dtmfgen, MS_DTMF_GEN_PLAY_CUSTOM,
                    (void*)&dtmf_cfg);
        }
        ms_usleep(20000);
    }
}

We compile and run the program. If everything works correctly, we should see behavior similar to this after running:

$ ./mstest4
ALSA lib conf.c:4738:(snd_config_expand) Unknown parameters 0
ALSA lib control.c:954:(snd_ctl_open_noupdate) Invalid CTL default:0
ortp-warning-Could not attach mixer to card: Invalid argument
ALSA lib conf.c:4738:(snd_config_expand) Unknown parameters 0
ALSA lib pcm.c:2266:(snd_pcm_open_noupdate) Unknown PCM default:0
ALSA lib conf.c:4738:(snd_config_expand) Unknown parameters 0
ALSA lib pcm.c:2266:(snd_pcm_open_noupdate) Unknown PCM default:0
ortp-warning-Strange, sound card Intel 82801AA-ICH does not seem to be capable of anything, retrying with plughw...
Press the command key, then Enter.
To exit the program, enter 0.
ortp-warning-alsa_set_params: periodsize:256 Using 256
ortp-warning-alsa_set_params: period:8 Using 8

Press any keys from "1" to "6", confirming with the "Enter" key, you should get an output like this:

2
Command sent: 2
                      Command received: V-
1
Command sent: 1
                      Command received: V+
3
Command sent: 3
                      Command received: C+
4
Command sent: 4
                      Command received: C-
0
$

We can see that the command tones are successfully sent and the detector detects them.

In the next article, we will turn to transmitting audio signals over Ethernet using the RTP protocol and apply it in our remote control.

Source: habr.com