flicker-free graphics

ay sound:

The QS Sound Board is based around a AY-3-8910 Programmable Sound Generator IC, which provide 3 independent tone channels, 1 noise channel and 2 general purpose 8-bit I/O ports. These features are accessed through 16 8-bit registers. The QS Sound Board accesses the AY-3-8910 using two memory mapped I/O ports at addresses $7FFE (32766) and $7FFF (32767). Port $7FFF selects the sound chip register to access and port $7FFE is used to set the data for the selected register. Using memory mapped I/O allows the sound board to be controlled from BASIC by poking to these two memory locations.

The Quicksilva documentation for the sound board claims it is driven by a 1.78977 MHz clock (which happens to be half the NTSC 'colour' crystal frequency of 3.579545 MHz). Since the circuit for the sound board takes the clock signal exposed at the edge connector of the ZX80 or ZX81 and halves it, this would imply that the clock frequency of the ZX80/81 was 2 x 1.78977 = 3.57954 MHz. This is wrong. The ZX80/81 is clocked at approximately 3.21 MHz and so results in the sound board being clocked at approximately 1.605 MHz.