SD-1 sound

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It’s time to get some music out of the synthesizer.

I’ve already implemented and validated SPI communication with SD-1 chip:

SD-1 .0..1..2..3..4..5..6..7..8..9..A..B..C..D..E..F
[00] 01 00 00 01 01 00 00 00 00 F8 00 00 00 00 00 00
[10] 00 00 00 00 00 00 00 40 00 F0 00 3F 00 01 00 00
[20] 00 00 00 10 00 00 00 00 00 00 00 00 00 00 00 00
[30] 00 00 10 00 00 00 00 00 00 00 00 00 00 00 00 00
[40] 00 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[50] 00

Let’s run the example code then.

The signal looks super clean, even on the breadboard:

SD-1 sound on oscilloscope

After connecting both output channels of the chip to left and right speaker, the volume significantly dropped. I was really surprised, until I saw this:

SD-1 channels on oscilloscope

It turns out, it is by design.

SD-1 output schematic

The fix is simple. I’ve connected non-inverted output to both LEFT and RIGHT input channels of the mixer.

Here is an example of mixing PWM and FM together: