[Coco] Cartridge Port Sound

[Gene Heskett]

On Sunday 27 December 2015 15:21:07 Darren A wrote:

> On Sun, Dec 27, 2015 at 11:46 AM, Gene Heskett wrote:
> > On Sunday 27 December 2015 13:15:28 Darren A wrote:
> > > I want to connect an IO pin of an AVR micro controller to the SND
> > > input line on the CoCo cartridge port. The IO pin will be
> > > outputting PCM audio. Can anyone describe the proper way to do
> > > this? The Service Manuals mention that it needs to be AC coupled.
> >
> > Yes.  Try a .05 capacitor in series with it, if no bass, try a .5.
> >
> > But the coco's audio  doesn't know what to do  with PCM audio, so
> > we'll need to know the output clocking speed of the AVR, with an eye
> > toward using a simple low pass filter to integrate the encoded but
> > digital signal into something resembling an analog audio. If its too
> > slow, say less that 50 kilohertz for the clock, the filter might get
> > more complex than a simple rc rolloff.
>
> It is playing audio sampled at 8 kilohertz using a carrier frequency
> of 62.5 kilohertz.
>
> - Darren

That I'd think, could be easily filtered, perhaps by the coco's own 
circuitry.  Biggest problem might be aliasing, depending on how steep a 
brick wall was in front of the digitizer A/D.  At 62.5 kilohertz, 
aliasing can't be a huge problem if it weren't for the fact that is so 
obvious to the human ear even at .05% equivalent distortion.  But thats 
a recording problem, not playback, because once it has been introduced 
into the digital data stream, there is not a known way to subtract it 
back out. In recording, its common to have a brick wall filter whose HF 
cutoff at 22 kilohertz is at least 60 db down.  And it should have unity 
gain at 15 or 16 kilohertz.

Cheers, Gene Heskett
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