Instructor Notes

March 24, 2008

Midterm

• too long
• best course of action?

Final Project Proposals

• what do we need to cover?
  • windows
  • random
  • scoring
  • ???
• other questions?

GameBoy Audio: General

• 4 sound circuits

• 1 & 2 are PWM, 3 plays wave patterns, and 4 is noise

• 2 sound channels (stereo), called SO1 (left) and SO2 (right)
• There is a line called Vin which can accept input from a microphone. From there you can route it back to either SO1 or SO2
• channels 1,2 and 4 have envelope functions, meaning you can have them automatically get louder or softer over time
• channel 1 has a sweep function, meaning you can get it to sweep up in pitch (chirp) or down automatically
• All channels have a duration parameter, or can be left continuously on

• gbhw-snd.inc is an include file to help your code be more self-documenting when coding sound in your apps

• hello-noise.asm demonstrates some basic concepts for producing sound on the GameBoy

General I/O Registers

• AUDVOL
  • Controls volume of SO1 and SO2 (range 0 through 7, 7 is the loudest)
  • Routes Vin to SO1 and/or SO2
• AUDTERM
  • select which of the 4 circuits goes to which of the 2 output channels
• AUDENA
  • turn all sound on or all sound off
  • read only: find out which sound circuits are on

I/O Registers for Circuits 1 and 2

• AUD1LEN, AUD2LEN
  • set duty cycle and sound length
  • Sound length is 6 bits and = (64-x)*(1/256) seconds

  • Duty cycle. From gbspec lines 1220-1223:

                  Wave Duty: 00: 12.5% ( _--------_--------_-------- )
                             01: 25%   ( __-------__-------__------- )
                             10: 50%   ( ____-----____-----____----- ) (default)
                             11: 75%   ( ______---______---______--- )
    

• AUD1ENV, AUD2ENV
  • set an envelope to get either louder or softer
  • set initial volume of envelope (4 bit)
  • set envelope rate (3 bit). One step = x * (1/64) seconds
• AUD1SWEEP
  • circuit 1 only
  • sweep time (3 bit). Higher is slower
  • sweep up or down
  • change of frequency per shift: x(t) = x(t-1) +- x(t-1)/(2^n)
• AUD1LOW, AUD2LOW
  • lower 8 bits of 11 bit number (x) representing a frequency: freq = 131072/(2048-x) Hz
• AUD1HIGH, AUD2HIGH
  • choose if sound is continuous or indeed only the length you specified in AUD1LEN or AUD2LEN
  • upper 3 bits of 11 bit number (x) (see AUD1LOW, AUD2LOW)
  • trigger sound

Sound Circuit 3

overall

• Plays wave pattern ram at $ff30 to $ff3f
• stores in nibbles, reading high nibble first. 32 nibble memory
• play continuous sounds by continually shutting off circuit and reloading memory

  • tutorial for this

I/O Registers

• AUD3ENA
  • turn circuit 3 on or off
• AUD3LEN
  • 8 bits to set sound length (256-x)*.5 seconds
• AUD3LEVEL
  • bits 5 and 6:
    • 00: mute
    • 01: full volume
    • 02: 1/2 volume
    • 03: 1/4 volume
• AUD3LOW, AUD3HIGH
  • load 11 bit number representing equation for freq
  • trigger sound
  • choose whether or not continuous

Sound Circuit 4

overall

• uses binary polynomials to generate pseudo-random numbers (CRC check)
• mess with the parameters and get some interesting effects

I/O registers

• AUD4LEN --- just like AUD1LEN and AUD2LEN
• AUD4ENV --- just like AUD1ENV and AUD2ENV
• AUD4POLY: Parameters for the pseudo random number generator
  • shift clock frequency (4 bits - )
  • selection of polynomial counter step (7 or 15)
  • dividing ratio of freqs (3 bits)
• AUD4GO
  • set whether or not continuous
  • trigger sound