Kelly Murta has made available some excellent games here: http://zx81.eu5.org/
All games were originally released for the Brazilian TK85 clone of the ZX81.
One of the games is 'Patrulha Espacial' (Space Patrol), which is a cracking shoot 'em up style game. This can be played with the EightyOne emulator but only if configured as a TK85 or as a 60Hz ZX81 ('NTSC TV' option in EightyOne). This is because the game reads the keyboard port but does not mask off the top 3 bits. Bit 6 indicates whether the ZX81 is set to 50Hz or 60Hz operation, and the game expects it to be 0, i.e. 60 Hz operation.
I've produced a patched version that allows it to work on any ZX81. There is an unused area of memory in the program (which looks to be the unused end of a REM statement that holds the machine code) and so I have placed a patch here that discards the upper 3 bits and then sets bit 5 to 1. This results in values that the game is expecting. The changes are as follows:
$450D: CALL $5EC5
$5EC5: AND $1F
$5EC7: OR $20
$5EC9: LD ($4503),A
$5ECC: RET
And here is the patched version of the game:
"Patrulha Espacial" Patch for 50Hz ZX81
Re: "Patrulha Espacial" Patch for 50Hz ZX81
When I tried "Patrulha Espacial" and "Missão Andrômeda" I was surprised to hear my Chroma Interface-equipped ZX81 play music over the TV! I don't have any sound equipment, so that was like when your cat suddenly starts talking to you...
Activate "Beeper Sound" in the EightyOne emulator, then you will hear the intro tune.
I assume these two games were designed for the TK-82 which seems to have a beeper where you can hear the tape I/O sounds, and that's how the music is probably generated (the TK-85 even has a three-voice sound generator).
Can the sound be heard on a standard ZX81 by connecting headphones or active speakers? If so, to which port, MIC or EAR?
Activate "Beeper Sound" in the EightyOne emulator, then you will hear the intro tune.
I assume these two games were designed for the TK-82 which seems to have a beeper where you can hear the tape I/O sounds, and that's how the music is probably generated (the TK-85 even has a three-voice sound generator).
Can the sound be heard on a standard ZX81 by connecting headphones or active speakers? If so, to which port, MIC or EAR?
Re: "Patrulha Espacial" Patch for 50Hz ZX81
Doing this will also cause EightyOne to output a buzz when displaying a normal picture, which is caused by the TV sync pulses the ZX81 generates. Chroma does not output the TV sync pulses as sound if it determines that a standard TV picture is being generated. But when the ZX81 is performing another operation, e.g. loading/saving, or as in this case deliberately timing its IO output then it renders these as sound. EightyOne should probably be enhanced to exhibit this behaviour when it has Chroma support enabled....
I'm not very familiar with the differences of the TK-8x range, so this is interesting to hear.
Yes, through the MIC socket. If you have a cassette recorder that has a monitor function then you can set it to record and you should hear the sounds.