CSNG (File Format)
The CSNG format contains MIDI data. It appears in Metroid Prime 1 and 2. It is essentially MusyX's GameCube SNG music format, with a custom header.
Format
All offsets are relative to the start of the main header (after the custom header).
Overall, SNG functions similar to a Type-1 (multi-track) MIDI file, with (up to) 64 tracks merging their events into 16 General-MIDI sequencer channels. In addition to the multi-track structure, SNG supports storing MIDI regions as indexed patterns, so songs with repetitive refrains can save on memory by referencing patterns more than once.
Timings are represented in ticks. Unlike MIDI, the tick-resolution is fixed at 384
ticks per beat (e.g. 120 beats-per-minute works out to 384 * 120 / 60 = 768 ticks-per-second).
Custom Header
This 0x14-byte header isn't part of the MusyX format; it appears at the start of the file. After parsing this the rest of the file is copied into a buffer and then passed to the MusyX functions.
| Offset | Size | Description |
|---|---|---|
| 0x0 | 4 | Magic; (always 0x2) |
| 0x4 | 4 | MIDI Setup ID |
| 0x8 | 4 | SongGroup ID |
| 0xC | 4 | AGSC ID |
| 0x10 | 4 | SNG File Length |
| 0x14 | MusyX data starts | |
Header
| Offset | Size | Description |
|---|---|---|
| 0x0 | 4 | Track Index Offset; usually 0x18 for GCN games |
| 0x4 | 4 | Region Data Index Offset |
| 0x8 | 4 | Channel Map Offset |
| 0xC | 4 | Tempo Table Offset; 0x0 if tempo doesn't change |
| 0x10 | 4 | Initial Tempo; (commonly 0x78 or 120 beats per minute) |
| 0x14 | 4 | Unknown |
| 0x18 | End of header | |
Region Info
The track index has offsets relating the first region info for each track.
There is a sequence of at least 2 region info structures populating each track, last one acting as terminator:
| Offset | Size | Description |
|---|---|---|
| 0x0 | 4 | Start Tick; time-point to begin executing region data |
| 0x4 | 4 | Unknown; commonly 0xffff0000 |
| 0x8 | 2 | Region Data Index; used to select region data via the region data index; value is negative on a dummy region info to terminate track regions |
| 0xA | 2 | Unknown |
| 0xC | End of region info | |
Region Data
Here begins a free-form blob of indexed region data. It starts with a variable-length u32 array of SNG offsets for each region, then the region data itself.
Region Data Header
| Offset | Size | Description |
|---|---|---|
| 0x0 | 4 | Region Data Header Size; size of the header after this field (always 0x8) |
| 0x4 | 4 | Pitch Wheel Data Offset; (absolute SNG-offset) 0x0 if no pitch-wheel messages in region |
| 0x8 | 4 | Mod Wheel Data Offset; (absolute SNG-offset) 0x0 if no mod-wheel messages in region |
| 0xC | End of region data | |
Region Commands
After the region data header, the actual playback commands begin. There are only 3 types of commands in SNG: note, control change, and program change.
Delta Time Encoding
Just like MIDI, each command starts with a delta time value telling the sequencer how many ticks to wait after the previous command. Unlike MIDI, Factor5 uses a fixed 16-bit value to store the delta times. In case of a delta time greater than 65535, no-op commands with a 65535 delta time are inserted into the command stream until the target time is attained.
In a theoretical Python streaming API, the delta time can be decoded with no-ops automatically consumed like so:
def DecodeDeltaTime(streamIn):
total = 0
while True:
term = streamIn.ReadU16()
next = streamIn.PeekU16()
if next == 0:
# Automatically consume no-op and continue accumulating time value
total += 0xffff
streamIn.seekAhead(2)
continue
total += term
return total
No-Op Command
When the two bytes following the delta-time are both zero, this is a no-op command.
As mentioned before, no-ops are useful for accumulating delta times greater than 65535.
Note Command
When the two bytes following the delta-time != 0xffff, and the high-bits of both bytes are unset, this is a note command.
Unlike MIDI, which has separate commands for note-on/note-off, SNG attaches a note length value to a note-on command, which is then able to track its own lifetime.
| Offset | Size | Description |
|---|---|---|
| 0x0 | 1 | Note; AND with 0x7f for the value |
| 0x1 | 1 | Velocity; AND with 0x7f for the value |
| 0x2 | 2 | Note Length; count of ticks before note-off issued by sequencer |
| 0x4 | End of note | |
Control Change Command
When the two bytes following the delta-time != 0xffff, and the high-bits of both bytes are set, this is a control change command.
See the standard MIDI control numbers for details.
| Offset | Size | Description |
|---|---|---|
| 0x0 | 1 | Value; AND with 0x7f for the value |
| 0x1 | 1 | Control; AND with 0x7f for the value |
| 0x2 | End of control change | |
Program Change Command
When the two bytes following the delta-time != 0xffff, and the high-bit of the first byte is set, this is a program change command.
See the standard MIDI program numbers for details.
| Offset | Size | Description |
|---|---|---|
| 0x0 | 1 | Program; AND with 0x7f for the value |
| 0x1 | 1 | Padding; always zero |
| 0x2 | End of program change | |
End Of Region
When the two bytes following the delta-time == 0xffff, this region has no more commands.
Continuous Pitch / Modulation Data
If the pitch or mod offsets in a region are non-zero, they point to a buffer of (delta-tick, delta-value) pairs. The delta times are encoded as unsigned 7/15 bit values and the delta values are signed 7/15-bit values. The values are promoted to 15-bit if they exceed their 7-bit range. In this case, the highest bit is set on the first byte in the pair (0x80). The decoder must track the absolute time and value, summing each consecutive update for the current time/values.
Similar to the command stream, when the delta time exceeds 15 bit range (>32767), extra pairs with a zeroed value delta are inserted.
def SignExtend7(value):
return (value & 0x3f) - (value & 0x40)
def SignExtend15(value):
return (value & 0x3fff) - (value & 0x4000)
def DecodeSignedValue(streamIn):
byte0 = streamIn.ReadU8()
if byte0 & 0x80:
byte1 = streamIn.ReadU8()
return SignExtend15(byte1 | ((byte0 & 0x7f) << 8))
else:
return SignExtend7(byte0)
def DecodeUnsignedValue(streamIn):
byte0 = streamIn.ReadU8()
if byte0 & 0x80:
byte1 = streamIn.ReadU8()
return byte1 | ((byte0 & 0x7f) << 8)
else:
return byte0
def DecodeDeltaPair(streamIn):
ret = [0,0]
while ret[1] == 0:
ret[0] += DecodeUnsignedValue(streamIn)
ret[1] = DecodeSignedValue(streamIn)
return ret
Channel Map
This is a simple u8 table mapping 64 SNG tracks to 16 MIDI channels for instrument selection via the SongGroup MIDI-Setup.
Tempo Table
When the SNG has a non-zero tempo table offset, this song features tempo changes. The change events are simple absolute-tick / BPM pairs.
| Offset | Size | Description |
|---|---|---|
| 0x0 | 4 | Tick; absolute time-point to perform tempo change |
| 0x4 | 4 | Tempo; new tempo in BPM |
| 0x2 | End of tempo change | |