Piano

Spectral Piano is an interactive piano simulator that maps computer keyboard keys to musical notes, displaying a graphical piano keyboard on screen and playing notes via the BEEP command. Lines 14–25 define eight custom UDG characters (UDGs \a through \h, codes 144–151) used to draw black and white piano keys, each stored as 8 bytes of pixel data POKEd into UDG memory. The program supports three octave offsets (bottom, middle, top) controlled by keys I, O, and P, and nine note durations (0.1–0.9 seconds) controlled by keys 1–9. White keys A–J play semitone offsets 0–11 from middle C, while black keys W, E, T, Y, U play the corresponding sharps and flats; pressing a key briefly swaps the UDG graphic to show a “pressed” appearance before restoring it.

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Program Structure

The program is divided into several functional blocks:

1. Lines 10–25: Entry point calls the introduction subroutine at line 1000, then loads eight UDG characters (144–151) with piano-key graphics via READ/POKE USR.
2. Lines 30–43: Initialise display colours and three key variables: a$ (octave label), len (note duration), and o (octave offset in semitones).
3. Lines 100–200: Draw the on-screen piano keyboard using PLOT, DRAW, PRINT AT, and UDG characters.
4. Lines 300–600: Main polling loop — reads INKEY$ for octave keys (I/O/P), duration keys (1–9), and note keys (A–J, W/E/T/Y/U), then calls BEEP and returns to line 300.
5. Lines 1000–1200: Tutorial subroutine presenting a multi-screen introduction with INPUT prompts and animated FLASH effects.
6. Lines 9950–9999: Utility tail: a PAUSE 0 trap, clean-up, STOP, and SAVE/VERIFY statements.

UDG Design

Eight UDGs (\a–\h, character codes 144–151) are defined by the DATA at lines 18–25 and POKEd with POKE USR CHR$ c+n, a. They represent the top and bottom halves of both white and black piano keys in “up” and “down” (pressed) states:

UDG	Code	Role
\a	144	White key top half (left side of pair)
\b	145	White key top half (right side of pair)
\c	146	White key top half, shorter (black-key gap)
\d	147	White key top half, shorter (right)
\e	148	White key bottom half / pressed state indicator
\f	149	White key bottom half / pressed (right)
\g	150	Restore graphic (left)
\h	151	Restore graphic (right)

The inner loop at line 16 uses the expression USR CHR$ c+n where c is the UDG base character code and n is the byte row (0–7), which is a standard technique for bulk UDG initialisation via a nested FOR loop and READ.

Keyboard Mapping and Note Logic

The white keys A, S, D, F, G, H, J map to semitone offsets 0–4 and 5–6 (C D E F G A B) while black keys W, E, T, Y, U map to semitone offsets 1, 3, 6, 8, 10 (C♯, D♯, F♯, G♯, A♯). The variable o shifts the entire octave by −12, 0, or +12 semitones. A typical note-play line reads:

IF INKEY$="a" THEN PRINT AT 14,8;"\e\f": BEEP len,0+o: PRINT AT 14,8;"\g\h"

The pattern is: replace the resting UDG pair with the “pressed” pair, sound the note, then immediately restore — giving a visual key-press animation without any delay loop.

Display Techniques

The piano graphic at lines 140–200 uses a combination of PLOT/DRAW for the outer border and dividing lines, and PRINT AT with PAPER 7 to fill white-key areas. Black keys are rendered at odd column positions in the loop FOR n=9 TO 20 STEP 2 using PAPER 0; INK 7. Line 170 uses IF n=13 THEN NEXT n to skip the column where no black key exists between E and F — a concise inline skip idiom.

The introduction subroutine uses OVER 1 to underline the title “Spectral Piano” by printing underscores on the same character cells, and FLASH 1 to animate the “MIDDLE C” label before clearing it.

Notable Bugs and Anomalies

• Unreachable lines 9950–9960: The PAUSE 0, PAPER 7, and CLS lines after 9950 are never reached during normal execution; they appear to be a manual reset/cleanup sequence intended to be run directly from the editor or via GO TO 9950 by the programmer.
• Line 190 column variable: After the inner loop FOR m=7 TO 10, NEXT m exits with m=11; PRINT AT m,n; at line 190 therefore prints at row 11, which is inside the keyboard graphic area — this places the short-key UDGs one row lower than visually expected for the topmost black-key row.
• Line 1030 loop structure: IF n=12 THEN NEXT n inside a FOR loop is used to skip column 12 (the E–F gap), mirroring the technique at line 170, but here NEXT n increments n to 14 before the body executes again, correctly skipping the gap column.
• Black-key note gap: There is no key mapped to the semitone between B and the next C (no H♯/C♭), which is correct musically, but the jump from h (semitone 11) to j (semitone 11 — line 500 uses offset 11) means both H and J would play the same pitch; j at line 500 plays BEEP len,11+o while h at line 485 also plays BEEP len,9+o — so H is A and J is B, which is correct. The apparent duplication is not present.

Main Loop Efficiency

The polling loop at lines 300–600 re-evaluates every IF INKEY$=… statement independently on each iteration, meaning all key checks are sequential with no GO SUB dispatch. This is typical of simple Sinclair BASIC games but means every pass executes up to 22 INKEY$ reads. The piano keyboard is also fully redrawn (lines 110–200) on every loop iteration, which causes visible flicker and is unnecessarily slow; the keyboard graphic could instead be drawn once and only the individual key cells updated.