audio controller noise with serial communication

Hi, I’ve built an audio switcher/controller using an Arduino Mega.

It uses the serial ports to communicate with the devices to know which to switch to (turn audio output on for one, off for all others), and then a simple mixer circuit with 1watt amp.

The problem is the serial communication is “heard” on the audio.

Basically, the serial communication is transmitting “in the air”, and it is being picked up by the audio lines. I know to be the case because even if I have the serial leads from the arduino “attached to nothing”, it picks it up. If I shorten the leads, it works, but I need them to be long enough to reach the devices…

Any ideas on a simple solution?

Thanks

Basically the switch is more of a mixer, but inputs are turned off, when not selected.

There is a input for a hd radio, an ipod and an aux port. The audio inputs are brought together using 10k resistors, and then into a small amp to correct the attenuation (also the receiving amp needs a stronger signal).

There are 3 serial ports in use on the arduino - one talks to the hd radio, one talks to the ipod and another communicates with a ibus device (used to talk on the automotive bus).

There is a 12v power supply (simulate car battery) that is then directed to a 12v-5v DC/DC switching power supply that is used to power the arduino, and the small amplifier.

When the arduino perform an serial communication -even over the USB link (e.g. downloading a progam to the arduino), noise is heard over the audio lines.... :(

The device is basically breadboarded, and i've tried to keep the leads as short as possible.

Does this give you more information ? What else would be helpful ?

Also, there are a few other circuits - one charger the ipod when connected (also off the switching amp), and the other circuit converts TTL to RS232 so the arduino can talk with the hd radio.

I think (sorry newbie...) that the arduino is generating noise on the ground line, but not on the audio lines, so it is picked up as sound... since there share a common ground.