I've built a talking clock using the Adruino, the LoL Shield for a matrix of LEDs and a SpeakJet chip to generate the voice.
Everything works superbly, but I get a ton of hum and hiss on the audio circuit due to the LED display. The switching on and off of the 124 LEDs hundreds of times a second is clearly piling buckets of noise into the amplifier. I know the source is the digital circuit, because the frequency of the noise is closely correlated to changes on my LED display.
What's the best way of removing the noise?
My best idea so far is to somehow switch off the audio circuit using a HIGH/LOW from the digital circuit when audio is not needed. The SpeakJet has a pin that goes HIGH when it is busy speaking, which would be ideal to use. My knowledge of analog circuits is pathetic though, so I don't know how to use that pin to turn the audio circuit off.