sound.c

#include "header.h"
#include "io.h"
#include "tc_dos.h"


void get_vol(ubyte iobyte);       // prototypes for sound functions
void get_freq(ubyte iobyte);

uint freqbits[4];                 // arrays holding frequencys
ubyte vol[4];                     // and volumes for each channel
ubyte soundyesno = 1;

void sound_byte(ubyte iobyte) {      // if a noise program byte, return
    if ((iobyte & 0xE0) == 0xE0) return;

    if ((iobyte & 0x90) == 0x90) get_vol(iobyte);  // if a volume byte
    if ((iobyte & 0x90) == 0x80) get_freq(iobyte); // if a frequency byte
    if (!(iobyte & 0x80)) get_freq(iobyte);      // if a frequency byte
    if (soundyesno) update_sound();            // update sound

}


void get_vol(ubyte iobyte) {    // used to extract volume data from a byte

    ubyte channel;

    switch (iobyte & 0x60) {            // select channel
        case 0x00:
            channel = 3;
            break;
        case 0x20:
            channel = 2;
            break;
        case 0x40:
            channel = 1;
            break;
        default:
            channel = 0; // Added to prevent compiler warning.  Check behaviour. 2016
    }

    vol[channel] = ((ubyte) 15) - (iobyte & ((ubyte) 0xF));   // update channel
}

void get_freq(ubyte iobyte) {      // used to extract frequency data from
    // a byte
    static ubyte channel = 0xFF;

    if (iobyte & 0x80) {              // select channel

        switch (iobyte & 0x60) {
            case 0x00:
                channel = 3;
                break;
            case 0x20:
                channel = 2;
                break;
            case 0x40:
                channel = 1;
                break;
        }

        freqbits[channel] &= 0;             // update high order bits of channel
        freqbits[channel] |= (iobyte & 0xF);
    }

    if (!(iobyte & 0x80)) {          // if a `low order' frequency byte

        freqbits[channel] &= 0x000F;
        freqbits[channel] |= (((uint) (iobyte & 0x3F)) << 4); // update low order bits
    }
}

/*
This function has the fairly difficult task of selecting a suitable single
frequency, which best describes the combined sound of three
variable frequency, variable volume frequencies.

*/

void update_sound(void) {

    uint c, totvol = 0;
    double totfreq = 0;

    for (c = 1; c < 4; c++) {    // step through each channel
        totvol += vol[c];     // keep a running volume count
        if (freqbits[c] == 0) continue;   // check to avoid divide by zero

        totfreq += vol[c] * (4000000. / (32. * freqbits[c]));
    }

    if (totvol == 0) {        // if no channels are turned on, switch off
        nosound();        // all sound
        return;
    }

    totfreq /= totvol;       // normalise sound
    sound(totfreq);        // and generate it
}