EEEN 462 - ANALOGUE COMMUNICATION SYSTEMS
ANALOG Telephony Fundamentals Quiz
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Question 1: What is the typical voltage range for the ringing signal in analog telephony?
Correct Answer: c) 90-110 V AC
The ringing signal is an AC voltage typically between 90 and 110 volts, used to alert the subscriber of an incoming call. This higher voltage is necessary to ensure the bell or ringer operates reliably.
Question 2: What is the purpose of the hybrid coil in a telephone set?
Correct Answer: b) To separate the transmit and receive audio paths
The hybrid coil (or hybrid transformer) is used to convert the 2-wire local loop to the 4-wire circuit used in the telephone set, allowing simultaneous two-way communication without feedback (echo).
Question 3: What is the standard DC voltage supplied to the subscriber loop by the central office?
Correct Answer: c) 48 V
The central office battery supplies a nominal -48V DC to the subscriber loop (negative with respect to ground) to power the telephone and indicate off-hook status. This voltage is sufficient to overcome line resistance while being safe for users.
Question 4: Which signaling method is used for dialing in analog telephony when you hear pulses (clicks) as you dial?
Correct Answer: b) Pulse dialing
Pulse dialing (or loop disconnect) interrupts the loop current a number of times corresponding to the digit dialed (e.g., 10 pulses for '0'). This was the original dialing method before DTMF became common.
Question 5: What is the frequency range of the voice channel in analog telephony?
Correct Answer: b) 300-3400 Hz
The standard voice channel in analog telephony is bandlimited to 300-3400 Hz, which is sufficient for intelligible speech while conserving bandwidth. This range is often referred to as the telephone bandwidth.
Question 6: What is the function of the sidetone in a telephone set?
Correct Answer: b) To allow the speaker to hear their own voice in the receiver
Sidetone is the intentional feedback of a portion of the transmitter audio to the receiver, which helps the speaker regulate their voice level and provides a more natural conversation experience. Without sidetone, people tend to speak too loudly.
Question 7: In analog telephony, what does the term "off-hook" condition indicate?
Correct Answer: c) The handset is lifted (active)
Off-hook means the handset is lifted from the cradle, closing the loop and drawing current from the central office, indicating a request for service. This is the active state of the telephone.
Question 8: What is the primary purpose of the dial tone?
Correct Answer: b) To indicate that the system is ready to receive dialed digits
The dial tone is a continuous tone (typically 350 Hz and 440 Hz) that lets the caller know the telephone exchange is ready to accept dialing. It confirms that the line is working and the central office is prepared to process the call.
Question 9: Which of the following is the standard impedance of analog telephone lines?
Correct Answer: d) 600 ohms
Traditional analog telephone lines are designed to have a characteristic impedance of 600 ohms, which matches the impedance of the telephone set and the central office equipment for maximum power transfer and minimum signal reflection.
Question 10: What is the function of the carbon microphone in an analog telephone?
Correct Answer: a) To convert sound waves into electrical signals by varying resistance
The carbon microphone uses carbon granules that change resistance with pressure from sound waves, modulating the current in the telephone loop. This simple but effective technology was used in telephones for decades before being replaced by electret microphones.