Differences

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--- hardware:audio [2023/12/23 16:15] – clyde
+++ hardware:audio [2025/03/01 20:39] (current) – [Effective sample rate] clyde
@@ Line 22: / Line 22: @@
 The DAC is not directly writable by the ACP. Any write by the ACP to an address at $8000 or above will also be mirrored to a buffer register. This buffer register is copied to the DAC at every IRQ signal.
+==== Audio Control Registers ====
+Write-only registers. $2000 and $2001 trigger on any write, regardless of value.
+^ Addr  ^ Use                                      ^
+| $2000 | Write to reset audio coprocessor.      |
+| $2001 | Write to send NMI to audio coprocessor |
+| $2006 | Audio enable and sample rate           |
 ===== Programming =====
@@ Line 29: / Line 38: @@
 The IRQ timer determines the sample rate by setting how many cycles lapse between DAC updates. The number of cycles is equal to the value of the register times four, since the IRQ timer is clocked at the main CPU frequency while the audio processor clock runs four times as fast.
-The main CPU clock is the same as the NTSC color carrier frequency, approximately 3.5795Mhz or exactly 315/88 MHz. So the audio rate with the most cycles available will be 315000000/(255*88) = 14037 Hz
+The main CPU clock is the same as the NTSC color carrier frequency, approximately 3.5795Mhz or exactly 315/88 MHz. So the audio rate with the most cycles available will be 315000000/(256*88) = 13982 Hz
 Conversely the IRQ rate can go as high as 3.5795MHz but without time for any CPU operations this isn't very useful
+Here's a table with some example rate settings close to some commonly used sample rates
+^ Hex ^ Sample Rate (Hz)   ^ Closest Common Rate  ^  Difference ^ ACP cycles per sample ^
+| **$FF** | **13983** |  N/A (Lowest setting)  | N/A   | 1024 |
+| **$EF** | **15980** | 16000 | 19.9  | 896 |
+| **$D0** | **22233** | 22050 | 183.2 | 644 |
+| **$B7** | **31960** | 32000 | 39.8  | 448 |
+| **$A8** | **44192** | 44100 | 91.9  | 324 |
+| **$A5** | **47099** | 32000 | 900.7 :( | 304 |
+| **$94** | **87306** | 88200 | 894.0 :( | 164 |
+| **$92** | **96744** | 96000 | 744.5 :( | 148 |
+==== Initializing the audio system ====
+For consistent and predictable (i.e. "correct") behavior the program code should be written to the Audio RAM while the ACP is paused. Then before it is resumed, it should be sent a reset signal. If using the lowest audio rate described above, the process is as follows:
+  - Write zero to $2006, the audio rate register
+  - Copy program data to Audio RAM
+  - Write anything to $2000, the audio reset trigger
+  - Write 255 to $2006
+Whichever value you choose for audio rate, the highest bit of $2006 should be set to enable the ACP.
+Example ASM code:
+<code ASM>
+LDA #0
+STA $2006 ; audio rate/enable register
+JSR CopyMyAudioProgramIntoAudioRam
+LDA #0
+STA $2000 ; audio reset
+LDA #255
+STA $2006 ; 255 sets enable bit and lowest sample rate / highest cycles per sample
+</code>