Tri-Voice (Beep)

TRI-VOICE is a three-channel music player that simulates polyphonic sound using the single-voice BEEP command by rapidly interleaving notes from three independent voices. Music data for each voice is stored as raw bytes in memory starting at addresses 50000, 50114, and 50372 respectively, with each note encoded as a pair of bytes: pitch value and duration counter. The program uses 254 and 255 as sentinel values — 254 signals a rest/pause for a given voice (substituting another voice’s current note to keep BEEP busy), and 255 marks end-of-stream. The music data block (“TRI-MUSIC”) is 495 bytes loaded at address 50000 (5E4 hex) and was likely produced by a companion tool called MUSIcomp V1.1, which stores note values offset by 36 semitones. A notable bug exists at line 45, where the condition checks `B>=254` twice instead of checking `C>=254`.

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Program Structure

The program is divided into a main loop and three symmetric subroutines, one per voice:

1. Initialisation (lines 6–20): Variables and voice data pointers are set, and the first note for each voice is loaded via GO SUB 100, GO SUB 200, GO SUB 300.
2. Main loop (lines 30–70): Each iteration decrements the three duration counters (A1, B1, C1), fetches new notes when a counter reaches zero, applies pause-substitution logic, plays the three notes with BEEP, then jumps back to line 30.
3. Note-fetch subroutines (lines 100–320): Three identical routines read a pitch byte and a duration byte from the relevant pointer (V1, V2, V3) and advance the pointer by 2.
4. Loader / saver (lines 9000–9999): Loads the binary music block “TRI-MUSIC” to address 50000 (hex 5E4 = decimal 1508, but here used as 5E4 meaning 5×10⁴ = 50000 in Sinclair floating-point scientific notation), then saves both the BASIC and the code block.

Voice Data Format

Music data for each voice is stored as consecutive byte pairs in a flat binary block loaded at address 50000. The three voices begin at fixed offsets:

Variable	Start address	Offset from 50000
V1	50000	0
V2	50114	114 bytes
V3	50372	372 bytes

Each note entry is two bytes: the first is the pitch (encoded with a +36 offset from MUSIcomp V1.1 format, hence A-36 etc. in the BEEP calls), and the second is a loop-iteration counter controlling how many main-loop cycles that note is held. Sentinel values 254 (rest) and 255 (end of voice) are reserved.

Tri-Voice Simulation Technique

True simultaneous polyphony is impossible with a single BEEP channel, so the program approximates it by playing each voice’s note in rapid succession within every loop iteration (line 60). The durations .014 and .019 seconds are short enough that the ear tends to fuse the three pitches into a chord. Voice 1 and Voice 3 each get 14 ms; Voice 2 gets 19 ms per cycle.

Pause/Rest Substitution Logic

When a voice is resting (pitch byte ≥ 254), the program substitutes another voice’s current pitch so that BEEP is never called with an undefined sentinel value. The substitution chain at lines 46–57 is:

• Lines 46–48: Handle cases where two voices are simultaneously resting — the resting voices borrow the active voice’s pitch.
• Line 50: If only Voice 1 is resting, it borrows Voice 2’s pitch.
• Line 55: If only Voice 2 is resting, it borrows Voice 3’s pitch.
• Line 57: If only Voice 3 is resting, it borrows Voice 1’s pitch (post-substitution value of A).

The duration counters (A1, B1, C1) are deliberately not altered during substitution, so each voice’s timing advances independently of the borrowed pitch.

Notable Bugs and Anomalies

• Line 45 — duplicate condition: The line reads IF A>=254 AND B>=254 AND B>=254. The third operand checks B again instead of C. This means the all-three-voices-resting guard never fires correctly when only C alone is the third resting voice; the intended condition was C>=254.
• Line 48 — self-assignment: LET C=B: LET B=B — the second assignment is a no-op. The intent was presumably to assign some other value to B, or it may simply be redundant.
• Line 42 vs line 100/200/300 termination: The end-of-music check (IF A=255 THEN STOP) only monitors Voice 1. Voices 2 and 3 will simply freeze at their last counter position if they finish before Voice 1, which is an acceptable design constraint for this format.
• 5E4 as a load address: Sinclair BASIC accepts scientific notation in numeric literals, so 5E4 evaluates to 50000 — a compact way to express the load address without typing five digits.

Key BASIC Idioms

• Multiple statements on one line with : to reduce line overhead and speed up execution.
• PEEK used directly in arithmetic expressions (LET A=PEEK V1) rather than requiring an intermediate variable.
• Pointer arithmetic by incrementing V1, V2, V3 by 2 after each read, acting as a simple sequential data iterator entirely within BASIC.
• The SAVE … LINE 9000 idiom sets the auto-run line so the loader executes immediately on LOAD.