Posted By
SukkoPera
on 2023-09-04
04:33:14
|  Re: Covox Speech Thing - parallel port adapter
Yeah, the 256kB RAM expansion makes the difference. I have now corrected the Second Mod page, adding that, while the demo will apparently load with just 64kB, 256kB are required for the music to play correctly.
First Mod also sounds great, by the way.
I'm not sure how many physical machines exist today, that are able to play these demos! Not many, I guess!
The board already had a place for a low-pass-acting capacitor. I hadn't soldered it yet when I shot the video since I wanted to test different values for it. The original project had 10nF, I ultimately settled to 1.8nF, but I guess it goes down to taste, I will add a note about this.
So, my opinion is that this thing sounds GREAT! I'm not sure how usable it is beyond dedicated demos though, as pushing data to it fast enough is probably quite CPU-intensive, but it's very cheap and easy to build. We have to discuss what to do with it, there are a few options with it:
1. Keep it a standalone board
2. Make it a Digiblaster-like addon for ReSeed/SID cards
3. Merge it with another of the boards I will be announcing soon
4. Do any of the above with a real DAC chip
Due to option 3, I guess it's too early for a final decision  .
What I'm not satisfied with is the amplifier circuit. I have used this LM386-based circuit because it is supposedly good-sounding, it has bass-boost, it was ready-made and I had no knowledge of amplifiers. The problem with it is that the MINIMUM gain is 20, and since the signal coming out of the resistor ladder is more than 1V peak-to-peak, it must be attenuated quite a bit or it will distort badly. Now, attenuating a signal just to amplify it again is plain silly IMHO (it kills the S/N ratio), and the bass boosting is not very effective either. I've studied a bit about opamps and I think I can now come up with an amplifier that is better suited for the job (forget about bass boost though). So, whatever option we choose, it will have a brand new amplifier circuit.
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