DSP envelopes

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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 is the volume level reached after the decay concludes.
  • Release begins at key-off, lowering to 0 with an exponential 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 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 SSSR RRRR Sustain level (S), release rate (R).

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.
  • Release at period[R]: envelope -= 1, then envelope -= envelope >> 8.

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.
  • Release 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 Release
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[1]. (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.

References

  1. apudsp_jwdonal.txt - Anomie's S-DSP document.