Many production encoders have a fixed list of supported bitrates. I'm familiar with Dolby Encoding Engine, which lists the following in its documentation:
Code:
32, 40, 48, 56, 64, 72, 80, 88, 96, 104,
112, 120, 128, 144, 160, 176, 192, 200,
208, 216, 224, 232, 240, 248, 256, 272,
288, 304, 320, 336, 352, 368, 384, 400,
448, 512, 576, 640, 704, 768, 832, 896,
960, 1008, 1024,
For mono: 32 (minimum), 1024 (maximum)
Default: 64
For stereo: 96 (minimum), 1024 (maximum)
Default: 128
For 5.1: 192 (minimum), 1024 (maximum)
Default: 192
For 7.1: 384, 448, 504, 576, 640, 704,
768, 832, 896, 960, 1008, 1024
Default: 384