GENMIDI

From Doom Wiki

GENMIDI Is a WAD lump that contains instrument data for the DMX Sound library to use for OPL synthesis. Given an *.OP2 extension, it can be used in MUSPlayer.

The lump is a database of OPL register values, used to program the OPL chip found in the Adlib and Sound blaster sound cards. It contains 175 entries, one for each of the 128 standard General MIDI instruments and 47 percussion effects.

1 Lump format
2 Use in modern ports
3 External links

[edit] Lump format

[edit] Header

The lump has an 8-byte ASCII header, containing the text "#OPL_II#".

[edit] Instrument data

The header is followed by 175 36-byte records of instrument data.

Each record has a 4 byte header of the following form:

Offset	Size (bytes)	Description
0	2	Flags (see below). Little endian.
2	1	Fine tuning - This normally has a value of 128, but can be adjusted to adjust the tuning of the instrument.
3	1	Note number used for fixed pitch instruments (see below)

The flags value is a bitfield containing several flags that control the record. These are:

Bit	Hex	Description
0	0x0001	Fixed pitch - Instrument always plays the same note. Most MIDI instruments play a note that is specified in the MIDI file, but some (most notably percussion instruments) always play the same fixed note.
1	0x0002	Unknown - used in instrument #65 of the Doom GENMIDI lump.
2	0x0004	Double voice - Play two voices simultaneously on an OPL3 chip. If this is not set, only the first voice is played.

Following the header are two 16 byte records of OPL voice data. If the double voice bit is set in the header, both voices will be played simultaneously on an OPL3 chip; otherwise, only the first set of voice data is played.

The voice data has this format:

Offset	Size (bytes)	Description
0	1	Modulator Tremolo / vibrato / sustain / KSR / multi
1	1	Modulator Attack rate / decay rate
2	1	Modulator Sustain level / release rate
3	1	Modulator Waveform select
4	1	Modulator Key scale level
5	1	Modulator Output level
6	1	Feedback
7	1	Carrier Tremolo / vibrato / sustain / KSR / multi
8	1	Carrier Attack rate / decay rate
9	1	Carrier Sustain level / release rate
10	1	Carrier Waveform select
11	1	Carrier Key scale level
12	1	Carrier Output level
13	1	Unused
14	2	"Base note offset" (unused?)

This data is essentially the same as the data found in Soundblaster SBI files; the differences are that the data is ordered differently, and the key scale and output level fields are split into two separate bytes within GENMIDI. In the SBI format, they are combined as bitfields within a single byte. For this reason, and because of the extra unused fields, the GENMIDI instrument data is longer (16 bytes) than SBI data (11 bytes).

[edit] Instrument names

Following the instrument data is 175 32-byte ASCII fields containing the names of the standard General MIDI instruments. It is not clear how these names are used within the sound code, if at all.

[edit] Use in modern ports

Most modern operating systems do not provide the ability to program the OPL chip directly; indeed, many modern computers may not even contain OPL-compatible sound chips. It is therefore common for source ports to simply use the standard MIDI playback API, and the GENMIDI lump is not used. There are a few source ports which do use the GENMIDI lump; particularly as it is necessary when performing OPL emulation.