# Salmon Run Stream Sound Demo

A faithful recreation of the stream/water sound effect from Bill Williams' **Salmon Run** (APX, 1982) for the Atari 8-bit.

## Background

This demo recreates the ambient stream sound heard in Salmon Run during idle gameplay. The original sound was reverse-engineered by disassembling the game and analyzing the POKEY register writes.

## How It Works

The stream sound uses:
- **AUDCTL = $00** (standard 64kHz clock)
- **AUDF1 = RANDOM & $1F** (random frequency 0-31)
- **AUDC1 = $A4** (pure tone distortion, volume 4)
- **DLI timing** - frequency updated via Display List Interrupts, not a tight loop

The key insight is that the sound texture comes from the **DLI update timing** - updating once per scanline creates a different character than updating as fast as the CPU can loop.

## Controls

| Key | Function |
|-----|----------|
| **1** | Set volume to 1 (quiet) |
| **2** | Set volume to 2 |
| **3** | Set volume to 3 |
| **4** | Set volume to 4 (original) |
| **+** | Increase DLI skip rate (fewer updates, smoother) |
| **-** | Decrease DLI skip rate (more updates, buzzier) |
| **START** | Exit program |

## DLI Skip Rate

The background color changes to indicate the current skip rate:
- **Skip 1** = Update every DLI (most frequent, buzziest)
- **Skip 2-8** = Update every Nth DLI (smoother texture)

Experiment with the skip rate to find the sound that best matches your memory of the original game.

## Technical Details

### Display List Structure
```
$F0, $F0, $F0    ; 3x 8 blank lines with DLI
$42, <addr>      ; Mode 2 + LMS
$82 x 23         ; 23 mode 2 lines with DLI
$41, <addr>      ; JVB (jump and wait for vblank)
```

Each `$82` byte triggers a DLI, which updates AUDF1 with a new random value.

### Original Salmon Run Parameters
From disassembly analysis:
- AUDF1 updated in DLI at $5000
- AUDC1 = $A4 (from variable $4032)
- AUDC2 = $A0 (silent, from variable $4033)
- AUDCTL = $00

## Files

- `salmon_run_stream_best.s` - Assembly source (ca65 format)
- `salmon_run_stream_best.xex` - Atari executable

## Building

```bash
ca65 -t atari salmon_run_stream_best.s -o salmon_run_stream_best.o
ld65 -C atari.cfg -o salmon_run_stream_best.xex salmon_run_stream_best.o
```

## Credits

- **Original Game:** Bill Williams, Salmon Run (APX, 1982)
- **Reverse Engineering & Demo:** Benj Edwards using Claude Code with Opus 4.5, January 2026