ADC bias injection

[dkaukov]

• removes clipping
• makes louder

To inject bias you need hardware modification:

    DRA818 AOUT-+---- GPIO34 (ADC Input)
                |
               [R1] 22kΩ
                |
 GPIO26 (DAC2) -+
                |
               [C1] 0.1µF
                |
               GND

Fixes: #148

[SmittyHalibut]

What voltage does 138 on the DAC generate? Line 191.

V2 hardware (and v1.9 when I get time to lay it out) already biases to 0.59vDC using a simple resistor divider on the regulated 3.3v supply. Vref is 1.1v, so 0.59v is just about the center. Meaning, v2 hardware should work well without any modifications.

[SmittyHalibut]

What DC voltage does this generate?

[dkaukov]

Hi @SmittyHalibut,

The SA818 datasheet lists the audio output amplitude at 0.7v, though in practice it's closer to 2v Vpp (probably datasheet states RMS value).

To accommodate this, I opted for ADC_ATTEN_DB_12, which provides an input range of 150 mV to 2450 mV (Espressif ADC Reference).

Theoretically, the ideal bias point would be at the center of this range:
[(2450 + 150) / 2 = 1.3v]

However, in practice, I’ve found 1.75v to work better for my boards. By "better," I mean that in 12-bit mode, it reads around 2048, which aligns with the center of the ADC range.

[dkaukov]

@SmittyHalibut This is my measurements after bias injection: