How can I make a digital audio panning controlled by a microcontroller?

Question

I've been trying to pan an audio signal between 2 speakers, in a way that I could program it to create an autopan effect.

It seems an easy thing to do, but I have been struggling a lot with it and some help would be appreciated. Here is what I've tried to do:

1: Using Arduino with digital pot:

I tried to use the MCP4141 (100K) digital pot with an Arduino through SPI communication. It did work, but the resistance is not enough as in its highest resistance the sound is still audible. I even tried to use 2 pots in parallel to double the resistance, still, 200K was not enough and I realised I needed a much higher value. I tried to use a 1000K pot, the AD5241BRZ1M, which is the only one that seems to be available at 1000K (1M) resistance. However, not much information is available about it and I could not make it work with an Arduino. After many tries I had to give up.

Now I'm considering using a Teensy, but it seems to expensive for my project (teensy + audio board) and I don't know if it would work.

Is there any other solution?? Are there other types of potentiometers I'm missing?

Thanks.

Below is my attempt of using it with MCP4141 (with only 1 pot and 1 speaker, as if I make this work I can add the other later).

^{simulate this circuit – Schematic created using CircuitLab}

MCP4141 datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/22059a.pdf

Answer 1

How can I make a digital audio panning controlled by a microcontroller?

Try using this circuit: -

But scale up the resistors to suit your pot. The down side of using a digital pot is the wiper resistance. For the MCP4141 (100K) you used, it has a wiper resistance of up to 300 ohms, so there will be a small amount of leak through on the "muted" channel. I estimate it will be about 0.3/164 kohm if the 8k2 is scaled up to 162 kohm. In log terms that's about -55 dB. Is this enough? Only you can say.

Answer 2

Since you say the audio is already in digital form, do the gain adjustments digitally. For each incoming sample, multiply it by (1-PAN) to the first speaker and PAN to the second. PAN is a variable that goes from 0 to 1 to control how much the sound is panned from the first speaker to the second.

Added

I was trying to show the conceptually simple way to do this. However, as Colin_s pointed out in a comment, linearly blending the voltage won't give you perceived constant amplitude during the pan. You need to keep the sum of the squares of the voltages constant, not the sum of the voltages as in the example above.

Again though, you already have a digital processor, so this is very easy to do. You can either compute the correct fraction into each speaker on the fly, or use a pre-computed lookup table.

Note that doing a constant-power pan is something the analog solutions don't do well, at least not easily. This is better and more easily done digitally.

Answer 3

As an alternative to the digital potentiometer, consider using an analog VCA (voltage controlled amplifier). You can drive the voltage control input with an analog output from your micro.

You can find integrated VCAs designed for the audio industry which have an inherently log control law - this will address difficulties and limitations associated with linear R-DACs. Check out THAT corporation.

This approach will have some associated analog design challenges, but will offload the amount of work to do in the digital domain!

Answer 4

It appears from your drawing that you are using the potentiometer as a simple series resistor. For a full-range volume control, you need to use the pot as a voltage divider, like so:

^{simulate this circuit – Schematic created using CircuitLab}