🥇Learning the Mediastreamer2 VoIP engine. Part 9

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

duplex intercom

In the past article a duplex intercom was announced, and in this one we will do it.

The scheme is shown in the title figure. The bottom filter chain forms the transmission path that starts with the sound card. It gives samples of the signal from the microphone. By default, this happens at a tempo of 8000 samples per second. The data bit depth used by the media streamer's sound filters is 16 bits (this is not important, if you wish, you can write filters that will work with a larger bit depth). The data are grouped into blocks of 160 readings. Thus, each block has a size of 320 bytes. Next, we feed the data to the input of the generator, which is "transparent" to the data in the off state. I added it in case you get tired of talking into the microphone during debugging - you can use the generator to "shoot through" the path with a tone signal.

After the generator, the signal goes to the encoder, which converts our 16-bit samples according to the µ-law (G.711 standard) into eight-bit ones. At the output of the encoder, we already have a data block half the size. In general, we can transfer data without compression if we do not need to save traffic. But here it is useful to use an encoder, since Wireshark can only play audio from an RTP stream when it is compressed according to the µ-law or a-law.

After the encoder, the lighter data blocks go to the rtpsend filter, which will put them in an RTP packet, set the necessary flags and give them to the media streamer for transmission over the network in the form of a UDP packet.

The upper chain of filters forms the receiving path, the RTP packets received by the media streamer from the network enter the rtprecv filter, at the output of which they already appear in the form of data blocks, each of which corresponds to one received packet. The block contains only payload data, in the last article in the illustration they were shown in green.

Next, the blocks go to the decoder filter, which converts the one-byte samples in them into linear, 16-bit ones. Which can already be processed by media streamer filters. In our case, we simply give them to the sound card to be played on the speakers of your headset.

Now let's move on to the software implementation. To do this, we will merge the receiver and transmitter files that we delimited before. Before, we used fixed port and address settings, but now we need the program to be able to use the settings that we specify at startup. To do this, we would add the functionality of processing command line arguments. After that, we can set the IP address and port of the intercom with which we want to communicate.

First, let's add a structure to the program that will store its settings:

struct _app_vars
{
  int  local_port;              /* Локальный порт. */
  int  remote_port;             /* Порт переговорного устройства на удаленном компьютере. */
  char remote_addr[128];        /* IP-адрес удаленного компьютера. */
  MSDtmfGenCustomTone dtmf_cfg; /* Настройки тестового сигнала генератора. */
};

typedef struct _app_vars app_vars;

The program will declare a structure of this type with the name vars.
Then we add a function to parse command line arguments:

/* Функция преобразования аргументов командной строки в
* настройки программы. */
void  scan_args(int argc, char *argv[], app_vars *v)
{
    char i;
    for (i=0; i<argc; i++)
    {
        if (!strcmp(argv[i], "--help"))
        {
            char *p=argv[0]; p=p + 2;
            printf("  %s walkie talkienn", p);
            printf("--help      List of options.n");
            printf("--version   Version of application.n");
            printf("--addr      Remote abonent IP address string.n");
            printf("--port      Remote abonent port number.n");
            printf("--lport     Local port number.n");
            printf("--gen       Generator frequency.n");
            exit(0);
        }

        if (!strcmp(argv[i], "--version"))
        {
            printf("0.1n");
            exit(0);
        }

        if (!strcmp(argv[i], "--addr"))
        {
            strncpy(v->remote_addr, argv[i+1], 16);
            v->remote_addr[16]=0;
            printf("remote addr: %sn", v->remote_addr);
        }

        if (!strcmp(argv[i], "--port"))
        {
            v->remote_port=atoi(argv[i+1]);
            printf("remote port: %in", v->remote_port);
        }

        if (!strcmp(argv[i], "--lport"))
        {
            v->local_port=atoi(argv[i+1]);
            printf("local port : %in", v->local_port);
        }

        if (!strcmp(argv[i], "--gen"))
        {
            v -> dtmf_cfg.frequencies[0] = atoi(argv[i+1]);
                printf("gen freq : %in", v -> dtmf_cfg.frequencies[0]);
        }
    }
}

As a result of parsing, the command line arguments will be placed in the fields of the vars structure. The main function of the application will collect the transmitting and receiving paths from the filters, after connecting the ticker, control will be transferred to an infinite loop which, if the generator frequency was set to non-zero, will restart the test generator so that it works without stopping.

