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Let’s Make a Game!

Step 13: Add Sound Effects

By Darrell Spice, Jr. (adapted by Duane Alan Hahn, a.k.a. Random Terrain)

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Original Blog Entry

A quiet game is playable, but it's more fun with sound. The TIA produces 2 channel sound. The channels are known as channel 0 and channel 1. There are 3 registers for each channel to control the sound produced:


It's common to have a hand full of routines that handle sound effects. For Collect, these routines can be found in the new source code file sfx.asm. To add the file to collect.asm we use the include command:

       include sfx.asm

Besides the file, we also need to allocate 2 RAM variables to keep track of the sound effects:

; indexes for sound effect driver    
SFX_LEFT:       ds 1    ; stored in $B1
SFX_RIGHT:      ds 1    ; stored in $B2

The routines in sfx.asm are:


sfx.asm also includes a couple data tables that SFX_UPDATE uses to update the 6 TIA registers. Yes, only 2 tables even though there are 3 registers per channel to update. AUDCx and AUDVx both only require a nybbles worth of data, so their information is combined into a single byte to save ROM.


The tables are defined like this for Collect:

    .byte 0, 31 ; collide
    .byte 0,  0,  0,  0,  1,  1,  1,  2,  2,  2,  3,  3,  3 ; collect
    .byte 0,  8,  8,  8,  8,  8,  8,  8,  8,  8,  8,  8,  8,  8,  8,  8 ; ping
    .byte 0, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31 ; game over

    .byte 0,$8f ; collide
    .byte 0,$6f,$6f,$6f,$6f,$6f,$6f,$6f,$6f,$6f,$6f,$6f,$6f ; collect
    .byte 0,$41,$42,$43,$44,$45,$46,$47,$48,$49,$4a,$4b,$4c,$4d,$4e,$4f ; ping
sfxPING = *-SFX_CV-1
    .byte 0,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf ; game over

Each .byte line contains the data for a single sound effect. The two tables are used together, so data in the first .byte line of SFX_F goes along with the data in the first .byte line in SFX_CV. The number of values must be the same in each table and each .byte line. The first value in each .byte line should be 0, it denotes end-of-sfx (though if you have a long-duration sound effect you could span it over multiple .byte lines).


Table SFX_CV looks a little complicated because of the extra lines such as sfxPING = *-SFX_CV-1. All those are doing is calculating the value to be used when you trigger a sound effect. You can name your sound effects whatever you want, just make sure it's followed by = *-SFX_CV-1 (also make sure you have a space before and after the equal sign).


Lets look at a single sound effect (that's been slightly changed from above for clarity) to explain how the data is used:

    .byte 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12 ; collect

    .byte 0,$64,$65,$66,$67,$68,$69,$6a,$6b,$6c,$6d,$6e,$6f ; collect

To trigger the sound effect, you'll use this in your code:

ldy #sfxCOLLECT     ; select sound effect
        jsr SFX_TRIGGER     ; and trigger it

By triggering the sound effect, the value of sfxCOLLECT will be stored in one of the RAM variables. The value of sfxCOLLECT points to the 12 in the SFX_F table and the $6f in the SFX_CF table. When SFX_UPDATE is called (via jsr SFX_UPDATE), the 12 goes into AUDFx while the $6f will be split into two parts with the $6 going into AUDCx and the $f going into AUDFx. Lastly the pointer will be updated so it now points to 11 and $6e. After the next update they'll point to 10 and $6d, and so on until they point to 0 which means the end of the sound effect.


The order of the sound effects, as listed in the tables, is used to denote priority. The first sound effect has the lowest priority, so if both channels are busy and you try to trigger sound effect sfxCOLLIDE, nothing will happen. If both are busy and you try to trigger sfxGAMEOVER, the last sound effect, then one of the current sound effects will be aborted so sfxGAMEOVER can be heard.


SFX_UPDATE is called during Overscan:

        sta WSYNC       ; Wait for SYNC (halts CPU until end of scanline)
        lda #2          ; LoaD Accumulator with 2 so D1=1
        sta VBLANK      ; STore Accumulator to VBLANK, D1=1 turns image output off
    ; set the timer so the total number of scanlines ends up being 262
        lda #35
        sta TIM64T
        jsr SFX_UPDATE      ; update sound effects

While SFX_TRIGGER is called from a number of locations throughout the program. One of them is a tick sound that plays during the last 8 ticks of the timer:

        lsr TimerPF+5   ; PF2 right side, reversed bits so shift right
        rol TimerPF+4   ; PF1 right side, normal bits so shift left
        ror TimerPF+3   ; PF0 right side, reversed bits so shift right
        lda TimerPF+3   ; only upper nybble used, so we need to put bit 3 into C
        ror TimerPF+2   ; PF2 left side, reversed bits so shift right
        rol TimerPF+1   ; PF1 left side, normal bits so shift left
        ror TimerPF     ; PF0 left side, reversed bits so shift right
        lda TimerPF+1   ; PF1 from left side
        and #%00011111  ; check the lower 5 bits
        bne NoTickSfx   ; branch if there's a value in the lower 5 bits
        ldy #sfxPING    ; else do a sound effect
        jsr SFX_TRIGGER

