Tone Dialer

Tone Dialer generates DTMF-style dual-tone multi-frequency signals through the SOUND command to dial telephone numbers from the keyboard or from stored memory. The program stores up to 20 phone numbers with 10-character names in two DIM arrays, `a$` (numbers, 30 chars each) and `b$` (names, 10 chars each). Digits 0–9, `#`, and `*` each map to a pair of low (`l`) and high (`h`) frequency values at lines 1000–1011, which are fed into the SOUND command with specific register settings for tone generation. A “p” character can be inserted into a number to introduce a roughly 1.5-second pause, useful for PBX access codes. The program supports manual dialing, redial of the last number, memory storage and recall, and saving the program with stored numbers to tape via `SAVE x$ LINE 12`.

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Program Structure

The program is organized into a menu-driven main loop and several subroutine blocks:

• Lines 5–19: Initialization — arrays, border/paper/ink, splash screen, instruction display, variable setup.
• Lines 20–40: Main input loop — reads a keypress and dispatches to manual dialing, memory dial, store, redial, or clear.
• Lines 100–140: SOUND output subroutine — plays a dual-tone and returns or loops.
• Lines 1000–1011: Frequency lookup table — each line sets l (low frequency register value) and h (high frequency register value) then falls through to line 100.
• Lines 2000–2050: Memory list display subroutine and dial-from-memory entry point.
• Lines 3000–3080: Phone number storage routine.
• Lines 4000–4030: Redial routine — iterates over r$ character by character.
• Lines 5000–5030: Save-to-tape routine.
• Line 9999: CLEAR then save the program shell without data.

Array Storage

Two fixed-dimension string arrays hold the phone book:

Array	Dimensions	Purpose
a$(20,30)	20 entries × 30 chars	Phone number strings
b$(20,10)	20 entries × 10 chars	Name/label strings

Because these are fixed-length string arrays, all entries are always exactly padded to their declared length with spaces. This is why the vacancy check at line 2035 compares a$(j) against a 30-space string literal, and the check at line 2120 compares against an 11-space string — the latter is a bug: the comparison string is only 11 characters long rather than the correct 30, so the “NO # AT” branch will never trigger correctly for truly empty slots.

Tone Generation

DTMF tones are produced via the SOUND command using the AY-3-8912 sound chip registers. Line 100 writes to registers 0, 1, 2, 3, 7, 8, and 9, setting two tone channels with separate period values (l for the low tone on channel A, h for the high tone on channel B) along with volume and mixer settings. Line 130 silences both channels. The 12 DTMF tones map as follows:

Key	l (reg 0/1)	h (reg 2/3)
0	117	82
1	158	91
2	158	82
3	158	75
4	143	91
5	143	82
6	143	75
7	129	91
8	129	82
9	129	75
#	117	75
*	117	91

Dispatch Technique

For digit keys, line 40 uses GO TO VAL n$+1000 to compute the target line number arithmetically — VAL "0" through VAL "9" yield 0–9, which are added to 1000 to land directly on the correct frequency-setup line. Similarly, the redial loop at line 4015 uses GO SUB 1000+VAL r$(i). This is a clean and efficient dispatch idiom that avoids a chain of IF statements for each digit.

Redial Buffer

Every dialed character (digit, #, *, or p) is appended to r$ at line 35. Pressing r replays the entire string character by character through the redial loop at lines 4005–4020. Pressing c goes back to line 12 (the CLS/menu display), but notably does not reset r$ — the clear only refreshes the display. The actual r$="" initialization is at line 18, executed only once at startup. When dialing from memory, r$ is loaded with the stored number at line 2140, so redial also works after a memory dial.

Key BASIC Idioms

• PAUSE 5 at line 120 controls the inter-tone gap duration (approximately 100 ms).
• IF INKEY$ <>"" THEN GO TO 110 at line 110 waits for any held key to be released before timing the tone, preventing runaway dialing.
• POKE 23609,10 at line 19 sets the keyboard repeat delay system variable.
• INPUT ... LINE b$(Q) at line 3040 captures the full input line including spaces into the fixed array slot.
• SAVE x$ LINE 12 at line 5020 saves the entire program (with the populated a$ and b$ arrays) and arranges for it to auto-run at line 12 on load, preserving the stored phone book.

Bugs and Anomalies

• Case mismatch at line 28: The main loop reads into n$ (lowercase), but line 28 tests N$ (uppercase). On the Spectrum, N$ and n$ are the same variable — BASIC is case-insensitive for variable names — so this works correctly, but the inconsistency is confusing.
• Empty-slot check at line 2120: The comparison string " " is 11 spaces, not 30, so it will never match the 30-space padded empty slot in a$(Q), making the “NO # AT” guard ineffective.
• Vacancy counter at line 2035: The 30-space comparison string appears correct for detecting empty a$ slots, but the count v is used at line 2100 to detect “no numbers stored yet” only when v=20. If even one slot has been filled, the all-empty check is bypassed correctly.
• Line 105 dead code: IF n$="r" OR n$="m" THEN PRINT r$(i); — when dialing manually, n$ will be a digit, #, or *, so this branch is never taken from the manual path. From the redial/memory paths, the subroutine is called via GO SUB and the condition may apply, but i is only defined inside the FOR loop at line 4005, making this potentially unreliable outside that context.
• Memory dial busy-redial flow: Line 2150 jumps to 4005 (inside the redial FOR loop) with r$ set to the stored number, bypassing the “REDIAL” print at line 4003. The instruction text on line 15 mentions pressing r after a busy signal during memory dial, which then triggers the standard redial of r$.