a52dec-0.7.4.tar.gz

Using the liba52 API -------------------- liba52 provides a low-level interface to decoding audio frames encoded using ATSC standard A/52 aka AC-3. liba52 provides downmixing and dynamic range compression for the following output configurations: A52_CHANNEL : Dual mono. Two independant mono channels. A52_CHANNEL1 : First of the two mono channels above. A52_CHANNEL2 : Second of the two mono channels above. A52_MONO : Mono. A52_STEREO : Stereo. A52_DOLBY : Dolby surround compatible stereo. A52_3F : 3 front channels (left, center, right) A52_2F1R : 2 front, 1 rear surround channel (L, R, S) A52_3F1R : 3 front, 1 rear surround channel (L, C, R, S) A52_2F2R : 2 front, 2 rear surround channels (L, R, LS, RS) A52_3F2R : 3 front, 2 rear surround channels (L, C, R, LS, RS) A52_LFE : Low frequency effects channel. Normally used to connect a subwoofer. Can be combined with any of the above channels. For example: A52_3F2R | A52_LFE -> 3 front, 2 rear, 1 LFE (5.1) Initialization -------------- sample_t * a52_init (uint32_t mm_accel); Initializes the A/52 library. Takes as a parameter the acceptable optimizations which may be used, such as MMX. These are found in the included header file 'mm_accel', along with an autodetection function (mm_accel()). Currently, the only accelleration implemented is MM_ACCEL_MLIB, which uses the 'mlib' library if installed. mlib is only available on some Sun Microsystems platforms. The return value is a pointer to a properly-aligned sample buffer used for output samples. Probing the bitstream --------------------- int a52_syncinfo (uint8_t * buf, int * flags, int * sample_rate, int * bit_rate); The A/52 bitstream is composed of several a52 frames concatenated one after each other. An a52 frame is the smallest independantly decodable unit in the stream. buf must contain at least 7 bytes from the input stream. If these look like the start of a valid a52 frame, a52_syncinfo() returns the size of the coded frame in bytes, and fills flags, sample_rate and bit_rate with the information encoded in the stream. The returned size is guaranteed to be an even number between 128 and 3840. sample_rate will be the sampling frequency in Hz, bit_rate is for the compressed stream and is in bits per second, and flags is a description of the coded channels: the A52_LFE bit is set if there is an LFE channel coded in this stream, and by masking flags with A52_CHANNEL_MASK you will get a value that describes the full-bandwidth channels, as one of the A52_CHANNEL...A52_3F2R flags. If this can not possibly be a valid frame, then the function returns 0. You should then try to re-synchronize with the a52 stream - one way to try this would be to advance buf by one byte until its contents looks like a valid frame, but there might be better application-specific ways to synchronize. It is recommended to call this function for each frame, for several reasons: this function detects errors that the other functions will not double-check, consecutive frames might have different lengths, and it helps you re-sync with the stream if you get de-synchronized. Starting to decode a frame -------------------------- int a52_frame (a52_state_t * state, uint8_t * buf, int * flags, sample_t * level, sample_t bias); This starts the work of decoding the A/52 frame (to be completed using a52_block()). buf should point to the beginning of the complete frame of the full size returned by a52_syncinfo(). You should pass in the flags the speaker configuration that you support, and liba52 will return the speaker configuration it will use for its output, based on what is coded in the stream and what you asked for. For example, if the stream contains 2+2 channels (a52_syncinfo() returned A52_2F2R in the flags), and you have 3+1 speakers (you passed A52_3F1R), then liba52 will choose do downmix to 2+1 speakers, since there is no center channel to send to your center speaker. So in that case the left and right channels will be essentially unmodified by the downmix, and the two surround channels will be added together and sent to your surround speaker. liba52 will return A52_2F1R to indicate this. The good news is that when you downmix to stereo you dont have to worry about this, you will ALWAYS get a stereo output no matter what was coded in the stream. For more complex output configurations you will have to handle the case where liba52 couldnt give you what you wanted because some of the channels were not encoded in the stream though. Level, bias, and A52_ADJUST_LEVEL: Before downmixing, samples are floating point values with a range of [-1,1]. Most types of downmixing will combine channels together, which will potentially result in a larger range for the output samples. liba52 provides two methods of controlling the range of the output, either before or after the downmix stage. If you do not set A52_ADJUST_LEVEL, liba52 will multiply the samples by your level value, so that they fit in the [-level,level] range. Then it will apply the standardized downmix equations, potentially making the samples go out of that interval again. The level parameter is not modified. Setting the A52_ADJUST_LEVEL flag will instruct liba52 to treat your level value as the intended range interval after downmixing. It will then figure out what level to use before the downmix (what you should have passed if you hadnt used the A52_ADJUST_LEVEL flag), and overwrite the level value you gave it with that new level value. The bias represents a value which should be added to the result regardless: output_sample = (input_sample * level) + bias; For example, a bias of 384 and a level of 1 tells liba52 you want samples between 383 and 385 instead of -1 and 1. This is what the sample program a52dec does, as it makes it faster to convert the samples to integer format, using a trick based on the IEEE floating-point format. This function also initialises the state for that frame, which will be reused next when decoding blocks. Dynamic range compression ------------------------- void a52_dynrng (a52_state_t * state, sample_t (* call) (sample_t, void *), void * data); This function is purely optional. If you dont call it, liba52 will provide the default behaviour, which is to apply the full dynamic range compression as specified in the A/52 stream. This basically makes the loud sounds softer, and the soft sounds louder, so you can more easily listen to the stream in a noisy environment without disturbing anyone. If you do call this function and set a NULL callback, this will totally disable the dynamic range compression and provide a playback more adapted to a movie theater or a listening room. If you call this function and specify a callback function, this callback might be called up to once for each block, with two arguments: the compression factor 'c' recommended by the bitstream, and the private data pointer you specified in a52_dynrng(). The callback will then return the amount of compression to actually use - typically pow(c,x) where x is somewhere between 0 and 1. More elaborate compression functions might want to use a different value for 'x' depending wether c>1 or c<1 - or even something more complex if this is what you want. Decoding blocks --------------- int a52_block (a52_state_t * state, sample_t * samples); Every A/52 frame is composed of 6 blocks, each with an output of 256 samples for each channel. The a52_block() function decodes the next block in the frame, and should be called 6 times to decode all of the audio in the frame. After each call, you should extract the audio data from the sample buffer. The sample pointer given should be the one a52_init() returned. After this function returns, the samples buuffer will contain 256 samples for the first channel, followed by 256 samples for the second channel, etc... the channel order is LFE, left, center, right, left