[Coco] Homebrew CoCo Games

Zippster zippster278 at gmail.com
Mon Dec 14 14:36:00 EST 2015

The cart sound line and the DAC output are switched between just like the
joystick lines, and with the same selection lines.  So you’d have to use a similar
strategy with the chip generated sound if you can or use an external jack.

Maybe that would actually work out better and you wouldn’t have to do all the switching?
Does the new hires joystick routine need more time reading the different stick inputs?
I didn’t read too deeply into that one.

- Ed

> On Dec 14, 2015, at 1:25 PM, Nick Marentes <nickma at optusnet.com.au> wrote:
> 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
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