YMF262 open questions

This page records chip behavior that still needs reliable evidence. These questions do not block the documented OPL3 feature set or require a physical card. A software reference can resolve them.

Four-operator output routing

What is known

Yamaha's 1994 YMF262 datasheet shows each four-operator algorithm's carrier signals combined at one voice output. The application manual lists C0 and C3 as paired channel settings and says only the feedback field in C0–C2 is effective in four-operator mode. The Ad Lib Gold Developer Toolkit says the card's stereo-left and stereo-right bits (D4 and D5 in Cx) must match in Cx+3; a four-operator voice is disabled for a side when the bits do not match. This resolves the driver's stereo rule. Yamaha's datasheet names four output selectors, CHA through CHD, but does not say how both Cx and Cx+3 route a combined voice across all four digital outputs.

The current gameframe-opl3 model sums a four-operator pair into one voice output and applies the first channel's route nibble. The exact four-output behavior remains unconfirmed. Keep the documented AdLib Gold left/right rule separate from the unresolved mapping to YMF262 outputs A–D.

Software reference test

No physical card is needed. Use a YMF262 reference that exposes all four digital outputs, or that can isolate A/B and C/D into separate stereo captures.

1. Enable native mode with 0x105 = 1 and pair channels 0 and 3 with 0x104 = 1.
2. Program the four-operator algorithms in C0 and C3. Use algorithms 0, 1, 2 and 3, setting the upper connection bit in C0 and the lower connection bit in C3.
3. Set the Toolkit's D4/D5 stereo bits in C0 and C3 to match, then mismatch one bit at a time. Confirm the documented left/right mute rule. Keep this result distinct from the four-output routing check.
4. Set the four Yamaha output selectors in C0 and C3 to A, B, C, D, or mismatched pairs. Capture each output. Repeat with the selectors equal, swapped and individually muted.
5. For each algorithm, mute selected operators with total level and leave one carrier audible at a time. This shows whether output selectors follow the first channel, the second channel, or a carrier group.
6. Repeat with a second pair in bank one to check that the result is the same in both banks.

Compare the four output words for each register sequence with FourChannelSample. Keep the reference binary and captures outside GameFrame. Record the reference name, version, settings and sample hashes before changing the implementation.

YAC512 floating DAC transfer

The new gameframe-yac512 crate models the digital boundary. Yamaha's YAC512 datasheet specifies a 16-bit serial input, FSEL high for offset binary and low for two's complement, and falling SMP1/SMP2 edges that latch channels 2/1. The YMF262 stream sends 16-bit offset-binary words LSB first. Yamaha describes a 10-bit DAC mantissa and seven-step analog shifter but does not provide the input-to-floating conversion formula in the available datasheet.

Software-only checks

1. For both FSEL modes, feed patterns 0x0000, 0x0001, 0x7fff, 0x8000 and 0xffff one bit at a time, least-significant bit first. Check the latched raw word and its signed input interpretation.
2. Pulse SMP1 and SMP2 independently. Confirm SMP1 updates channel 2, SMP2 updates channel 1, and the other channel keeps its previous word. Change FSEL between two latches and confirm that the first word retains its captured format.
3. Feed each YMF262 frame's DOAB and DOCD serial levels through separate receivers using the board's documented SMP wiring. Verify the four raw words and channel ordering across multiple frames. The two unspecified clocks per word may use either logic level; the next full 16-bit word replaces them before its latch.
4. To implement the floating transfer, first obtain a reliable formula or a named black-box reference with YAC512 conversion. Compare all 65,536 input codes in both FSEL modes. If no reference or formula is available, keep the analog conversion explicitly open; this does not block the digital receiver or the YMF262 chip.

Physical DAC testing is optional. These checks do not depend on a physical sound card.

Remaining checks

• Compare the YMF262 waveform levels and envelope timing with a named software reference. Physical hardware testing is optional.
• Check whether the selected software reference exposes the four native outputs. If it provides only stereo, isolate each output pair through the reference's documented DAC wiring.
• Keep YAC512 analog conversion and card-specific filtering separate from the digital YMF262 model unless a game-facing requirement needs them.
• The AdLib Gold card's MMA (YMZ263) sample, MIDI and timer paths are implemented in gameframe-ymz263 and connected through gameframe-adlib-gold. Software tests cover generated PCM, FIFO thresholds, timers, MIDI timing and card interrupt status. Compare those paths with the original GSS Wave/MIDI driver under an instrumented NE emulator. Record port transactions, DMA setup, FIFO requests and timer/IRQ cadence, then compare those traces with synthetic sequences. No physical card is required.

Licence: CC0-1.0.