MP3 files¶
The crew in the rooms speak their lines from MP3 files on the discs' archives, %s.mp3 of the
names the crew's tables give, such as rm06p.mp3 on the Reliant's disc and ym19p.mp3 on the
Yamato's (The crew). The game hands a file to Miles whole, named
.mp3 (AIL_set_named_sample_file), and Miles's MP3 decoder, MP3DEC.ASI, plays it. Every MP3
file the discs hold is MPEG-2 audio, Layer III, at 22,050 Hz in joint stereo, 64 kilobits a second.
In OpenReliant¶
formats/mp3.zig reads a file's frames, and FFmpeg's MP3 decoder
decodes each (Platform) into the WAVE file a voice plays.
Frames¶
A file is a run of frames. It may start with an ID3v2 tag, whose header of ten bytes holds ID3, a
version of two bytes, a byte of flags, and the length of the tag past the header in four bytes of
seven bits each, the highest first; a footer of ten bytes ends the tag where bit 4 of the flags is
set. It may end with an ID3v1 tag, TAG and 125 bytes more. Neither tag holds audio.
A frame starts with a header of four bytes, read as a big-endian word:
| Bits | Field |
|---|---|
| 31 to 21 | All set: the frame's sync |
| 20 to 19 | The version: 11 MPEG-1, 10 MPEG-2, 00 MPEG-2.5, 01 none |
| 18 to 17 | The layer: 01 Layer III |
| 16 | Clear where a checksum of two bytes follows the header |
| 15 to 12 | The bit rate's index |
| 11 to 10 | The sample rate's index |
| 9 | The padding: a byte more in the frame |
| 7 to 6 | The channel mode: 11 one channel, else two |
The bit rates in kilobits a second, by their index from 1 to 14, are 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256 and 320 for MPEG-1, and 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144 and 160 for the others; index 0 is a free rate and 15 none. The sample rates by their index from 0 to 2 are 44,100, 48,000 and 32,000 Hz for MPEG-1, half those for MPEG-2, and a quarter for MPEG-2.5.
A Layer III frame holds 1,152 samples a channel in MPEG-1, and 576 in the others. Its length in bytes, header and all, is 144 times the bit rate over the sample rate for MPEG-1, and 72 times for the others, plus the padding's byte, rounded down: 208 bytes, or 209, for the crew's lines. OpenReliant passes over bytes that start no frame of Layer III with a set bit rate, as a decoder finds its way back to the frames.