Playing Sound
To play sound from your calculator you either need 2.5mm Stereo Headphones (rare) or a 2.5mm <-> 3.5mm Stereo adaptor. The latter is available for about 70p (about 1 US dollar) from Maplin (UK) or Tandy / Radio Shack (UK and US). You can then simply plug the headphones into the linkport socket and hear sound. Note that your calculator may freeze for a second or two if you plug the headphones in when you're on the homescreen.
What is Sound?
Sound is just changes in pressure in the air. This is generated through headphones by a rapidly moving disc or similar. The purest form of sound uses a sine wave, as illustrated in the diagram. However, as we only have digital control of the linkport we can only generate a square wave, also in the diagram. This generates a slightly tinny sound. If you're a real purist you could attach two capacitors to your headphones to round off the square waves, but that's a bit OTT.
The frequency of a sound affects how high or low it is. This is shown in the lower half of the diagram below. The higher the frequency (number of 'cycles' per second, measured in Hz or Hertz) the higher the note. If you double the frequency you make a note one octave higher (playing games in physics when this was being explained lost me a mark in the prelims, grrr...) and if you half it you go down an octave.
One other thing to note is that the _pause, _getky
and _getkey ROM calls will not work properly when the linkport
is being used.
Generating Sound
We can generate sound by switching on and off the linkport rapidly. The code
to do this is simple - you just need a delay loop of the required value and
then toggle the status of the linkport. The problem is that higher frequency
notes take longer to play, so playing notes of the same length requires more
complicated maths. I recommend using the cpl instruction to
take the one's complement of the delay loop counter and looping that many
times.
General Sound Routine
The code below was written by me. It plays a sound out of the linkport. It takes an input in a which determines the frequency (high number, high frequency). It destroys af, bc and de but does not change hl, so you can use hl as a pointer to sound data, for example. I suggest disabling interrupts before calling this routine to get the best sound.
PlaySound: ld d,a ; value for NumberLoop cpl ; calculate value for FrequencyLoop ld c,a ; value for FrequencyLoop ld a,%11000000 ; value to send to linkport ld e,%00111100 ; value to toggle a with NumberLoop: ld b,c ; restore FrequencyLoop counter FrequencyLoop: djnz FrequencyLoop ; FrequencyLoop is to generate a delay ; before toggling linkport status xor e ; toggle a with e... out (7),a ; ...and write a to linkport dec d ; decrease NumberLoop counter jr nz,NumberLoop ; NumberLoop is to ensure that all ; notes are roughly the same length.
