• Audio IO Ports

The sound system has four channels which play signed eight bit samples. The frequency (period) of each channel can be independently set and has a resolution of 1 clock cycle (62.5 nS). There are 4 ports assigned to each channel covering location, length, period and volume.

Location:

• Sample data needs to be in signed raw 8 bit format and should be loaded into pages 4:7 of system memory (ie: $20000-$3FFFF). The location ports hold a 16 bit value which is an offset in words from the start of the section of memory, therefore samples must be start on even addresses.

Length:

• The length ports hold a 16 bit value equal to the number of words in each sample, therefore samples must always be an even number of bytes long.

Period:

• The period ports hold a 16 bit value equal to the number of systems clocks that must pass between each sample byte:

Volume:

• The volume ports hold a single byte value in the range $00 (silence) to $40 (full volume) - in linear steps).

Samples are set up to play in a similar way to the Amiga Hardware, ie: You set the location, length, period and volume registers of a sound channel, then start the channel playing (with the relevant sys_audio_enable port bit). Once the channel is playing, its location and length registers can be updated - the channel will only fetch the new values once its original length value has counted down to zero (or you stop the channel for a while and restart it). If the registers are not changed, the channel simply loops around playing the same sample data until it is stopped.

Advanced:

Its possible to use an interrupt to seamlessly switch between two or more sample buffers in order to play long samples without worrying about the exact timing. To do this you would start a channel playing, reload it with the second buffer ready for the first loop as normal, and then set up the audio interrupt so that it occurs upon the first loop (IE: just as the second buffer starts to play). In the IRQ service routine, you would set the channel location and length to point back to the first buffer, and on subsequent IRQs switch between the two buffers.

Note: The four audio loop flags are OR'd into a single interrupt source. It is up to the IRQ service routine to read bits 4:7 of the sys_audio_enable port, see which bits are set (and clear them on exit from the IRQ).