Posted By
Bionic
on 2023-02-26
18:52:21
|  Re: Solder's SIDcard Replica
The CD4520 is an interesting story. I received (one of?) the first prototypes from Christian and when I tried it it sounded absolutely horrible. Turned out he had not tested it with real SID music yet. The issue was that the SID was originally clocked with the Phi2 signal, that is unforturtunately not constant in the Plus/4. He came up with the fix deriving the clock for the MUX signal using the CD4520. I also implemented the same fix, deadbugging a CD4520 I found somewhere onto the PCB. Things worked fine. He later told me that using anything else than the CD4520 failed.
The clock frequency is, of course, still different from the C64. The result was still significantly better than the contemporary "frequency-converters" that were used to play C64 music on the Plus/4 at that time. So I guess nobody cared.
Ten years later, I built a prototype based on a CPLD (data synchronization is more complex when the clocks are mismatched) and a 8701+XTAL from a C64 that fixed that issue. I did not follow up further, unfortunately. But there are other cards available in the meantime.
There is a lot of 80ies electronic design legacy in solders card (TL497, GAL, discrete audio amp). I wonder how an accessible design would look like today? Maybe using a MCU to implement all the glue logic, to avoid the need to source logic ICs?
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