PWM sound

04 Sep 2024 - smokku

« SD-1 sound : RP2350 migration »

After RP2350 migration we have enough GPIO pins to spare some to play simple PWM sounds.

I have wired a simple audio circuit from RP2040 hardware design document and it works just fine.

This posed a question: How to mix it with output coming off SD-1 synthesizer.

• Synthesizer has two output channels.
• We have two PWM channels.

Mix them all together? Mix PWM1 to LEFT channel and PWM2 to RIGHT channel?

Maybe let the user decide? 🤔 So, I started researching digitally controlled audio mixers…

I have found some chips that would fit perfectly, but these are not produced for more than a decade. Fortunately there are some automotive chips still available, which could be repurposed for my goal.

I’ve decided on ROHM BD3461FS mixer. If I merge its 6 output channels into two L/R channels, it would work as we need.

This chip has some unusual pitch, thus putting it on breadboard is also unusual 😜

It is wired as follows:

• INF1/2 - Yamaha SD-1 synthesizer
• INR1/2 - line-in cinch (x2)
• INS1/2 - expansion port (to allow sound card type expansion)
• EXT1 - PWM channel 1
• EXT2 - PWM channel 2
• EXT3 - expansion port
• OUT*1/2 - line-out cinch (x2) - three pairs joined together, so there is a mix of all IN-puts.

Then I tried, whether playing sampled sound over PWM works…