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.
The lump has an 8-byte ASCII header, containing the text "#OPL_II#".
The header is followed by 175 36-byte records of instrument data.
Each record has a 4 byte header of the following form:
|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. This field only applies to the second voice of double-voice instruments; for single voice instruments it has no effect. The offset values are similar to MIDI pitch bends; for example, a value of 82 (hex) in this field is equivalent to a MIDI pitch bend of +256.|
|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:
|0||0x0001||Fixed pitch - Instrument always plays the same note. Most MIDI instruments play a note that is specified in the MIDI "key on" event, 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. This is used even on an OPL2 chip. If this is not set, only the first voice is played. If it is set, the fine tuning field (see above) can be used to adjust the pitch of the second voice relative to the first.|
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; otherwise, only the first set of voice data is played.
The voice data has this format:
|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|
|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|
|14||2||Base note offset. This is used to offset the MIDI note values. Several of the GENMIDI instruments have a base note offset of -12, causing all notes to be offset down by one octave.|
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).
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.
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. Ports with GENMIDI support include Chocolate Doom and ZDoom.