CSNG (File Format)

The CSNG format contains MIDI data. It appears in Metroid Prime 1 and 2. It is essentially MusyX's SON music format, with a custom header.

Format

All offsets are relative to the start of the main header (after the custom header).

Timings are represented in ticks, like MIDI. 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	SON File Length
0x14	MusyX data starts

Header

Offset	Size	Description
0x0	4	Version; always 0x18
0x4	4	Track Data Offset; (absolute SON-offset)
0x8	4	Channel Map Offset; (absolute SON-offset)
0xC	4	Tempo Table Offset; (absolute SON-offset) 0x0 if tempo doesn't change
0x10	4	Initial Tempo; (commonly 0x78 or 120 beats per minute)
0x14	4	Unknown
0x18	256	Track Header Offsets; (absolute SON-offsets) 64 elements, 0x0 if track not present
0x118	End of header

Track Header

This is a variable-length table of headers for each track

Offset	Size	Description
0x0	4	Start Tick; time-point to begin executing track data
0x4	4	Unknown; commonly 0xffff0000
0x8	2	Track Data Index
0xA	2	Padding
0xC	4	Start Tick; copy of start tick
0x10	4	Unknown; commonly 0xffff0000
0x14	4	Unknown; commonly 0xffff0000
0x18	End of header

Track Data

Here begins a free-form blob of indexed track data. It starts with a variable-length u32 array of SON offsets for each track, then the track data itself.

Track Data Header

Offset	Size	Description
0x0	4	Track Data Header Size; size of the header after this field (always 0x8)
0x4	4	Pitch Wheel Data Offset; (absolute SON-offset) 0x0 if no pitch-wheel messages on track
0x8	4	Mod Wheel Data Offset; (absolute SON-offset) 0x0 if no mod-wheel messages on track
0xC	End of header

Track Commands

After the track data header, the actual playback commands begin. There are only 2 types of commands in SON: note and control change.

Delta Time RLE

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 custom RLE scheme to adaptively scale the value's precision to reduce the value's size.

The RLE operates on 16-bit words, with the value 0xffff triggering a continuation, then a 'dead' 16-bit word skipped over, then the 0xffff is summed with the following RLE value, looping the decode algorithm.

In Python, decoding works like so:

def DecodeDeltaTimeRLE(in):
    total = 0
    while True:
        term = in.ReadU16()
        if term == 0xffff:
            total += 0xffff
            dummy = in.ReadU16()
            continue
        total += term
        return total

Note Command

When the two bytes following the delta-time != 0xffff, and the high-bit of the first byte is unset, this is a note command.

Unlike MIDI, which has separate commands for note-on/note-off, SON 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-bit of the first byte is 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

End Of Track

When the two bytes following the delta-time == 0xffff, this track has no more commands.

Continuous Pitch / Modulation Data

If the pitch or mod offsets in a track are non-zero, they point to a buffer of RLE-compressed (delta-tick, delta-value) pairs, decoding to signed 16-bit precision. The decoder must track the absolute time and value, summing each consecutive update for the current time/values.

The algorithm for this RLE is different than the delta-time one for commands. It may scale down to a single byte if able.

def DecodeRLE(in):
    # high-bit shift-trigger RLE, limited to 2 bytes
    term = in.ReadU8()
    total = term & 0x7f
    if term & 0x80:
        total = total * 256 + in.ReadU8()
    return total

def DecodeContinuousRLE(in, isValue):
    total = 0
    while True:
        # 1-2 byte RLE accumulated within continuable RLE
        term = DecodeRLE(in)
        if term == 0x8000:
            total += 0xffff
            dummy = in.ReadU8()
            continue
        total += term

        # values are signed deltas;
        # extending into the high-half of 15-bit precision
        if isValue:
            if total >= 0x4000:
                return total - 0xffff
            else:
                return total

        # times are always forward-deltas
        return total

Channel Map

This is a simple u8 table mapping 64 SON tracks to 16 MIDI channels for instrument selection via the SongGroup MIDI-Setup.

Tempo Table

When the SON 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