exxos's Atari, Amiga & retro forum

Last remaining thing to sort out (that i know of) on the TFMSX is the sound circuit.

I'll freely admit this is not my area of expertise. For the first 2 versions of the TFMSX i simply stole the H4 audio mixing circuit as shown below.


The trouble is that the FPGA implementation of the SCC isnt that happy with this (Thanks to BGOllie for lending me a MegaFlashROM+SCC). I think this is, in part, because its a Delta-Sigma DAC built on a Spartan 3 class FGPA chip and any downstream circuit that isnt suitable will interfere with the DACs behaviour. For those unfamiliar with Delta-Sigma its an algorithm, a digital pin then a cap and resistor in series.


I measured the resistor at 1.2K. I found this in some Sony MSX schematic that i cant remember which one now.


Which looks like it buffers the SOUND_IN inputs. My uni electronics theory has me thinking a FET input op-amp is the solution but i'm sure there is a more practical way to do this. I'm planning to break out the bread board and try some options but suggestions are appreciated.

terriblefire wrote: 02 Jan 2023 16:23 The trouble is that the FPGA implementation of the SCC isnt that happy with this

Not happy how exactly ?

Have you built the circuit you posted above (not the H4 one ?) If it's just a buffer then all should be fine really.

EDIT:

Oh delta-sigma is modulated :roll:

https://e2e.ti.com/blogs_/archives/b/pr ... -modulator

Couldn't you just output analog instead of digital ? Even "stepped" waveforms would probably be fine.

i've not built that circuit yet. Will do later.

It's unhappy in that there is no sound from the the SOUND_IN source

terriblefire wrote: 02 Jan 2023 16:52 i've not built that circuit yet. Will do later.

It's unhappy in that there is no sound from the the SOUND_IN source

SOUND_IN Does not really make sense because it is going to the base junction of the transistor which means it would have to be a output. Looks more like a left and right channel sort of thing been mixed into a single channel and buffered which then goes to the SOUND pin. I assume SOUND_IN is generated in the FPGA ? If you have nothing there than it would be a emulation problem.

SOUND_IN is on each cartridge (one on each) slot and is there so a cart can add sound themselves. I freely admit i know nothing here

Only thing I can suggest is just get the scope on it all and see what is going on.

I think this is one of those situations where SPICE is the answer

terriblefire wrote: 02 Jan 2023 20:51 I think this is one of those situations where SPICE is the answer

If you're referring to the amplifier circuit then it is nothing special. Just a basic mixer and a inverter buffer amplifier. You could probably just wire SOUND_IN to SOUND via a cap and it would be basically the same. Just not buffered.

exxos wrote: 02 Jan 2023 21:32

terriblefire wrote: 02 Jan 2023 20:51 I think this is one of those situations where SPICE is the answer

If you're referring to the amplifier circuit then it is nothing special. Just a basic mixer and a inverter buffer amplifier. You could probably just wire SOUND_IN to SOUND via a cap and it would be basically the same. Just not buffered.

Yeah thats what i tried. It works for original SCC carts but not the FPGA repros (Which work with the original MSX Machines).

i've actually managed to reproduce the issue in iCircuit. I get only a few picovolts of output

terriblefire wrote: 02 Jan 2023 22:19 Yeah thats what i tried. It works for original SCC carts but not the FPGA repros (Which work with the original MSX Machines).

i've actually managed to reproduce the issue in iCircuit. I get only a few picovolts of output

I don't know anything about the MSX unfortunately. Is there not any schematics for such a FPGA system ? maybe just needs a stupid pull-up on the pin or something ? I have no idea what the FPGA is trying to output. I know pretty much diddly squat about FPGA stuff as well.