Posted By
MMS
on 2023-03-22
04:27:11
 |  Re: Covox Speech Thing - parallel port adapter
For Covox, as circuit diagram visible, for 8 bit data, 2-9 pins used on the PC Centronics port.
You need Pin 25 for ground, and you need to get a 5V for the card. USB? 
Plus/4_____PC
--------------------
P0 ( B)___D0 (2)
P1 (K)____D1 (3)
P2 (4)____D2 (4)
P3 (5)____D3 (5)
P4 (6)____D4 (6)
P5 (7)____D5 (7)
P6 (J)____D6 (8)
P7 (F)____D7 (9)
GND(1)___GND(19 or 25)
The C64 Soundmax is pretty smart.
A similar smart design in the 90s could make the MOD playback on PC much easier and more available to the masses. I remember my 286/16Mhz machine was rather loaded with 4 channel MOD playback and the 8 channel was not possible at all. Because all the mixture and conversion into one single 8 bit stream requires a lot of CPU calculations. And that's why Amiga could easily do it, as it just uses the 4 independent HW channels, and mixing happened only in case of 8 channels
Here you can see, how even a 4.77MHz XT struggles to play a 4 channel MOD:
https://www.youtube.com/watch?v=nZX5NzSXUDc
The 4 independent 8 bit DAC channels seen in SoundMax, by the selection of A0 and A1 let the CPU just to send out the channel raw 8 bit data, without adding them together and then covert it back to a single 8 bit data. On PC this is the main reason (mixing) why an XT cannot handle it well, because they try to use the Soundblaster's single 8 buit DAC too (like an internal Covox).
In case of Soundmax, all the mixing done by few passive components without the overhead on the CPU.
With SoundMax or Covox you do not need to convert samples to 4 bit (like on Pigmy's player), or do the conversion internally, just you can use the originals as they are made on Amiga. SoundMax makes the playback much less CPU intensive.
A similar design to SoundMax on PC (via Centronics port) could make the MOD playback smooth even on IBM XT machines or even possible on original IBM PC 5150, I suppose. (original IBM PC 8088/4.77MHz is ~7x faster than a C64 in pure calculations, like PI, and had the half the calculation speed as an A500 at 7 MHz).
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