My board is designed to output audio and control servos, and I've just found that when driving a medium sized servo I get a lot of static on my audio output every time it changes direction, and some static when it's just turning as well. Even with a tiny servo, I can hear static if I'm preventing it from turning.Most hobby servos used brushed DC motors, and brushes create electrical noise both onto the DC and ground wires and admitted as low level RF noise. Filtering of the servo wires and your audio signals may be the only solution. Running the servos on a separate voltage source from the arduino and audio stuff might help also.I thought the servos would have a reverse protection diode built in as well as some capacitors to reduce noise since noise on the data lines could potentially result in crashed airplanes, but if they're in there, they don't seem to be doing a very good job. Servo motor using brushed DC motors will create some level of noise. R/C receivers are designed to work with servos so they have the proper internal noise filtering not to be effected by servo generated noise. I also don't seem to have done a very good job at isolating my audio from this sort of noise in my circuit. I did make sure to have only one entry and exit point for the current into the audio portion of the circuit, and it's on the other side of the board from the servo ports, and the power source is between the two, so I thought that would be sufficient to avoid this issue, but apparently my efforts were either insufficient, or the noise is being radiated from the servo line and the audio line is picking it up or something. But I suspect it's direct transfer on the board itself.Hard to say, a good oscilloscope can be used sometimes to identify the noise which is the first step to identifying the best method of filtering/protecting something from it.You can see the layout of my board here. The servo is attached to port 9. So the current for the servo either flows in from the top of the board from the mosfet on the top right, or under the microcontroller and past the switch, I'm not sure. The audio portion is in the lower right, and there is a fairly straight line for the positive plane to the mosfet, but the ground plane current has to go around the LED pins: http://shawnswift.com/arduino/mighty-layout.png This was as good as I could get it, and as far as noise from other sources goes, like a vibration motor I've got attached via a mosfet and with a protection diode... That works just fine. I don't hear any noise at all from it. And I'm switching it on and off rapidly. It's just these darn servos, which are plugged into the same ports that are the issue.Is it perhaps just the servos are inadequately noise protected? I mean I hardly did anything for my vibration motor other than that diode. Might there be some way to modify a servo cable to reduce the noise? Put a choke on it for example?Vibration motors generally are much lower current then the DC motors used in servos so maybe that would account for your observations?
You are generating audio only?
In that case the chief suspect is the supply rail - you should power audio DAC + amps from a separate powerrail with its own regulator and at lots of decoupling (0.1iF + 10uF + 100uF for instance). Separate voltage regulator is _vital_ if yourDAC's reference voltage is the supply rail (and it needs to be linear, not switch-mode).
The layout looks good and you don't appear to have low-level audio signals that could be picking up RFI.