[Coco] Homebrew CoCo Games

[Nick Marentes]

On 15/12/2015 4:24 AM, Darren A wrote:

> I don't believe there is any mixing of the DAC audio with the cartridge
> port SND line.  There are four audio sources; the DAC, the Cassette port
> input, the cartridge port SND line and the single-bit sound line. The first
> three are routed through an analog switch so only one is active at a time.
> The single-bit sound can be mixed with one of the other sources.
>
> The selector lines for the analog switch are controlled by a couple of
> outputs on PIA0. These are the same lines used to select the joystick axis
> for sampling. If you have a device playing sound through the cartridge port
> and want to be able to read the joysticks at the same time then sound
> interruption would likely be an issue.
>
> - Darren
>


That is correct regarding the joystick and analogue switch for selecting 
the DAC. They do "clash" but what I have done in my game is bounce the 
switch between joystick and DAC sound. As long as you save the DAC value 
prior to disabling the Sound Enable ($FF23), then read the joystick (you 
don't need a 6bit read), then restore the DAC value and re-enable the 
Sound Enable, you can have DAC sound AND joystick read.

If you don't restore the DAC to the previous value prior to the Sound 
Enable flip, you will get a buzz from the sound.

It's not the greatest hardware design but that's what we've got to work 
with.

The SND IN line from the cartridge connector... doesn't that feed into 
some place after the analogue switch closer to the CoCo's sound outputs?

How does the Sound & Speech cartridge do it?

Nick