From: Rhett Aultman Date: Sun, 2 Aug 2020 20:08:22 +0000 (-0400) Subject: APU init and audio synthesis X-Git-Url: https://git.kitaultman.com/?a=commitdiff_plain;h=refs%2Fheads%2Flab_6;p=nes_coding.git APU init and audio synthesis This lovely new lab shows how to set up the APU registers into a state suitable for working with the NES audio subsystem. It also introduces some of the basics about how to play a sound out of the APU, specifically the concepts of period, volume, length counters, step-per-frame counter, etc. Practically, the little guy now makes a rumbling, huffing noise as he runs. And I changed the animation cycle parameters to give him a little hustle. --- diff --git a/src/sprite.asm b/src/sprite.asm index 8c9bd6b..de5627b 100644 --- a/src/sprite.asm +++ b/src/sprite.asm @@ -1,53 +1,31 @@ -; Drawing sprite animation +; Audio Processing Unit (APU) Basics ; -; Since we previously drew a sprite by allocating its 6 tiles in the Object -; Attribute Memory (OAM), this time we get fancy and make it animate a little. -; Specifically, I'm using the 4-cell walk animation for the little person -; that was already laying around in the pattern table. +; This lab introduces some very basic code for initializing the RP2A03 APU +; (used by the NTSC NES) and generating a single sound. Again, this is done +; not with a focus on efficacy or efficiency but instead for readability and +; documentation focused on explaining the roles of various registers. ; -; There are, assurredly, more efficient ways to implement this, but this one -; was coded up with the layout of the pattern table in mind. The pattern -; table layout is really more of the "easy to reason about" sort rather than -; one chosen for tighter code. +; First off, there's the initialization of the registers to clear, known +; states. This is done in the main routine prior to turning on the NMI. +; recall from previous materials that the NMI provides a heartbeat for the +; video processing and display, and as such, it's better to do all +; initialization prior to turning on the NMI. Since we're going to be playing +; only one (pretty ugly) sound, we also go ahead and configure that in the +; main intitialization code, too. ; -; In order to load the sprite, we're now using a subroutine called -; "load_sprite" which knows how to convert the current frame number and an -; initial x and y coordinate and load a 2x3 tile sprite from them. You'll -; notice that this information is basically "passed" using "global variables." -; The 6502 has limited registers making the use of the stack for parameter -; passing pretty tricky. Effective parameter passing is probably a lab on -; its own, so I skipped it. +; Essentially, making a sound through the synthesizers works in a common way +; across all of them: you write in paramters describing the period (the inverse +; of the frequency) and the volume. After that, you write a value into the +; length counter bits which describes how many ticks of the frame counter +; (running at 240 Hz on the 4-frame config used here) the sound should last. ; -; Because the pattern table entries needed to draw the sprite correctly are -; not arranged in a linear way, I created a little array called "anim" which -; describes each frame of the animation in a way that's coherent to the -; algorithm used in load_sprite. It takes 6 pattern table entries to draw -; the character, so advancing to the next step in the animation is a matter -; of adding different multiples of 6 to "anim". +; Muting a channel can be performed through the DMC_LEN_CNT_CTRL_STA register. +; The bottom 5 bits enable/disable the length counters for the synthesizers, +; thus effectively muting them. ; -; To animate, you must change the image as time progresses, meaning you also -; need a timer. This is where the other major point of this lab comes in-- -; the non-maskable interrupt (NMI). The NMI fires every time the PPU starts -; drawing another frame on the TV screen. This gives us a "heartbeat" for our -; code and also serves as a general sense of time. NTSC refreshes 60 fields -; (half-frames) per second, so we know that each trigger of the NMI is 1/60th -; of a second, and we can therefore decide how long to devote to each part -; of an animation. -; -; Also kindly note that we go ahead and do the OAM DMA immediately at the -; beginning of the NMI hander. This is because the NMI signals the beginning -; of something called "vertical blanking" in the NTSC and PAL standards. The -; OAM must be ready to go at end of vertical blanking so the image can be put -; on the screen, so we do it first and make sure we don't delay. After that, -; we set up the next frame of animation. -; -; Finally...this is the first lab where we need RAM in order to track changing -; variables! Astute observers might have noticed that all the memory we've -; declared up to this point has been RAM. Since the variables we