Bit Rate Reduction

Bit Rate Reduction, or BRR, also called Bit Rate Reduced, is a name given toan audio compression method used on the SPC700 sound coprocessor used in the SNES, as well as the audio processors of the Philips CD-i and the PlayStation. The method is a form of ADPCM. Bit Rate Reduction, or BRR, also called Bit Rate Reduced, is a name given toan audio compression method used on the SPC700 sound coprocessor used in the SNES, as well as the audio processors of the Philips CD-i and the PlayStation. The method is a form of ADPCM. BRR compresses each consecutive sequence of sixteen 16-bit PCM samples into a block of 9 bytes. From most to least significant, the first byte of each block consists of four bits indicating the range of the block (see below) which controls the size of steps between the 16 possible values such that minute changes can be recorded if the 16 values are closer together but minute changes are lost if the 16 values are far apart, two bits indicating the filter (see below), and two bits of control information for the SPC700. The remaining eight bytes consist of 16 signed 4-bit nibbles which correspond to the 16 samples, packed in a big-endian manner. As 32 bytes of input become 9 bytes of output, the BRR algorithm yields a 3.56:1 compression ratio. A nibble n in a block with filter f {displaystyle f} and range r {displaystyle r} should be decoded into a PCM sample s t {displaystyle s_{t}} using the following second-order linear prediction equation: Here, s t − 1 {displaystyle s_{t-1}} and s t − 2 {displaystyle s_{t-2}} are the last-output and next-to-last-output PCM samples, respectively. The filter type f {displaystyle f} is translated into IIR prediction coefficients k {displaystyle k} using the following table: These calculations are all done in signed 16.16 fixed-point arithmetic.