How to use PCM audio

[kliepatsch]

Hi everyone,

I am new to the X16, have been trying around a little. The next thing that I would like to get to work is the PCM audio. However, this turned out to not work for me. Could anyone help me with that?

I tried the following simple test (check if FIFO Full bit will be set eventually):

Quote

10 VPOKE 0,$9F3B,$8F : REM RESET, MAX VOLUME
20 VPOKE 0,$9F3C,0 : REM SET SAMPLE RATE TO ZERO
30 VPOKE 0,$9F3D,127 : REM WRITE DATA
40 A=VPEEK(0,$9F3B) AND 128 : REM CHECK IF BUFFER IS FULL
50 IF A=0 GOTO 20 : REM CONTINUE UNTIL BUFFER IS FULL
60 END

As recommended in the VERA documentation, I set the sample rate to zero in order for my data to not disappear while I am writing data into the FIFO.
The FIFO Full bit (which I am checking in line 40) seems to be set in line 10 by the reset command, and therefore the program exits immediately. If I leave line 10 away, the FIFO Full bit isn't set and the program gets stuck forever. But what I would expect is that the FIFO Full bit will be set only if the buffer is full.

Playback didn't work, either. (Edit: to check if playback works, I used a different test ... just wanted to mention it)

Thanks in advance.

Edited October 8, 2020 by kliepatsch
clarification

[SlithyMatt]

On closer look, you are mixing up CPU RAM and VRAM. The PCM registers are in the former, so replace your VPOKEs with regular POKEs. The PCM doesn't actually use VRAM at all, despite being built into the VERA. The 4kB buffer is built-in, and not externally addressible.

[SlithyMatt]

You are only writing a single sample value. You need to write a whole lot of values to make a sound, and then set the sample rate based on your sample. Don't confuse the PCM with the PSG, it won't generate a waveform for you.

[kliepatsch]

Thanks for the quick answer. Yes, this simple test wasn't designed to create an audible sound. I just wanted to check if the FIFO Full bit mechanism actually works as I expect.

[SlithyMatt]

P.S. - You can programmatically create a sawtooth wave very easily by just having a single variable that's always incrementing from -128 up to 127 and back again, if you want to hear something.

[kliepatsch]

2 minutes ago, SlithyMatt said:

On closer look, you are mixing up CPU RAM and VRAM. The PCM registers are in the former, so replace your VPOKEs with regular POKEs. The PCM doesn't actually use VRAM at all, despite being built into the VERA. The 4kB buffer is built-in, and not externally addressible.

Thanks! That was it.

[kliepatsch]

Here's a minimal working example for PCM for anyone interested:

Quote

10 POKE $9F3B, $8F : REM RESET, MAX VOLUME
20 POKE $9F3C, 0 : REM ZERO SAMPLE RATE
30 FOR I=1 TO 4096
40 A%=INT(127*SIN(I*0.05))
50 IF A%<0 THEN A%=A%+256 : REM CONVERT TO 8 BIT SIGNED
60 POKE $9F3D, A%
70 NEXT I
80 POKE $9F3C, 128 : REM MAX SAMPLE RATE

It should compute for a while and eventually create a short sine wave beep.

Edited October 8, 2020 by kliepatsch