DSP envelopes

The envelope value of each S-DSP voice is driven by either an ADSR envelope, or a gain control. This gives an additional way to automatically shape the volume of the voice over time, aside from its VOL registers.

The ADSR describes a 4 stage envelope:

• Attack begins at key-on, rising from 0 to full over a chosen amount of time.
• Decay lowers from full to a chosen Sustain Level.
• Sustain exponential decay from Sustain Level to 0 (if the Sustain Rate is non-zero).
• Release begins at key-off, lowering to 0 with an fixed decay.

Internally the envelope is an 11-bit value multiplied by by the voice output. The ENVX value that can be read from the DSP contains only the high 7 bits.

See:

• ADSR
• GAIN

ADSR Envelope Timings

See: S-SMP ADSR

Name	Address	Bits	Notes
ADSR (1)	$X5	EDDD AAAA	ADSR enable (E), decay rate (D), attack rate (A).
ADSR (2)	$X6	LLLR RRRR	Sustain level (SL), sustain rate (SR).

At a rate according to the period table the following action is performed, and the envelope is clamped to 0-2047 ($7FF):

• Attack at period[A*2+1]: adds 32, or if A=$F adds 1024 ($400).
• Decay at period[D*2+16]: envelope -= 1, then envelope -= envelope >> 8.
• Sustain at period[SR]: envelope -= 1, then envelope -= envelope >> 8.
• Release: envelope -= 8 every sample.

This table of timings gives the resulting time taken by the above operations:

• Attack is the time from 0 to full.
• Decay is the time from full to sustain level.
• Sustain is the time from full to 0.

ADSR Attack A Time (ms) $0 4100 $1 2600 $2 1500 $3 1000 $4 640 $5 380 $6 260 $7 160 $8 96 $9 64 $A 40 $B 24 $C 16 $D 10 $E 6 $F 0

ADSR Decay D Time (ms) 0 1200 1 740 2 440 3 290 4 180 5 110 6 74 7 37

ADSR Sustain R Time (ms) R Time (ms) $00 Infinite $10 1200 $01 38000 $11 880 $02 28000 $12 740 $03 24000 $13 590 $04 19000 $14 440 $05 14000 $15 370 $06 12000 $16 290 $07 9400 $17 220 $08 7100 $18 180 $09 5900 $19 150 $0A 4700 $1A 110 $0B 3500 $1B 92 $0C 2900 $1C 74 $0D 2400 $1D 55 $0E 1800 $1E 37 $0F 1500 $1F 18

Gain Timings

See: S-SMP GAIN

Name	Address	Bits	Notes
GAIN	$X7	0VVV VVVV 1MMV VVVV	Mode (M), value (V).

At a rate according to the period table the following action is performed, and the envelope is clamped to 0-2047 ($7FF):

• Linear gain adds or subtracts 32.
• Bent gain adds 32 if below 1536 ($600), or 8 if above.
• Exponential is two steps: envelope -= 1, then: envelope -= envelope >> 8.

This table gives times taken between 0 volume and full volume (or the reverse):

GAIN
Decrease Linear		Decrease Exponential		Increase Linear		Increase Bent
V	Time (ms)	V	Time (ms)	V	Time (ms)	V	Time (ms)
$80	Infinite	$A0	Infinite	$C0	Infinite	$E0	Infinite
$81	4100	$A1	38000	$C1	4100	$E1	7200
$82	3100	$A2	28000	$C2	3100	$E2	5400
$83	2600	$A3	24000	$C3	2600	$E3	4600
$84	2000	$A4	19000	$C4	2000	$E4	3500
$85	1500	$A5	14000	$C5	1500	$E5	2600
$86	1300	$A6	12000	$C6	1300	$E6	2300
$87	1000	$A7	9400	$C7	1000	$E7	1800
$88	770	$A8	7100	$C8	770	$E8	1300
$89	640	$A9	5900	$C9	640	$E9	1100
$8A	510	$AA	4700	$CA	510	$EA	900
$8B	380	$AB	3500	$CB	380	$EB	670
$8C	320	$AC	2900	$CC	320	$EC	560
$8D	260	$AD	2400	$CD	260	$ED	450
$8E	190	$AE	1800	$CE	190	$EE	340
$8F	160	$AF	1500	$CF	160	$EF	280
$90	130	$B0	1200	$D0	130	$F0	220
$91	96	$B1	880	$D1	96	$F1	170
$92	80	$B2	740	$D2	80	$F2	140
$93	64	$B3	590	$D3	64	$F3	110
$94	48	$B4	440	$D4	48	$F4	84
$95	40	$B5	370	$D5	40	$F5	70
$96	32	$B6	290	$D6	32	$F6	56
$97	24	$B7	220	$D7	24	$F7	42
$98	20	$B8	180	$D8	20	$F8	35
$99	16	$B9	150	$D9	16	$F9	28
$9A	12	$BA	110	$DA	12	$FA	21
$9B	10	$BB	92	$DB	10	$FB	18
$9C	8	$BC	74	$DC	8	$FC	14
$9D	6	$DD	55	$BD	6	$FD	11
$9E	4	$BE	37	$DE	4	$FE	7
$9F	2	$BF	18	$DF	2	$FF	3.5

Period Table

The rate of DSP envelope events are controlled by a common table of 32 periods. Each entry is how many S-SMP clocks elapse per envelope operation. The table is arranged in groups of 3.

Additionally, each column of periods appears to have a delay offset applied to it, affecting when the operation occurs. If counter is counting down the number of S-SMP clocks elapsed since reset, the envelope operation is applied when the following is true:

• 0 == (counter + offset[rate]) % period[rate]

The counter begins at 0 after reset, and decrements on each S-SMP clock, wrapping to $77FF (30,720) when it would go below 0. (The first clock after reset will wrap.)

DSP Period Table / +0 +1 +2 0 Infinite 2048 1536 3 1280 1024 768 6 640 512 384 9 320 256 192 12 160 128 96 15 80 64 48 18 40 32 24 21 20 16 12 24 10 8 6 27 5 4 3 30 2 1 -

DSP Period Offset / +0 +1 +2 0 Never 0 1040 3 536 0 1040 6 536 0 1040 9 536 0 1040 12 536 0 1040 15 536 0 1040 18 536 0 1040 21 536 0 1040 24 536 0 1040 27 536 0 1040 30 536 0 -

Note that most of the offsets given above are effectively much smaller, given that they are modulo (%) with their associated period, but the modulo-equivalent larger values shown here demonstrate the symmetry between columns.