samson7point1
Posts: 22
Joined: 17 Apr 2018, 01:17

Stereo CD support theory

08 Jul 2019, 22:55

So while I wait however long it’s going to be before the Neo SD Pro AES is available to order/preorder I have to peck at the Mono CD audio thing just a little. Apologies if this has been addressed elsewhere.

I get that CD audio can’t be fed into the console through the cart slot, but what would the real level of effort be to add a couple ports and a mixing circuit to the Neo SD Pro cart itself? If one were so inclined, is there anywhere to tap stereo CD audio from the Neo SD Pro board?

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neodev
Posts: 291
Joined: 09 Apr 2018, 14:47

Re: Stereo CD support theory

09 Jul 2019, 13:40

It's not possible, as the analog signal is not present anywhere on the cart. It's all digital, transferred to the FPGA for ADPCM encoding and read as ADPCM by the YM chip, that generates the final analog signal. Obtaining an analog signal would require a DAC and I2S pins on the MCU to transfer the digital audio to the DAC and then to analog.

samson7point1
Posts: 22
Joined: 17 Apr 2018, 01:17

Re: Stereo CD support theory

10 Jul 2019, 05:24

What I know about FPGAs couldn’t fill a thimble, but I’m trying to follow along. It seems like the FPGA can’t be used to convert ADPCM to I2S? What’s the limitation on the MCU doing it? Are you saying it’s not possible because there aren’t any available output pins, or that it doesn’t work like that without reprogramming?

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neodev
Posts: 291
Joined: 09 Apr 2018, 14:47

Re: Stereo CD support theory

10 Jul 2019, 12:40

Well, it can be done on FPGA or on MCU (SSDS3 and MegaSD do I2S through the FPGA), but there are no enough pins on the FPGA or ARM for that. Currently, the NeoSD PRO PROG FPGA has like 1 free pin, and you need 4 do drive a DAC (DATA,L/R,SCLK,BCLK). On MCU it's harder because not any pin can be used for I2S, only the one with the I2S peripheral connected can, and these pins are already being used for other peripherals.

samson7point1
Posts: 22
Joined: 17 Apr 2018, 01:17

Re: Stereo CD support theory

15 Jul 2019, 04:20

Thanks for the explanation!

Looks like that would require a major redesign. What comes after Pro? NeoSD Super Pro? (I’m kidding...unless you might actually do it.)

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