Genesis is 16-bit sampled music, sorry! :p
EDITED:
Yeah, I think there is Sample system like SNES: It uses PCm, and 2minutes loop of plain music would be a little big for a cartrige, no?
Okay, I'm tired, I seem to be trolling, but I don't want to be hurting...

I think I misunderstood myself:
1-I think that MD/genesis uses FM and sample system.
2- longs plain audio files would be a little big for a cart, wouldn't it?
3- Excuse-me for my rudeness, I'm not accostumed with a neutral community (was on forums with an atmosphere more "only with firends")
4- And I think that Prizm... Oh, nevermind, maybe someone will show us a little good project, with beautiful graphics and marvellous music... GO FOR IT!

Given the Prizm and FX-9860G series got much faster CPUs than the 84+, I wonder if better sound than the 8 bit chiptunes from some 83+/84+ programs could be produced, such as Sega Genesis, Turbografx-16/PCEngine or PC88?

What quality do you expect ?
The uart running 1 bit D/A mode (without any HW support) is very expensive.
Assume we want output 8bit samples on 22 KHz mono (the worst quality which may be acceptable).
So we have 8 bit samples => 256 different values, so to send 1 sample we have to transmit 255 bits (round up to 256 bits =>  32 bytes).
This means we need to prepare 704000 bytes each second and overclock uart to 7040000 bps (count 10 bits for byte because of start bit and stop bit). The uart has fifo of 64 bytes, so the interrupt should fire each (less than) 90 us.
This all seems to be OK, but the 9860 is simply not capable of doing this.
My player runs uart on ~ 1.8 Mbps well, I tried double speed when measuring, but simple loop like
volatile i;
...
while(i)  //timer will reset after 1 second
{
com_transmit(0);
count++;
}was not able to push bytes quick enough.

and register i?

The delay is in "com_transmit(0);" Communication with hardware is always much slower than memory access.

Just open the COM port on 9600 bps, no parity, 8 bits and output byte 0x55 in a loop.

Thanks

Interessed, Pierrot? I think you could have enough experience to lead us, nah?

Hehe :-,

One of the best French programmers

I know! That's why I said 'hehe' Because he does have enough experience (look at gravity duck)