Audio Circuit (Neo Geo AES)

the neo geo aes has a fairly complex audio circuit worth exploring the audio is generated from several places the game cartridge (over sdac, sdpadc and sdradc) the z80 / neo c1 / neo d0 (over sddc) z80 cpu the z80 cpu works in conjunction with the neo c1 to do the interactive data control to the ym2610 sound processor important if the !nmi (pin 17) has a damaged trace to neo d0 then its read (pin 35) will never be high, and so it will never read in and process sound data causing no audio to play neo c1 / neo d0 audio the neo c1 is a pin mapping controller to allow correct communications between the z80 and other parts of the system, that will need to be working and connected for it to allow audio to be processed and dispatched by the z80 the neo d0 is responsible for generating the audio and controlling the ym2610 sound processor it uses the z80 in assistance to control the ym2610, this includes generating the audio for the bios intro the neo d0 controls the !read, !write and !cs pins of the ym2610 the neo c1 / neo d0 / z80 control the sddc pins of the ym2610 the d0 7 pins are interactive data which controls generating sounds using a process (the z80) game cartridge audio the audio from the game cartridge comes in on sdac, sdpadc and sdradc adac adac0 and 1 are address pins for 2 bit digital audio so they toggle between 00, 01, 10 and 11 sdp / sdr adc the sdr is the pcb a channels address/data signals for generating pcm audio for channel a the sdp is the pcb b channels address/data signals for generating pcm audio for channel b both are digital sound data as 8bit data notice the changing digital data as audio plays when the game is quiet it looks like this (pin 10 of ym2610 as example) notice the completely static repeating pulses a good test is to remove the game, boot to blank screen bios, and a working ym2610 should show a single pulse or small peaks like so and a faulty ym2610 often looks like this on pin 10 with no game inserted (high then pulsing low, often stable or eratic) ym2610 analog stream the ym2610 has a software generated sound mode called ssg if you use a diagnostics bios with an m1 eeprom installed in a game cartridge to test the z80 sound circuit, it will only test the ssg mode, not the digital pcm modes so bare that in mind when the analog mode is used, the anout (pin 27) of the ym2610 will output analog audio which goes straight into the final audio mix and out of the av port and headphone port ym2610 digital stream the resulting combined audio from the game cartridge, z80 and neo d0 are output as a single bit stream on pin 31 (opo) which is the input to the ym3016 digital to analog converter on pin 4 (sd) some basic checks for operation using fatal fury special ground should have continuity to ground 5v should be on vcc pins 19/28/62 !test pin 40 should be 5v phase s (pin 64) should output 2 685mhz square wave phase m (pin 63) should be an 8mhz clock input rmpx (pin 20) should be 111 9khz square wave !roe (pin 21) should be the invert of rmpx (inverted 111 9khz wave) analog out (pin 27) should be 2v dc sh1 (pin 29) and sh2 (pin 30) should be 55 94khz 4 2v square wave ic (pin 33) is !reset and should be 5v data pins d0 d7 (pins 2 to 9) should be pulsing about 4 2v to 5v a0 (pin 60) and a1 (pin 61) should toggle 4 2v !read (pin 59) should toggle low constantly but with long gaps !write (pin 58) should toggle low constantly but with long gaps !cs (pin 57) should toggle low constantly but with long gaps !irq (pin 56) should toggle low, with typical gaps of 5 10ms radx (pins 10 to 17) should be pulsing about 4 2v to 5v rax (pins 22, 23, 35, 36) should be pulsing about 4 2v to 5v the high ra22 (pin 37) and ra23 (pin 38) are usually 0v pax (pins 41 44), should be 4 2v dc until used, then pulse like on intro !poe (pin 46) should sit at 4 2v dc until pax pins toggle, and then toggle !pmpx (pin 47) should sit at 0v dc until pax pins toggle, and then toggle padx (pins 48 55) should sit at 0v dc (noisy) until pax pins toggle, and then toggle opo digital stream the pin to observe for digital audio output is pin 31, opo with no sound we get a pulse high every 9us (111 9khz) then when audio plays it shows the streamed data after this the ym3016 has the job of converting this digital stream to analog audio output read, write, chip select the !read, !write and !cs pins are controlled by the neo d0 if you see issues with them, make sure the pins are connected to the neo d0 if the conditions of the neo d0 are not satisfied it might also not drive the pins low and could indicate a problem with the z80, ram, d0, c1 or traces between them all, usually the data or address bus ym3016 digital to audio the ym3016 expects a 2 685mhz clock on pin 5 as well as the two sample signals on pins 7 and 8 at 55 94khz (generated by the ym2610) and the audio digital stream on pin 4 (sd) it will then output the audio on channels 1, 2 and common (pins 10, 11 and 12 respectively) there is also a tobuf output on pin 13 that goes through a buffer and back into com without the op amp chip installed (pc458c / pc471 / 4558 / 4559) therefore no input into com, the audio still works but is more gritty, so i suspect the tobuf is a filter of some sort for the audio tobuf is a 1vpp audio signal rb (pin 16) is similar to tobuf as an output to the op amp, and input fed back into bc/mp (pins 14/15) audio output finally after the ym3016 has converted the digital audio into analog left and right, it passes these left and right signals from channel 1 (pin 10, left) and channel 2 (pin 11, right) through an op amp (pc458c / pc471 / 4558) at this point the buffered left and right audio splits off one path goes through the volume slider on the front of the console, then into the headphone amp (njm2066, or discrete circuitry) the other path sends the left and right audio channels through capacitors mixed together, then through one more op amp (pc458c / pc471 / 4558) and off to the av port pin