Composition Editor

This program is a three-voice composition editor that allows users to enter, edit, and play back musical notes using the TS2068’s hardware sound chip (AY-3-8910). Notes are encoded as two-byte values stored in fixed-length string arrays, with each pair of characters holding the register values needed to set pitch on one of the chip’s three tone channels. Note names (c, d, e, f, g, a, b and their sharps) are pre-mapped to numeric indices via single-letter variable assignments at line 26, and a 3D array `n` translates note/octave pairs into the corresponding AY register byte values. Playback iterates through up to 400 note slots per voice, writing directly to AY registers via consecutive `SOUND` statements with a variable speed pause between steps. An edit mode allows real-time single-step navigation through a voice using key presses, with inline note replacement via subroutines at lines 1000 and 1050.

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Program Analysis

Program Structure

The program is organized around a main menu at line 50 that dispatches to five sections via a computed GO TO at line 55: GO TO 80+opt*20. This maps menu options 1–5 to lines 100, 120, 140, 160, and 180 respectively. The five sections cover entering notes for Voice A, Voice B, Voice C, editing, and playback.

Option	Target Line	Function
1	100	Enter Voice A notes
2	120	Enter Voice B notes
3	140	Enter Voice C notes
4	160/200	Edit a voice
5	180	Play back composition

Note Encoding and the `n` Array

A 3D numeric array n (implicitly dimensioned elsewhere or pre-populated) maps a note index and octave number to two AY-3-8910 register bytes. Line 16 seeds one entry: n(6,1,1)=228 and n(6,1,2)=4, which represents the coarse and fine pitch bytes for one note. At line 26, single-letter variables are assigned integer indices: c=1, cc=2, d=3, dd=4, e=5, f=6, ff=7, g=8, gg=9, a=10, aa=11, b=12. This allows the user to type note names like f1 or g2, from which n1 (note index, all characters except the last) and n2 (octave, last character) are extracted via VAL.

String Array Storage

Notes are stored in fixed-length two-character string arrays a$, b$, and c$, each dimensioned (1000,2) as noted in the REM at line 20. Each element pair holds the two AY register bytes as characters via CHR$, and they are retrieved during playback with CODE a$(j,1) and CODE a$(j,2). A rest is represented by storing CHR$ 0 in both elements.

Playback Mechanism

Playback at lines 190–194 uses consecutive SOUND statements to load AY registers directly. Registers 0–1 carry Voice A pitch, 2–3 carry Voice B pitch, and 4–5 carry Voice C pitch (with register 4 decremented by 1 at line 192, possibly to compensate for an off-by-one in the note table). Register 7 sets the mixer, and registers 8–10 set channel volumes. The loop runs to note 400 and restarts from the same start position, creating a loop. Pressing x during playback silences the chip and returns to the menu.

Edit Mode

Edit mode (lines 200–229) lets the user navigate through notes of a single voice in real time. A computed GO TO voice*10+200 at line 205 dispatches to the appropriate voice editor (210 for A, 220 for B). Within each editor, the current note plays continuously via SOUND, and INKEY$ polling at lines 216/226 moves forward ("7") or backward ("6") through note slots. Pressing "i" triggers a GO SUB to a note-input routine at line 1000 or 1050. There is no edit handler for Voice C — line 230 does not exist, so selecting voice 3 in edit mode would cause an error.

Key BASIC Idioms

• Computed GO TO for menu dispatch: GO TO 80+opt*20 and GO TO voice*10+200.
• Boolean arithmetic for navigation: LET j=j+(INKEY$="7")-(INKEY$="6") increments or decrements j based on key state.
• INPUT ... LINE k$ used to accept unquoted string input including note names.
• The last entered note is remembered in z$ and reused when the user presses ENTER without typing a new note (lines 107, 127, 147).
• VAL n$ used to parse a numeric start position from string input.

Notable Techniques

The note string is parsed by splitting on length: k$(1 TO (LEN k$-1)) gives the note name/index portion, and k$(LEN k$) gives the octave digit. Since note indices are assigned to variables (f=6, etc.), the user can type something like f1 or g2, and VAL "f1"(1 TO 1) would return the value of variable f — a clever use of BASIC’s VAL evaluating variable names within strings.

The dummy FOR/NEXT loop at lines 22–24 (iterating 1 to 1000 with no body) is used as a progress indicator during a “Configuring memory now” message, serving as a visible delay while appearing to perform initialization work.

Bugs and Anomalies

• Line 20 is a REM containing the DIM statements for a$, b$, and c$. These arrays are never actually dimensioned, so accessing them at runtime would produce an error unless the arrays were pre-existing in memory from a prior session.
• Voice C has no edit subroutine — GO TO voice*10+200 with voice=3 targets line 230, which does not exist.
• The typo “Compostion” (missing ‘i’) appears in the title at line 10, and “promt” (missing ‘p’) appears in the entry prompts at lines 100, 120, and 140.
• Line 16 pre-populates one cell of array n before the array is dimensioned, which would cause an error on a fresh run; the array must be pre-loaded separately.
• At line 192, CODE c$(j,1)-1 subtracts 1 from the Voice C coarse pitch register value, which may be intentional tuning compensation but is inconsistent with voices A and B.
• Variable name case inconsistency: JJG and jjg are used interchangeably (lines 101–103, 203), which in Sinclair BASIC are treated as the same variable since variable names are case-insensitive for numeric variables.