Audioscan

Audioscan is a signal visualization program that reads audio input from the ear (tape input) socket and displays it as a waveform on screen in one of three modes: line graph, bar graph, or point graph. A short machine code routine is POKEd into memory at address 65368 and called via USR to sample the audio signal, returning a value proportional to the input level. The main scan loop iterates across all 256 horizontal pixel columns, plotting each sample and optionally drawing connecting lines or bars using DRAW with INK color attributes. A freeze feature lets the user pause the display and choose to return to the menu, restart the scan, or end the program.

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Program Analysis

Program Structure

The program is organized into a small number of functional blocks. Lines 100–120 contain the two drawing subroutines for line and bar graph modes. Lines 200–275 form the main menu and scan loop. Lines 300–330 handle the freeze/pause menu. Lines 400–425 handle the machine code data and installation.

Lines	Purpose
2–3, 100	REM comments (title, source, program name)
102	Commented-out DEF FN (unused in final program)
103–105	Initialization: install MC, jump to menu
110	Subroutine: line graph (DRAW between successive samples)
120	Subroutine: bar graph (DRAW vertical bar downward)
200–275	Menu display, option selection, main scan loop
300–330	Freeze display, m/r/e key handler
400–425	Machine code DATA, POKE installation at 65368

Machine Code Routine

The 18-byte machine code routine is loaded into RAM starting at address 65368 (near the top of the 64K address space) using a simple READ/POKE loop in lines 410–420. The routine is called via USR tone in the scan loop at line 255. Disassembling the DATA values reveals the routine samples the ear port (IN A,(254), i.e., 219,254), tests bit 6 of the result (203,119 = BIT 6,A), conditionally increments a counter register, loops, and returns the accumulated count in BC — providing a crude analog measurement of the audio signal level over a fixed number of samples. The 55 (SCF) and 75 (LD C,E) bytes suggest result packing, and 17,0,0 loads DE with zero as a loop counter.

Offset	Byte	Mnemonic (approx.)
0	1	LD BC,nn (lo)
1	0	(BC low byte)
2	255	(BC high byte = 255)
3	17	LD DE,nn
4	0	(DE=0)
5	0
6	219	IN A,(n)
7	254	port 254 (ear/keyboard)
8	203	CB prefix
9	119	BIT 6,A
10	32	JR NZ,+1
11	1	(offset)
12	19	INC DE
13	16	DJNZ (loop BC times)
14	247	(offset -9)
15	55	SCF
16	75	LD C,E
17	201	RET

Scan and Display Loop

The main scan loop (lines 255–270) iterates n from 0 to 255, corresponding to all 256 horizontal pixel columns. For each column, USR tone samples the audio and returns a level l. A point is PLOTted at (n, INT(l/1.5)), scaling the raw machine code result into the screen’s 176-pixel vertical range. For modes 1 and 2, the appropriate subroutine is called via the computed GO SUB at line 260: GO SUB 100+(q*10), which resolves to line 110 (line graph) or 120 (bar graph). Mode 3 (point graph) skips the GO SUB entirely, leaving just the PLOT.

Drawing Subroutines

The line graph subroutine at line 110 uses DRAW INK 1; to connect the previous sample position (x,y) to the current one with a colored line, then updates x and y for the next iteration. The bar graph subroutine at line 120 uses DRAW INK 2;0,-l/2 to draw a vertical bar downward from the plotted point. Both subroutines use INK color attributes (1=blue, 2=red) to visually distinguish the modes. The state variables x and y are initialized to 0 at line 250 before each full scan pass.

Freeze and Control Flow

Pressing the space bar during scanning sets i$=" " at line 265, branching to the freeze menu at line 300. The freeze menu uses PAUSE 0 to halt and then checks INKEY$ for m/r/e. Pressing m executes RUN to restart from the beginning (re-displaying the option menu). Pressing r jumps to line 270 (NEXT n), which effectively resumes the current scan mid-loop, though since n is at 255 this will immediately roll over to a new scan pass via line 275. Pressing e halts the program with STOP.

Notable Techniques and Anomalies

• The computed GO SUB 100+(q*10) at line 260 is an elegant dispatch idiom, avoiding IF/THEN chains for mode selection.
• Line 102 contains a DEF FN statement preceded by REM, making it a comment rather than an active definition — the function is never used in the program as written.
• The machine code address 65368 is hardcoded in line 410; this works because the routine fits comfortably in the high RAM area, but the program makes no attempt to verify available memory.
• The division by 1.5 used in both the PLOT and DRAW statements scales the raw sample value (0–255) to fit within the 176-pixel screen height, though at the extremes some clipping may occur if the signal is strong.
• The STOP at line 425 acts as a guard to prevent accidental fall-through into the DATA/installation block if the program were somehow re-entered after the GO SUB at line 103 returns.
• Pressing r to restart the scan jumps to line 270 (NEXT n), not to line 255; since the FOR loop variable n is at 255 when frozen, this causes an immediate loop completion and a new pass begins at line 275, which is functionally correct but slightly indirect.