In playing the game, I noticed it was harder to locate the box drawn by the ball, because it is the same color as the playfield, so I modified the program so it scores 2 points instead of just 1.

        bit CXP0FB          ; N=player0/playfield, V=player0/ball
        bvc notP0BL         ; if V is off, then player0 did not collide with ball
        ldy #0              ; which score to update
        ldx #4              ; which box was collected
        jsr Collect2ptBox   ; update score and reposition box
        bit CXP1FB          ; N=player1/playfield, V=player1/ball
        bvc notP1BL         ; if V is off, then player1 did not collide with ball
        ldy #1              ; which score to update
        ldx #4              ; which box was collected
        jsr Collect2ptBox   ; update score and reposition box

Collect2ptBox is a new routine that falls into CollectBox:

        lda #2              ; 2 point box
        .byte $2C           ; BIT with absolute addressing, trick that
                            ; causes the lda #1 to be skipped over
        lda #1              ; 1 point per box
        sed                 ; SEt Decimal flag
        clc                 ; CLear Carry bit
        adc Score,y         ; add to player's current score

The ROM and the source are at the bottom of my blog entry.




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Table of Contents for Let’s Make a Game!


Step 1: Generate a Stable Display

Step 2: Timers

Step 3: Score and Timer Display

Step 4: 2 Line Kernel

Step 5: Automate Vertical Delay

Step 6: Spec Change

Step 7: Draw the Playfield

Step 8: Select and Reset Support

Step 9: Game Variations

Step 10: “Random Numbers”

Step 11: Add the Ball Object

Step 12: Add the Missile Objects

Step 13: Add Sound Effects

Step 14: Add Animation





Useful Links

Easy 6502 by Nick Morgan

How to get started writing 6502 assembly language. Includes a JavaScript 6502 assembler and simulator.



Atari Roots by Mark Andrews (Online Book)

This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).



Machine Language For Beginners by Richard Mansfield (Online Book)

This book only assumes a working knowledge of BASIC. It was designed to speak directly to the amateur programmer, the part-time computerist. It should help you make the transition from BASIC to machine language with relative ease.



The Second Book Of Machine Language by Richard Mansfield (Online Book)

This book shows how to put together a large machine language program. All of the fundamentals were covered in Machine Language for Beginners. What remains is to put the rules to use by constructing a working program, to take the theory into the field and show how machine language is done.



6502 Instruction Set with Examples

A useful page from Assembly Language Programming for the Atari Computers.

Continually strives to remain the largest and most complete source for 6502-related information in the world.



Guide to 6502 Assembly Language Programming by Andrew Jacobs

Below are direct links to the most important pages.



Stella Programmer's Guide

HTMLified version.



Nick Bensema's Guide to Cycle Counting on the Atari 2600

Cycle counting is an important aspect of Atari 2600 programming. It makes possible the positioning of sprites, the drawing of six-digit scores, non-mirrored playfield graphics and many other cool TIA tricks that keep every game from looking like Combat.



How to Draw A Playfield by Nick Bensema

Atari 2600 programming is different from any other kind of programming in many ways. Just one of these ways is the flow of the program.



Cart Sizes and Bankswitching Methods by Kevin Horton

The "bankswitching bible." Also check out the Atari 2600 Fun Facts and Information Guide and this post about bankswitching by SeaGtGruff at AtariAge.



Atari 2600 Specifications

Atari 2600 programming specs (HTML version).



Atari 2600 Programming Page (AtariAge)

Links to useful information, tools, source code, and documentation.




Atari 2600 programming site based on Garon's "The Dig," which is now dead.



TIA Color Charts and Tools

Includes interactive color charts, an NTSC/PAL color conversion tool, and Atari 2600 color compatibility tools that can help you quickly find colors that go great together.



The Atari 2600 Music and Sound Page

Adapted information and charts related to Atari 2600 music and sound.



Game Standards and Procedures

A guide and a check list for finished carts.




A multi-platform Atari 2600 VCS emulator. It has a built-in debugger to help you with your works in progress or you can use it to study classic games.




A very good emulator that can also be embedded on your own web site so people can play the games you make online. It's much better than JStella.



batari Basic Commands

If assembly language seems a little too hard, don't worry. You can always try to make Atari 2600 games the faster, easier way with batari Basic.



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