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4. Audio
FIGnition supports single-channel, 1-bit audio. Audio out is generated by a timer and
therefore doesn't halt the CPU. It can be used for sound effects as well as music over an 8
octave range. The audio circuit can also be used for audio in.
4.1 Circuit Diagram
4.2 Operation
4.2.1 Circuit Details
There are two main challenges with designing a simple audio interface for FIGnition:
• FIGnition outputs 0v and 5v, but audio requires a -1v to +1v range.
• FIGnition would normally output pure square waves, but these have very high fre-
quency components which we should filter out.
FIGnition's audio circuit resolves the second question, but doesn't resolve the first one
properly. Nevertheless, in practical terms it works well when outputting simple audio.
Let's consider the initial case, with the audio turned off. Here, the resistors generate a
voltage of 2.5v into the capacitor. The output quickly charges up to nearly 2.5v, but then
drops as current transfers through the internal resistor. Thus when you connect audio
there is a faint click due to a brief pulse on the phono [Fig 4.2].
Fig 4.1
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