1. Beyond the Beep Sequencer-Controlled Music Synthesis for 8-bit Apple II’s Michael Mahon http://home.comcast.net/~mjmahon

2. The Apple II speaker (and cassette output) Can only toggle “1-bit output” Can’t choose “polarity”

3. Simple Sound Double-frequency timing loop  Squarewaves (beep, etc.) Actual frequency timing loop  Non-50% duty cycles, timbre (Red Book) Infinitely-clipped 1-bit sound  Distorted speech and music (Hex Dump Reader, Audex, etc.)

4. Complex Sound Ultrasonic timing loop: Software DAC Variable duty cycle pulses  Electric Duet (4 duty cycles = 2-bit precision)  Software Automatic Mouth (?)  DAC522 (32 duty cycles = 5-bit precision)

5. DAC Timing Constraints 11kHz = 92 Apple II cycles/sample  1/92cy = 11.092kHz, +67Hz, +0.6% Apple II timing resolution = 1 cycle 4 cycles required to flip speaker  Requires 8 cycles per pulse  4-cycle minimum pulse width  6-cycle practical minimum pulse width Sound amplitude =  width / period  32/46 = 70% of squarewave volume

6. DAC522 Pulse Generators  46 cycles  46 cycles 937 9 99 3888 3977 4066 31 03 02 01 00 Sample value 

7. 32 separate pulse generators  Each generator: Is aligned at a fixed offset on consecutive pages Generates two 6..37-cycle pulses in 92 cycles Counts samples to advance envelope pointer Fetches next sample and sets vector Tests for end of note Vectors to next generator  Computation is distributed between pulse edges DAC522 Two 46-cycle pulse periods, 5-bit precision Pulse frequency of 22kHz is (almost) inaudible

8. Other Issues Starting and stopping without “pops”  Solution: Never stop! “Ramp” waves to start and stop at 0 level Only end a note when sample value is 0..3 Generate continuous 0-level pulses –6 cycles every 46 cycles Generating many/long sounds at 11kB/second  Solution: Direct Digital Synthesis Resample wavetables on-the-fly Use envelope table for dynamics

9. Voice Envelope (Banjo) Wave shape and amplitude change with time Changes diminish with time Wave can usually be compressed significantly

10. Voice Compression Just 10 wave pages can represent a banjo 1 2 3 4 5 6 7 8 9 0.1.2 seconds.3.4.5 Envelope page:

11. Actual Voice Wave Pages Negative envelope has been subtracted Samples have been rounded to 5 bits (32 levels) 1 2 3 1 2 3

12. DAC522 Generator Code Generate pulse Generate pulse 2 Advance envelope Compute next sample Test for end of note 0840: 8D 30 C0 >41 rgen0 sta spkr ; <=== start time: 0 0843: EA >42 nop ; Kill 2 cycles 0844: 8D 30 C0 >43 sta spkr ; <==== stop time: 6 0847: EA >44 nop ; Kill 2 cycles 0848: EA >45 nop ; Kill 2 cycles 0849: EA >46 nop ; Kill 2 cycles 084A: EA >47 nop ; Kill 2 cycles 084B: 08 >48 php ; Kill 7 cycles 084C: 28 >49 plp 084D: E6 ED >50 inc scount ; Compute envelope 084F: F0 01 >51 beq *+3 ; If =, branch to iny 0851: A5 >51 dfb $A5 ; "lda $C8" to skip iny 0852: C8 >51 iny 0853: 18 >52 clc 0854: A5 EC >53 lda frac ; Compute next sample 0856: 65 FE >54 adc freq 0858: 85 EC >55 sta frac 085A: 8D 30 C0 >56 sta spkr ; <=== start time: 46 085D: 8A >57 txa 085E: 8D 30 C0 >58 sta spkr ; <==== stop time: 52 0861: 6D FF 00 >59 adca freq+1 0864: AA >60 tax 0865: B1 06 >61 lda (env),y ; Next sample page 0867: 8D 6C 08 >62 sta :ptr+2 086A: BD 00 00 >63 :ptr ldaa 0*0,x ; Fetch sample. 086D: 8D 7C 08 >64 sta :sw0+2 0870: AD 10 C0 >65 lda AKD 0873: C5 EF >66 cmp key ; Key changed? 0875: F0 03 >67 beq :sw0 ; -No, keep playing. 0877: 4C 77 10 >68 jmp newkey ; -Yes, handle it. 087A: 4C 40 08 >69 :sw0 jmp rgen0 ; Switch to rgen, T = 89

13. A2 DMS II / DMS Drummer II Single-voice multi-timbral real-time wavetable synthesizers Support up to 8 different “instruments” (one at a time)  Voice is represented as a set of single-cycle waveshapes selected by a table representing the envelope of the voice. Voice waves resampled on-the-fly to desired note pitch  Frequency = Integer.fraction sample index increment Wavetable waveforms start and end at zero amplitude to minimize pops “Resting” or idling sound is the zero-level pulse train

14. A2 DMS II Help Screen

15. DMS Drummer II Sequencer

16. DMS Drummer II Performance

17. Controlling the Synthesizers (the end test) A2 DMS II  Keyboard control  “Playback” control  Serial port control DMS Drummer II  Internal sequencer control (“drum machine”)  Keyboard control  Serial port control Serial port control allows external MIDI control

18. Questions and discussion...