What type of signaling uses pairs of audio tones?

• A. CTCSS
• B. DTMF
• C. PL tone
• D. Packet radio

Answer: (B)

The correct answer is DTMF, which stands for Dual-Tone Multi-Frequency. DTMF utilizes pairs of audio tones generated at specific frequencies. Each key on a DTMF keypad produces a unique combination of two audio tones—the row tone and the column tone—which allows for efficient signaling. This method is commonly used in telephone systems to send digits, such as when you press the numbers on a telephone keypad.

CTCSS (Continuous Tone-Coded Squelch System) uses a single sub-audible tone to control the squelch function of a receiver and does not involve pairs of tones. Similarly, PL tone, which stands for Private Line, is another name for CTCSS and functions in a similar manner. Packet radio, on the other hand, involves the transmission of digital data packets and does not utilize audio tones in the same way as DTMF. Thus, DTMF is specifically characterized by its use of pairs of audio tones for signaling purposes.