need are few, -; I've declared them in the "zero page", which is a RAM region already -; made available by the cc65 default NES configuration. This is all a fancy -; way of saying that the first 256 bytes of addressable space are RAM, and -; because they're the first 256 bytes, the 6502 can fetch them very quickly. +; There's enough in this lab to get your feet wet on the APU, but it doesn't +; cover more advanced topics like sweeps, envelopes, samples, or composing +; music. .define SPRITE_PAGE $0200 @@ -70,6 +48,45 @@ .define BGPALETTE_HI $3F .define BGPALETTE_LO $00 +; APU REGISTERS + +; Pulse/square generator 1 +.define PULSE_1_DUTY_VOL $4000 +.define PULSE_1_SWEEP $4001 +.define PULSE_1_PERIOD_LO $4002 +.define PULSE_1_PERIOD_HI $4003 + +; Pulse/square generator 2 +.define PULSE_2_DUTY_VOL $4004 +.define PULSE_2_SWEEP $4005 +.define PULSE_2_PERIOD_LO $4006 +.define PULSE_2_PERIOD_HI $4007 + +; Triangle wave generator +.define TRI_LINEAR_COUNTER $4008 +.define TRI_UNUSED $4009 +.define TRI_TIMER_LO $400A +.define TRI_COUNT_TIMER_HI $400B + +; Noise generator +.define NOISE_ENV_LEN_VOL $400C +.define NOISE_UNUSED $400D +.define NOISE_MODE_PERIOD $400E +.define NOISE_LENGTH_COUNTER $400F + +; Delta modulation (sample) player +.define DMC_FLAGS_RATE $4010 +.define DMC_VOL_DIRECT_LOAD $4011 +.define DMC_SAMPLE_ADDRESS $4012 +.define DMC_SAMPLE_LENGTH $4013 + +; On write: DMC enable, length counter enable +; On read: DMC interrupt, frame interrupt, length counter status +.define DMC_LEN_CNT_CTRL_STA $4015 + +; Frame counter mode (4 or 5 frame), frame counter interrupt enable/disable +.define FRAME_CNT_MODE_INT $4017 + ; Mandatory iNES header. .segment "HEADER" @@ -178,6 +195,16 @@ zero_oam: lda #$1e sta PPUMASK +; Initialize the APU. + jsr init_apu + +; Since we're going to use only one sound in this demo, let's go ahead and +; configure it. + +; Enable noise tone mode, lowest tone supported (deep rumble) + lda #$8F + sta NOISE_MODE_PERIOD + ; generate NMI lda #$80 sta PPUCTRL @@ -198,7 +225,7 @@ nmi: ; existing sprite. inc frame_count lda frame_count - cmp #15 + cmp #5 bne done lda #0 sta frame_count @@ -210,23 +237,35 @@ nmi: adc #6 ; Each frame is an offset of a multiple of 6 cmp #24 ; After 4 frames, wrap around (4*6 = 24) bne dont_cycle_anim +; Make a little sound on every cycle through the animation + +; Load up 2 cycles of the length counter + lda #$10 + sta NOISE_LENGTH_COUNTER + +; Full volume, run the length counter + lda #$1F + sta NOISE_ENV_LEN_VOL + +; Reset the animation cycle lda #0 + dont_cycle_anim: sta current_frame lda sprite_x clc - adc #4 + adc #2 sta sprite_x dont_reset_x: sta sprite_x + + done: jsr load_sprite rti ; Return from the NMI (NTSC refresh interrupt) - - ; load_sprite consults current_frame to determine the offset into anim ; and then draws the data in that row of anim into a 2x3 rectangle .proc load_sprite @@ -282,6 +321,85 @@ load_loop: rts .endproc +; Initialize the Audio Processing Unit (APU) +; This will load the APU with default values guaranteed +; to not make sound. +; +; It's more effective to do this as a table of bytes and +; load the registers with a loop, but tis will give us a +; chance to discuss what they do. +; +; For more details, see the following pages: +; http://wiki.nesdev.com/w/index.php/APU_basics +; http://wiki.nesdev.com/w/index.php/APU_registers +; http://wiki.nesdev.com/w/index.php/APU_DMC + +.proc init_apu +; Note there are 2 pulse (square) wave units, and both are initialized +; to the same values. + +; Duty cycle 0, length counter halted (1), constant volume (1), vol 0 + lda #$30 + sta PULSE_1_DUTY_VOL + sta PULSE_2_DUTY_VOL + +; Sweep not enabled, no period, sweep upward (1), pitch shift step 0 + lda #$08 + sta PULSE_1_SWEEP + sta PULSE_2_SWEEP + +; Period 0 (no waveform) + lda #$00 + sta PULSE_1_PERIOD_LO + sta PULSE_1_PERIOD_HI + sta PULSE_2_PERIOD_LO + sta PULSE_2_PERIOD_HI + +; Halt tri linear counter, load 0 into counter + lda #$80 + sta TRI_LINEAR_COUNTER + +; Unusued register, 0 out + lda #$00 + sta TRI_UNUSED + +; Set frequency timer (frequency control) to 0 + sta TRI_TIMER_LO + sta TRI_COUNT_TIMER_HI + +; Length counter halt(1), constant volume(1), envelope period 0 + lda #$30 + sta NOISE_ENV_LEN_VOL + +; Not used, zero out + lda #$00 + sta NOISE_UNUSED + +; Set period and length to 0 + sta NOISE_MODE_PERIOD + sta NOISE_LENGTH_COUNTER + +; DMC (sample playback) control +; No IRQ generation, no loop, frequency 0 + sta DMC_FLAGS_RATE + +; No sample loaded directly to DMC + sta DMC_VOL_DIRECT_LOAD + +; No sample indirectly in memory, so 0 address and 0 length + sta DMC_SAMPLE_ADDRESS + sta DMC_SAMPLE_LENGTH + +; Disable DMC length counter, enable all 4 synths + lda #$0F + sta DMC_LEN_CNT_CTRL_STA + +; 4x frame (i.e. 240Hz) counter, disable frame counter interrupt + lda #$40 + sta FRAME_CNT_MODE_INT + +rts +.endproc ; The background colors are, in order: ; $0F: black