TV Check

TV Check is a TV and monitor alignment utility that displays five test patterns: color bars, horizontal lines, vertical lines, a crosshatch grid, and a dot grid. The program uses PLOT and DRAW commands to render the geometric patterns at pixel level, stepping in increments of 17 pixels to produce evenly spaced lines and dots across the 256×176 display area. Color bars are drawn using PRINT AT with PAPER attribute changes, cycling through all eight Spectrum colors with abbreviated labels read from DATA statements. The variable Z is initialized to 0 via NOT PI in the DATA list, a compact idiom for producing a zero value without the literal digit.

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Program Structure

The program is organized as a menu-driven dispatcher. Lines 30–80 set up the display and print the menu; line 90 waits for a valid keypress (1–5); line 100 uses GO TO 30+r*80 to jump to one of five pattern sections. The computed GO TO targets are:

Choice	Target Line	Pattern
1	110	Color Bars
2	190	Horizontal Lines
3	270	Vertical Lines
4	350	Crosshatch
5	430	Dots

After each pattern, the program waits for any keypress at line 150 (IF INKEY$="" THEN GO TO 150), then executes RUN at line 160 to restart from the beginning.

Variable Z and the NOT PI Idiom

The variable Z is used throughout as a convenient zero. It is initialized by reading the last DATA item, which is NOT PI. Since PI is non-zero, NOT PI evaluates to 0 in Sinclair BASIC. This avoids the need to write LET Z=0 and saves a few bytes while also serving as a mildly obfuscated constant. Z is then used wherever a literal 0 would appear in PLOT, DRAW, PAPER, and loop arguments, keeping the code compact.

Color Bar Pattern (Lines 110–180)

The color bar routine loops x from 0 to 7, calling the subroutine at line 170 for each color. The subroutine prints a column of 22 rows (the full character height) using PRINT AT y,Z; PAPER x;" ", producing a four-character-wide colored stripe. After printing the stripe, it overlays the color name from the c$() array in INK 9 (transparent/contrast ink). The position variable Z is incremented by 4 after each call at line 140 (LET Z=Z+4), advancing the column position across the screen.

Note that this modifies Z away from zero during the color bar routine. Since the program restarts via RUN, Z is re-read from DATA each time, but within a single run of the color bar section Z doubles as the current column index — a somewhat fragile reuse of the variable.

Geometric Pattern Routines

Horizontal lines (line 210–230) use PLOT and DRAW 254,Z to draw lines across the full screen width, stepping y from 3 to 173 in steps of 17 — giving approximately 10 evenly spaced horizontal lines.

Vertical lines (lines 290–300) use PLOT x,3 and DRAW Z,170 to draw vertical lines, stepping x from 0 to 255 in steps of 17 — approximately 15 lines across the width.

The crosshatch pattern (line 350–360) simply jumps to line 200, executing horizontal lines first, then falls through via IF r=4 THEN GO TO 290 at line 240 to also draw the vertical lines — combining both sets.

The dot grid (lines 450–460) uses nested loops over both axes at step 17, plotting a single pixel at each grid intersection.

Border Subroutine

The subroutine at line 490 draws a rectangular border around the display using four DRAW commands from the origin: right 255, up 175, left 255, down 175. This is called before showing the menu to frame the screen.

Data Initialization

Line 40 dimensions c$(8, PI). Since PI ≈ 3.14159, Sinclair BASIC truncates this to 3, giving a string array of 8 strings of length 3 — exactly matching the three-character color abbreviations (“Blk”, “Blu”, “Red”, etc.) read from DATA in line 50.

Anomalies and Notable Points

• Line 160 uses RUN rather than GO TO 30 to return to the menu; this fully reinitializes variables, which is necessary because Z was mutated during the color bar routine.
• The REM labels (lines 110, 190, 270, 350, 430, 480) use numbers that hint at the intended line numbers but are not structurally significant.
• Line 100’s computed GO TO targets line 110 for choice 1, but the REM at line 110 means execution immediately falls through to line 120 — the REM lines act as harmless no-ops.
• The PAUSE Z at line 80 (with Z=0) is PAUSE 0, which on the Spectrum waits indefinitely for a key — though the program then separately reads INKEY$ in a loop at line 90, making this pause redundant with the explicit wait loop.
• Lines 270 and 350 are REM/redirect stubs; the actual vertical line code begins at 280–300, and the crosshatch redirect at 360 jumps to 200.