[Coco] Homebrew CoCo Games

[Darren A]

On Sun, Dec 13, 2015 at 10:34 AM, Zippster wrote:

> There is no additional overhead for the CoCo after the AY (or whatever)
> has done it’s part and the signal is put on the SND IN line at the cart
> port.
>
> It’s all analog from there.  The cart port sound in line is then mixed with
> the DAC output at the DAC in the case of the CoCo 2/3, or at the
> MC14529B (between the DAC and the modulator) in the case of the
> CoCo1.
>
> Nothing would really be gained by a separate jack on the cart except
> another output jack, one that would be missing whatever the DAC would
> have contributed to the signal.
>
> This is my understanding from the schematics, data sheets, etc.
>
> - Ed
>



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