Moy Blog

The past weekend I spent some time writing a module for the FreeSwitch project in order to support the G729A codec in it. This codec is patent encumbered, however, my target was not to do this in software, but just create the software interface for FreeSwitch to talk with a PCI card manufactured by Digium that does the transcoding for G729A and G723.1 .

This is the data sheet for the TC400B board. The programming interfaces to access the encoders and decoders is not documented (or at least I could not find any documentation), but it’s enough to have available the source code for the module in Asterisk that uses that very same board.

This board expose its available encoders to the DAHDI / Zaptel core driver, which in turn exposes all transcoders registered by the boards through the Linux filesystem in /dev/dahdi/transcode or /dev/zap/transcode, depending on whether you have Zaptel or DAHDI drivers.

Here I want to explain the few interfaces required to use this board.

The first thing you usually want to do is verify that there is encoders and decoders available. This is done through an ioctl to request the information about the availability of these transcoders.

        struct dahdi_transcoder_info info = {0};

        fd = open("/dev/dahdi/transcode", O_RDWR);
        if (fd < 0) {

                fprintf(stderr, "Failed to open dahdi transcode device\n");
                exit(1);
        }
        for (info.tcnum = 0; !(res = ioctl(fd, DAHDI_TC_GETINFO, &info)); info.tcnum++) {

                printf("Found transcoder '%s', numchannels = %d, dstfmts = %d, srcfmts = %d.\n", info.name,
                                info.numchannels, info.dstfmts, info.srcfmts);
        }

        close(fd);

The driver will let you know the number of encoders, decoders and the source and destiny formats. The formats masks can be found in /usr/include/dahdi/kernel.h. In future versions that may change, I discussed this with one of the DAHDI developers, since I think that this should be in dahdi/user.h and not in kernel.h given that is a user space interface.

Once you know which transcoders are available you can request an encoder or decoder, or both.

        int encoder_fd, decoder_fd;

        struct dahdi_transcoder_formats g729_encoder;
        struct dahdi_transcoder_formats g729_decoder;

        g729_encoder.srcfmt = DAHDI_FORMAT_ULAW;

        g729_encoder.dstfmt = DAHDI_FORMAT_G729A;

        g729_decoder.srcfmt = DAHDI_FORMAT_G729A;

        g729_decoder.dstfmt = DAHDI_FORMAT_ULAW;

        encoder_fd = open("/dev/dahdi/transcode", O_RDWR);

        if (encoder_fd < 0) {
                printf("Failed to open transcode device\n");
                exit(1);
        }
        if (ioctl(fd, DAHDI_TC_ALLOCATE, &g729_encoder)) {

                printf("Failed to allocate encoder\n");
                close(encoder_fd);
                exit(1);
        }

        decoder_fd = open("/dev/dahdi/transcode", O_RDWR);
        if (decoder_fd < 0) {

                printf("Failed to open transcode device\n");
                exit(1);
        }
        if (ioctl(fd, DAHDI_TC_ALLOCATE, &g729_decoder)) {

                printf("Failed to allocate decoder\n");
                close(fd);
                exit(1);
        }

Finally, once allocated, you just have to write chunks of ulaw data and read chunks of decoded g729 data and viceversa. You can choose whether the device will accept ulaw or alaw to g729 or g723 manipulating the srcfmt and dstfmt members of dahdi_transcoder_formats. Just remember that ulaw or alaw encoded data requires 8 times more bytes than g729 encoded data, therefore if you write a frame of 20ms of alaw (160 bytes for a sampling rate of 8000hz) you will read just 20 bytes of g729 encoded data, of course, the same apply when you decode a g729 frame or for g723 (for which alaw and ulaw requires 12 times more space).

The module is available here under the MPL license. Also, the module was just commited to trunk yesterday in the Freeswitch SVN repository. I want to thank to Voiceway for sponsoring the module and Neocenter for providing the hardware to test it.