This is an adaptor that plugs into the PS1 parallel port and provides a digital audio output via 3.5mm optical or coax (cables not included). The pure 44.1KHz/16bit digital audio is picked up before the internal DAC, eliminating any noise caused by the circuitry inside the PS1.

The parallel port found on earlier PS1 models already provides most of the required signals to create a digital audio output (LRCLK, BCLK, MCLK, RESET) however SDATA (the actual serial sound data) is missing. Therefore this mod does require soldering 1 wire (or 2 for the SCPH-750X PU-22) inside the PS1 to take the SDATA signal to the parallel port.

V1 vs V2 and PU-22 notes:

Performance wise both V1 and V2 produce the same results. V1 of this mod required two wires to be installed and was not compatible with the PU-22 boards. The extra wire for V1 was used to bring the 16.934MHz audio MCLK signal to the parallel port but V2 now uses the CPU MCLK which is 33.87MHz and already available on the parallel port. This is halved using a clock divider inside the digital audio adaptor to generate a stable 16.934MHz clock required for the digital audio stream.

Removing the need for a second wire also freed up a pin on the parallel port (the PS1 normally has 2 free pins on the parallel port). And after a lot of poking around at the SCPH-750X PU-22 I found there is a way to make the DSP inside the PS1 output the SDATA stream which is normally inaccessible on all models after the PU-20 hence no digital audio mods were previously available for the PU-22 until now.

Enabling the SDATA output on the PU-22 requires pulling one of the pins on the DSP high, however doing this disables the analog audio output. The solution for this was to pull the pin high using the digital audio adaptor itself (hence the 2nd wire required for PU-22) so that when the adaptor is plugged in digital audio is available, and without the adaptor analog audio is available. This isn't an issue with the PU-20 and earlier revisions and the analog output is always available on those regardless of if the adaptor is plugged in or not.

Fun fact: the SCPH-900X PU-23 boards and the smaller PSOne models can also output digital audio in the same way the PU-22 does. However due to the lack of a parallel port on these models there's no easy way to create a digital audio mod for them :-(

Compatibility:

This mod is only compatible with models that have a parallel port and not compatible with PSIO modded PlayStations. Not only does the PSIO already occupy the parallel port, but it also re-routes a bunch of signals going to the port. This mod is however compatible with X-Station modded consoles, and should also work with picostation modded consoles.

Don't forget the SCPH-750X PU-22 analog audio output will only work when the digital audio adaptor is unplugged. Do not plug or unplug the digital audio adaptor while the console is powered on.

As this outputs a standard 44.1KHz/16bit digital audio signal, it should be compatible with any external DAC, receiver or upscaler that accepts digital audio (like the RetroTink 4K, 6X and OSSC Pro). While I have tested it with a number of devices without issue, including the RetroTink 6X CE, I do not have the OSSC Pro so it remains untested at present.

Parts/tools required:

• Soldering iron and solder
• JIS/Phillips head screwdriver
• Basic multimeter
• Wire strippers (a short length of 30AWG Kynar wire will be included with the adaptor)
• For coax output: A 3.5mm to RCA cable (if using a stereo cable, the digital audio will be on the left channel)
• For optical output: A 3.5mm mini-TOSLINK to TOSLINK adaptor or cable

Installation:

1. Open the console by removing the screws on the underside and lift off the top shell (make sure you don't have a disc in there).
2. Carefully unplug the connections to the optical assembly, power supply, controller ports and set aside the optical assembly.
3. Remove the screws holding the top metal plate (on the SCPH-100X you will also need to remove the controller ports).
4. Lift and remove the metal plate by grabbing it at the rear of the console and pulling up and towards the front.
5. For PU-7, PU-8, PU-18, PU-22 boards, remove the remaining screws holding the mainboard and remove the mainboard. For PU-20 boards, lift the copper shield and fold it back over the metal shield on the mainboard.
6. Solder the wire(s) from the points on the mainboard to the pins on the parallel port (see images above). Make sure to sever the jumper link on the PU-22 board
7. Reassemble the console, plug in the adaptor and hook it up to your amp, DAC, or compatible upscaler.
8. Crank the volume and party like it's 1999!

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