TV Channel

This program calculates the frequency parameters for US television channels, covering VHF (channels 2–13), UHF (channels 14–83), and cable TV bands. Given a channel number or letter-coded cable designator, it computes the lower and upper band-edge frequencies, picture carrier, color subcarrier (3.58 MHz offset), sound carrier (4.5 MHz offset), local oscillator frequency, and image frequencies assuming a 41–47 MHz IF strip. The program branches on whether the input is numeric or alphabetic to handle over-the-air versus cable channels, and prompts the user whether a given channel is UHF or cable when ambiguity exists. REM statements at lines 700–730 document an alternate formulation for converter-based setups with a different IF output channel.

Program Structure

The program is organized as a top-level input/dispatch loop with frequency calculation in a shared computation block. Execution flows as follows:

1. Line 2: Initialize C$="Y" as a sentinel to skip the INPUT at line 40 on the very first pass, using the value already in A$ (or whatever the runtime provides).
2. Lines 10–50: Clear screen, print header, and accept a channel identifier. If C$ is not a letter (i.e., it came from INKEY$ being a digit or “Y”), the program skips the INPUT LINE at line 40 and uses C$ directly as A$.
3. Lines 60–200: Determine channel type. If the first character’s CODE is above 64 (ASCII ‘A’), it’s a letter-coded cable channel; otherwise VAL A$ gives a numeric channel, and a chain of range checks dispatches to the correct base-channel/base-frequency pair.
4. Lines 300–360: Handle letter-coded cable channels (A–Z maps to channels 14–39 using CODE - 51), classifying them as Mid Band or Super Band.
5. Lines 400–540: Shared frequency computation and display block.
6. Lines 550–580: “More?” prompt using INKEY$ polling; loops back to line 10 unless “N” is pressed.
7. Line 600: Error handler for invalid channels.
8. Lines 700–730: REM-commented alternative formulas for converter-based IF setups.
9. Line 9999: GO TO VAL "80" — targets non-existent line 80, a known memory-optimization idiom using VAL for the literal.

Channel Band Dispatch Table

Channel Range	Band	Base Channel (BC)	Base Freq (BF, MHz)
2–4	VHF Low	2	54
5–6	VHF Low (high)	5	76
7–13	VHF High	7	174
14–83	UHF	14	470
A–M (cable Mid Band)	Cable Mid	14	120
N–Z (cable Super Band)	Cable Super	23	216

Frequency Calculation Block

Lines 400–540 contain the core math. All US analog TV channels are 6 MHz wide, so:

• LB = BF + (C - BC) * 6 — lower band edge in MHz
• UB = LB + 6 — upper band edge
• FV = LB + 1.25 — picture (video) carrier
• FC = FV + 3.58 — color subcarrier (NTSC 3.579545 MHz offset from picture carrier)
• FA = FV + 4.5 — sound carrier (standard 4.5 MHz FM intercarrier offset)
• FL = LB + 47 — local oscillator frequency, placing the picture carrier at 45.75 MHz IF
• IL = UB + 41*2 and IU = LB + 47*2 — image frequency range for a 41–47 MHz IF strip

Input Handling Idioms

Line 2 pre-loads C$="Y" so that the conditional at line 31 (IF C$<"A") is false on the first run, causing the program to fall through to INPUT LINE A$. On subsequent iterations, C$ holds the INKEY$ result from the “More?” prompt; if the user pressed a digit key, the condition routes directly to line 50, reusing that keypress as the channel. This is a compact but somewhat fragile reuse of the loop variable.

Line 31 uses IF C$<"A" to distinguish digit characters (codes 48–57) from letters (codes 65+). Since “Y” has a code of 89, the first-run sentinel correctly falls through to INPUT LINE.

INKEY$ polling loops appear at lines 145 and 560, the standard busy-wait keypress idiom.

Cable Channel Letter Mapping

At line 310, the formula C = A - 51 converts a letter’s character code to a cable channel number. For ‘A’ (code 65), this gives channel 14; for ‘Z’ (code 90), channel 39. The boundary check at line 300 (IF A>90) limits valid letters to A–Z. The Mid/Super Band boundary is set at cable channel 23 (letter ‘N’, code 78), matching the standard US cable TV band plan.

Bugs and Anomalies

• Line 150 checks IF B$(1)="C" for “Cable,” but numeric channels 5–13 also reach line 140‘s UHF/Cable prompt. Pressing anything other than “C” routes to UHF handling regardless of what letter was typed, with no validation.
• The Mid Band cable path at line 360 sets BC=14 and BF=120 but does not include a GO TO 400; it falls through naturally since line 400 is the next line. This works correctly but could be confused for a missing branch.
• Line 440: IL = UB + 41*2 and IU = LB + 47*2. The image frequency for a superheterodyne receiver is LO + IF, so image = signal + 2×IF. Using upper-band-edge plus 2×41 for the lower image limit and lower-band-edge plus 2×47 for the upper image limit is intentionally conservative (worst-case image window), not an error.
• Line 9999‘s GO TO VAL "80" targets line 80, which does not exist; the program never normally reaches this line.

REM Documentation

Lines 700–730 contain detailed REM comments explaining how to adapt lines 430–510 for a cable converter box scenario where the receiver’s IF is tuned to a specific output channel rather than the standard 41–47 MHz strip. This is unusually thorough inline documentation for a BASIC program of this length.