TFMSX Sound Circuit

[terriblefire]

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.

WhatsApp Image 2022-12-23 at 10.06.10.jpg

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.

scc_megarom.JPG

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

WhatsApp Image 2022-12-23 at 10.20.22.jpeg

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.

[exxos]

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.

[terriblefire]

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

[exxos]

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.

[terriblefire]

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

[exxos]

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

[terriblefire]

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

[exxos]

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.

[terriblefire]

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

[exxos]

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.