Analysis of AAC-LC code stream in dvsdk (2): adts_variable_header ()

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After adts_fixed_header (), this section is defined in the standard:

In FFMPEG, this section is processed with adts_fixed_header.CodeFFmpeg directly discards the three definitions copyright_identification_bit, copyright_identification_start, and adts_buffer_fullness.

This section consumes 28bit:

0 10 1f FC

The two headers consume a total of 56 bits and 7 bytes.

The obtained variable values are:

Here, frame_length specifies the data encoding length of this frame. The value of adts_buffer_fullness is 2047, that is, 0x7ff in hexadecimal notation. According to the standard:

A value of hexadecimal 7ff signals that the bitstream is a variable rate bitstream. In this case, buffer fullness is not applicable.

In FFMPEG, calculate the number of samples using the value of number_of_raw_data_blocks_in_frame:

HDR-> frame_length = size = get_bits (GBC, 13);/* aac_frame_length */
HDR-> number_of_raw_data_blocks_in_frame = RDB = get_bits (GBC, 2 );

HDR-> num_aac_frames = RDB + 1;

HDR-> samples = (RDB + 1) * 1024;

HDR-> bit_rate = size * 8 * HDR-> sample_rate/HDR-> samples;