It's not the prettiest thing to look at, but it does function as requested:
The device is yet another PIC-based 18F2455 USB/HID device.
The state of the device is reported back to the host PC over USB, as well as the audio input level (after being converted through the built-in A-to-D convertor).
Although relatively straight forward to design, we did have problems with one little bit, but it turned out to be a major headache and took a few hours to resolve. The problem we hit was the input voltage level was in quite a low range - not more than about 1 volt. This meant that when put onto an A-to-D pin that converts voltages from between 0V-5V into a value in the range 0-255, our input values were only ever in the region of 0-50 (20% of 5V ~ 20% of 255). Complicating things further, the audio in from the PC had a background noise level, so instead of a value range of 0-50, we were getting a value range of about 30-80.
But this was only at absolute peak volume, which didn't really occur very often, so our input value range hovered around a relatively low 30-50. If we turned the volume up on the PC, to get a higher peak voltage, we also increased the background hiss, so the value range went up to 50-80, but the size of the range didn't actually increase.
Although this relatively small range did create a working trigger threshold, we were uncomfortable about it being so narrow. We didn't want to mess about with op-amps and extra components, so set about finding a way of increasing the voltage range on the A-to-D output.
The answer was to provide an alternative reference voltage onto pin RA3 on the PIC (also labelled VREF+). To achieve this, we put the wiper of a potentiometer onto the input pin and connected the other two pins to power and ground (creating a voltage divider and allowing us to create a variable reference voltage of 0V-5V)
Another addition to the circuit was to include a 0.1uF capacitor in series with the left and right audio inputs. The idea being that as we're detecting white noise rather than regular, relatively smooth speech or sound, the wild fluctuations in the input voltage can be negated through the use of the capacitor. As we get a spike in the input voltage, this can charge the capacitor and when the white noise voltage level goes low, the input signal into the PIC can be sustained.
In practice this worked well, and coupled with the variable reference voltage gave us input values of 0-200+ but meant that after the audio was stopped, the input on the analogue pin retained a relatively high value. We added some "bleed" resistors across the capacitor pins of about 10K, so that when the audio signal is removed, the capacitors discharge relatively quickly (after about a second) and the input signal always returns to zero.
Frustratingly, we also spent about an hour trying to solve a problem that wasn't really present. With the extra capacitor in place, we noticed that there was a slight delay between starting the movie off (that contained the white noise soundtrack) and the device recognising the noise and triggering it's outputs.
After an hour of changing capacitor values, altering resistor values, changing analogue reference voltages, introducing software thresholds and so on, we unplugged the device and listened to the soundtrack as the movie played.
Sure enough, after hitting "play" on the movie player, the images moved instantly, but the sound only came in a fraction of a second later - the problem wasn't with the device at all, but the movie player software used to start and stop the audio signal!
Here's the latest, updated schematic, including last night's changes:
Audio Input Circuit_v2
note some input/output pins may have changed from the original design
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