Audio signal is generated by software and internal MCU peripherals. There is no dedictated sound controler in PIC peripherals, but some other resources are used instead. As the consequence to this compromise, audio output signal is pure binary (square wave), just like in the old computers and game consoles. To make the sound more pleasant, there is the passive, first order low-pass RC filter (R15-R19 and C9). There are five audio channels, each of them controlled by software. On-off, tone frequency and two-step volume control is supported. Three channels are typically used for music, one is for Jack's sound effects and one for his enemies sound effects.

The most convenient peripheral for audio signal generating is OC (output Compare), which is preset to Center-Aligned PWM mode. Unfortunately, there are only four OC channels in PIC24EP512GP202 (or any other 28-pin PIC MCU), and one of them is already used in horizontal sync generator, so we have only three for audio application. They are used for music.

The remaining two audio channels (sound effects) are created using TX units from UART1 and UART2 peripherals. TX buffers (U1TXREG and U2TXREG) are permanently loaded with 10101010s, so the whole transmited sequence, including Start and Stop bits, contains 0101010101s, repeated endlessly. BRG (Baud Rate Generator) determines the frequency, and On-Off is obtained by remapping Peripheral Pin Select to TX out or Port Latch.

In the Menu (or Pause) page, each channel is adjusted to Off, Low and High volume. The only difference which determines volume level, is OD (Open Drain) bit in ODCx register.

Music (or effect) script contains of 4-byte groups. First byte is for tone channel #1, then goes tone channel #2, tone channel #3 and duration (in 17.86 ms steps, which is equivqlent to 56 Hz, same as VGA vertical frequency). The table on the bottom is for tone pitch bytes (red numbers, 1-75), showing corresponding tones (blue, A2-B8) and frequencies (black, 110-7920 Hz). If the tone byte contains 0, it is pause (no tone on that channel), and if it contains 99, it is "no change" (previous tone continues). All table members must contain four bytes. Table terminator contains 0,0,0,255 and then follows one-word address (low endian) with new table which shall be executed. So "0,0,0,255" is opcode "jump" and the following word is jump address.

For sound effects, script contains 2-byte members. The first one is the tone pitch, and the second one is pitch in 17.86 ms steps (56 Hz). Terminator contains two zero bytes. There is no loop address like in the music script, as sound effects should be executed only once for each game event. Pause is represented as 0 (followed by pause duration byte which is >0), and there is no "equal" byte, as this is monotone effects table.

There are two global variables, named music.shift and effects.shift, they are used as global pitch shifters, for final frequency adjustment in one-tone steps. By default, they are 8 and 6. respectively.

The first example is for the part of Jumping Jack main theme (pp is pause and it equals to 00, ee means "equal" and it equals to 99 in the table), and the second one is sound effect for Jack's jump:

Note: Special credits go to Marko Antonić, who made the arrangements for music and adapted them for this media. Thanks to him, music sounds suprisingly good, even with this modest hardware.

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