SPU Reverb

Digital reverb unit of the SPU with its registers and its work area.

Summary

The SPU has a digital reverb that uses the sound buffer as its work area. Reverb is switched on or off voice by voice, and it can also be applied to the CD input.¹ Sony provides reverb as a set of templates whose parameters can be adjusted, and only one type of reverb can be active at a time.² Sony never documented the reverb registers. The names on this page are unofficial names from psx-spx.³

psx-spx describes the unit as a stereo Schroeder reverberator. It replaced the echo of the SNES sound chip, and its inputs from the CD-ROM drive and the expansion port can also be sent through it.⁴

Registers

Three registers outside the reverb area also control the reverb:⁵

The reverb configuration area holds 32 registers. The volumes are signed 16-bit values from -8000h to +7FFFh. The addresses and displacements are in 8-byte units like every sound buffer address in the SPU.⁵

The src and dst registers are relative to the current buffer address, and the disp registers are relative to the src registers. ESA sets the start of the work area, while the end is fixed at 7FFFEh. A write to ESA also sets the current buffer address.⁵

A voice with its EON bit set goes to the mixer directly and to the reverb as well, so the dry sound is heard at once and the reverb later.⁵

Reverb Formula

The reverb runs at 22050 Hz. In each cycle it reads the inputs, works the reflections, the comb filter and two all-pass filters, and writes the output:⁶

Lin = vLIN * LeftInput                ; voices with reverb enabled
Rin = vRIN * RightInput
; same side reflection
[mLSAME] = (Lin + [dLSAME]*vWALL - [mLSAME-2])*vIIR + [mLSAME-2]
[mRSAME] = (Rin + [dRSAME]*vWALL - [mRSAME-2])*vIIR + [mRSAME-2]
; different side reflection
[mLDIFF] = (Lin + [dRDIFF]*vWALL - [mLDIFF-2])*vIIR + [mLDIFF-2]
[mRDIFF] = (Rin + [dLDIFF]*vWALL - [mRDIFF-2])*vIIR + [mRDIFF-2]
; early echo (comb filter)
Lout = vCOMB1*[mLCOMB1] + vCOMB2*[mLCOMB2] + vCOMB3*[mLCOMB3] + vCOMB4*[mLCOMB4]
Rout = vCOMB1*[mRCOMB1] + vCOMB2*[mRCOMB2] + vCOMB3*[mRCOMB3] + vCOMB4*[mRCOMB4]
; late reverb, all-pass filter 1
Lout = Lout - vAPF1*[mLAPF1-dAPF1], [mLAPF1] = Lout, Lout = Lout*vAPF1 + [mLAPF1-dAPF1]
Rout = Rout - vAPF1*[mRAPF1-dAPF1], [mRAPF1] = Rout, Rout = Rout*vAPF1 + [mRAPF1-dAPF1]
; late reverb, all-pass filter 2
Lout = Lout - vAPF2*[mLAPF2-dAPF2], [mLAPF2] = Lout, Lout = Lout*vAPF2 + [mLAPF2-dAPF2]
Rout = Rout - vAPF2*[mRAPF2-dAPF2], [mRAPF2] = Rout, Rout = Rout*vAPF2 + [mRAPF2-dAPF2]
; output
LeftOutput  = Lout * EVOLL
RightOutput = Rout * EVOLR
BufferAddress = max(ESA, (BufferAddress + 2) and 7FFFEh)

Every product is divided by 8000h to stay in 16 bits, and every value written to memory is saturated to -8000h to +7FFFh. All addresses are relative to the current buffer address and wrap within ESA to 7FFFEh.⁷ The hardware works the left side in one 44.1 kHz cycle and the right side in the next, which is why the whole unit runs at half rate.⁷ Input to and output from the unit are resampled with a 39-tap FIR filter.⁸

The work area holds signed 16-bit samples. The left and right addresses should place all left samples in one half of the area and all right samples in the other half. As the buffer address advances, each half then overwrites the oldest samples of the other.⁷

vIIR works only from -7FFFh to +7FFFh. When it is -8000h the result written to memory is negated.⁹

Reverb Disable

ATTR bit 7 stops the writes to the work area, but the reads go on. With vAPF2 at zero the output is the samples read at the APF2 address. With vAPF2 set the APF1 stage is used as well, and with both APF volumes set the comb stage too. The reflection stages are not used while reverb is disabled.¹⁰ To silence the reverb output, EVOLL and EVOLR are set to zero or the work area is cleared.⁵

Work Area

The work area runs from ESA to the end of the sound buffer. Its size depends on the reverb mode, and Sony gives the sizes as follows:¹¹

The off mode takes 128 bytes when SpuReserveReverbWorkArea(SPU_ON) has been used and none otherwise.¹² ESA for a mode is therefore (80000h - size) / 8. For the hall mode this is (80000h - ADE0h) / 8 = EA44h.¹³

A change of mode sets the reverb depth to 0, because the size of the work area changes and old data in it would produce noise. The depth must then be set again with SpuSetReverbModeParam() or SpuSetReverbDepth().¹¹ Sony therefore asks that the mode be set well in advance and the depth only when reverb is needed. Otherwise the work area must be cleared with SpuClearReverbWorkArea() first.² The depth runs from -0x8000 to 0x7fff on each side, and a negative depth inverts the phase of the wet sound. Delay time and feedback (0 to 127) apply only to the echo and delay modes.¹²

The area below the reverb offset address can hold waveform data.² A program that uses reverb must set the mode to off and call SpuSetReverb(SPU_OFF) before it exits. Otherwise noise may occur the next time a program runs.¹⁴

Register Settings

psx-spx lists the register values of each template in the order of 1F801DC0h to 1F801DFEh:¹³

dAPF1  dAPF2  vIIR   vCOMB1 vCOMB2  vCOMB3  vCOMB4  vWALL    ; 1F801DC0h
vAPF1  vAPF2  mLSAME mRSAME mLCOMB1 mRCOMB1 mLCOMB2 mRCOMB2  ; 1F801DD0h
dLSAME dRSAME mLDIFF mRDIFF mLCOMB3 mRCOMB3 mLCOMB4 mRCOMB4  ; 1F801DE0h
dLDIFF dRDIFF mLAPF1 mRAPF1 mLAPF2  mRAPF2  vLIN    vRIN     ; 1F801DF0h

The sizes match the work area sizes in Sony's table. The Chaos Echo and Delay templates have the size of Sony's echo and delay modes, and their settings differ only in vWALL.¹³¹¹

Before reverb is turned on, EVOLL, EVOLR, EON and ATTR must be set and the work area cleared, so that no old data is heard. RAM_CTRL must also be set, or the reverb stays off.¹³ The off template uses memory offsets of 0001h rather than 0000h. With zero offsets, clearing the work area does not work reliably.¹³

References