These restarts will be needed by the generator due to its design features, for some reason it cannot generate a signal lasting more than 16 seconds. At the same time, it should be noted that its duration is specified by a 32-bit number.

The whole program will look like this:

/* Файл mstest8.c Имитатор переговорного устройства. */

#include <mediastreamer2/mssndcard.h>
#include <mediastreamer2/dtmfgen.h>
#include <mediastreamer2/msrtp.h>

/* Подключаем файл общих функций. */
#include "mstest_common.c"

/*----------------------------------------------------------*/
struct _app_vars
{
    int  local_port;              /* Локальный порт. */
    int  remote_port;             /* Порт переговорного устройства на удаленном компьютере. */
    char remote_addr[128];        /* IP-адрес удаленного компьютера. */
    MSDtmfGenCustomTone dtmf_cfg; /* Настройки тестового сигнала генератора. */
};

typedef struct _app_vars app_vars;

/*----------------------------------------------------------*/
/* Создаем дуплексную RTP-сессию. */
RtpSession* create_duplex_rtp_session(app_vars v)
{
    RtpSession *session = create_rtpsession (v.local_port, v.local_port + 1, FALSE, RTP_SESSION_SENDRECV);
    rtp_session_set_remote_addr_and_port(session, v.remote_addr, v.remote_port, v.remote_port + 1);
    rtp_session_set_send_payload_type(session, PCMU);
    return session;
}

/*----------------------------------------------------------*/
/* Функция преобразования аргументов командной строки в
* настройки программы. */
void  scan_args(int argc, char *argv[], app_vars *v)
{
    char i;
    for (i=0; i<argc; i++)
    {
        if (!strcmp(argv[i], "--help"))
        {
            char *p=argv[0]; p=p + 2;
            printf("  %s walkie talkienn", p);
            printf("--help      List of options.n");
            printf("--version   Version of application.n");
            printf("--addr      Remote abonent IP address string.n");
            printf("--port      Remote abonent port number.n");
            printf("--lport     Local port number.n");
            printf("--gen       Generator frequency.n");
            exit(0);
        }

        if (!strcmp(argv[i], "--version"))
        {
            printf("0.1n");
            exit(0);
        }

        if (!strcmp(argv[i], "--addr"))
        {
            strncpy(v->remote_addr, argv[i+1], 16);
            v->remote_addr[16]=0;
            printf("remote addr: %sn", v->remote_addr);
        }

        if (!strcmp(argv[i], "--port"))
        {
            v->remote_port=atoi(argv[i+1]);
            printf("remote port: %in", v->remote_port);
        }

        if (!strcmp(argv[i], "--lport"))
        {
            v->local_port=atoi(argv[i+1]);
            printf("local port : %in", v->local_port);
        }

        if (!strcmp(argv[i], "--gen"))
        {
            v -> dtmf_cfg.frequencies[0] = atoi(argv[i+1]);
                printf("gen freq : %in", v -> dtmf_cfg.frequencies[0]);
        }
    }
}

/*----------------------------------------------------------*/
int main(int argc, char *argv[])
{
    /* Устанавливаем настройки по умолчанию. */
    app_vars vars={5004, 7010, "127.0.0.1", {0}};

    /* Устанавливаем настройки настройки программы в
     * соответствии с аргументами командной строки. */
    scan_args(argc, argv, &vars);

    ms_init();

    /* Создаем экземпляры фильтров передающего тракта. */
    MSSndCard *snd_card =
        ms_snd_card_manager_get_default_card(ms_snd_card_manager_get());
    MSFilter *snd_card_read = ms_snd_card_create_reader(snd_card);
    MSFilter *dtmfgen = ms_filter_new(MS_DTMF_GEN_ID);
    MSFilter *rtpsend = ms_filter_new(MS_RTP_SEND_ID);

    /* Создаем фильтр кодера. */
    MSFilter *encoder = ms_filter_create_encoder("PCMU");

    /* Регистрируем типы нагрузки. */
    register_payloads();

    /* Создаем дуплексную RTP-сессию. */
    RtpSession* rtp_session= create_duplex_rtp_session(vars);
    ms_filter_call_method(rtpsend, MS_RTP_SEND_SET_SESSION, rtp_session);

    /* Соединяем фильтры передатчика. */
    ms_filter_link(snd_card_read, 0, dtmfgen, 0);
    ms_filter_link(dtmfgen, 0, encoder, 0);
    ms_filter_link(encoder, 0, rtpsend, 0);

    /* Создаем фильтры приемного тракта. */
    MSFilter *rtprecv = ms_filter_new(MS_RTP_RECV_ID);
    ms_filter_call_method(rtprecv, MS_RTP_RECV_SET_SESSION, rtp_session);

    /* Создаем фильтр декодера, */
    MSFilter *decoder=ms_filter_create_decoder("PCMU");

    /* Создаем фильтр звуковой карты. */
    MSFilter *snd_card_write = ms_snd_card_create_writer(snd_card);

    /* Соединяем фильтры приёмного тракта. */
    ms_filter_link(rtprecv, 0, decoder, 0);
    ms_filter_link(decoder, 0,  snd_card_write, 0);

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

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

    /* Если настройка частоты генератора отлична от нуля, то запускаем генератор. */   
    if (vars.dtmf_cfg.frequencies[0])
    {
        /* Настраиваем структуру, управляющую выходным сигналом генератора. */
        vars.dtmf_cfg.duration = 10000;
        vars.dtmf_cfg.amplitude = 1.0;
    }

    /* Организуем цикл перезапуска генератора. */
    while(TRUE)
    {
        if(vars.dtmf_cfg.frequencies[0])
        {
            /* Включаем звуковой генератор. */
            ms_filter_call_method(dtmfgen, MS_DTMF_GEN_PLAY_CUSTOM,
                    (void*)&vars.dtmf_cfg);
        }
        /* Укладываем тред в спячку на 20мс, чтобы другие треды
         * приложения получили время на работу. */
        ms_usleep(20000);
    }
}

Compiling. Further, the program can be run on two computers. Or one, as I will do now. We start TShark with the following arguments:

$ sudo tshark -i lo -f "udp dst port 7010" -P -V -O RTP -o rtp.heuristic_rtp:TRUE -x

If the launch field in the console only shows a message about the beginning of the capture, then this is a good sign - it means that our port is most likely not occupied by other programs. In another terminal, we launch an instance of the program that will imitate a "remote" intercom by specifying this port number for it:

$ ./mstest8 --port 9010 --lport 7010

As you can see from the text of the program, the default IP address is 127.0.0.1 (local loopback).

In another terminal, we launch the second instance of the program, which imitates a local device. We use an additional argument that enables the built-in test generator:

$ ./mstest8  --port 7010 --lport 9010 --gen 440

At this moment, in the console with TShark, packets transmitted towards the "remote" device should begin to flicker, and a continuous tone signal will be heard from the computer speaker.

If everything happened as written, then we restart the second instance of the program, but without the key and argument "-gen 440". The role of the generator will now be performed by you. After that, you can make some noise into the microphone, you should hear the corresponding sound in the speaker or headphones. There may even be acoustic self-excitation, turn down the volume of the speaker and the effect disappears.

If you ran it on two computers and did not get confused in IP addresses, then you will find the same result - two-way digital quality voice communication.

In the next article, we will learn how to write our own filters - plugins, thanks to this skill you will be able to apply the media streamer not only for sound and video, but also in some other specific area.

Source: habr.com

Exploring the Mediastreamer2 VoIP engine. Part 9

duplex intercom

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