This quiz covers fundamental concepts of stereo FM multiplexing, including modulation techniques, spectral components, and system design. Select the best answer for each question and check your understanding with detailed explanations.
1. What is the primary purpose of the pilot tone in stereo FM multiplexing?
- A) To provide a reference for the stereo demodulator
- B) To carry the difference (L-R) signal
- C) To improve signal-to-noise ratio
- D) To suppress carrier leakage
Correct Answer: A
The 19 kHz pilot tone serves as a reference signal for the stereo demodulator at the receiver. It's used to synchronize the demodulation process and regenerate the 38 kHz subcarrier needed to decode the stereo information.
2. What modulation technique is used for the (L-R) signal in stereo FM multiplexing?
- A) Frequency Modulation (FM)
- B) Double-Sideband Suppressed Carrier (DSB-SC)
- C) Single-Sideband Modulation (SSB)
- D) Amplitude Modulation (AM)
Correct Answer: B
The (L-R) difference signal is modulated using Double-Sideband Suppressed Carrier (DSB-SC) technique. This allows efficient use of bandwidth while enabling the receiver to demodulate the stereo information using a synchronized local oscillator.
3. What is the frequency of the pilot tone in stereo FM broadcasting?
- A) 15 kHz
- B) 19 kHz
- C) 38 kHz
- D) 67 kHz
Correct Answer: B
The pilot tone is transmitted at exactly 19 kHz. This frequency is precisely half of the 38 kHz subcarrier frequency, allowing receivers to easily double the frequency to regenerate the subcarrier needed for demodulation.
4. What is the frequency range of the DSB-SC modulated (L-R) signal in the stereo multiplex?
- A) 0-15 kHz
- B) 19-23 kHz
- C) 23-53 kHz
- D) 38-53 kHz
Correct Answer: C
The DSB-SC modulated (L-R) signal occupies the frequency range from 23 kHz to 53 kHz. This places it above the (L+R) signal (0-15 kHz) and the pilot tone (19 kHz), allowing for efficient spectrum utilization.
5. Why is DSB-SC used for the (L-R) signal instead of standard AM?
- A) To save power by suppressing the carrier
- B) To reduce bandwidth requirements
- C) To improve frequency response
- D) To simplify receiver design
Correct Answer: A
DSB-SC is used because it suppresses the carrier, saving significant transmission power. Since the carrier contains no information but consumes most of the power in standard AM, suppressing it allows more power to be allocated to the sidebands containing the actual (L-R) information.
6. How is the 38 kHz subcarrier generated at the receiver?
- A) By filtering the pilot tone
- B) By using a local oscillator
- C) By frequency doubling the 19 kHz pilot tone
- D) By dividing the main carrier frequency
Correct Answer: C
The receiver extracts the 19 kHz pilot tone and then doubles its frequency to generate the 38 kHz subcarrier needed to demodulate the DSB-SC (L-R) signal. This ensures perfect synchronization with the transmitter's subcarrier.
7. What is the purpose of pre-emphasis in FM broadcasting?
- A) To reduce high-frequency noise
- B) To compress the dynamic range
- C) To limit bandwidth
- D) To improve stereo separation
Correct Answer: A
Pre-emphasis boosts higher audio frequencies before transmission to improve the signal-to-noise ratio. At the receiver, de-emphasis reduces these frequencies by the same amount, thereby reducing high-frequency noise that would otherwise be prominent.
8. What is the maximum frequency deviation for stereo FM broadcasts in the United States?
- A) ±25 kHz
- B) ±50 kHz
- C) ±75 kHz
- D) ±100 kHz
Correct Answer: C
In the United States, the maximum frequency deviation for FM broadcasts is ±75 kHz. This deviation limit applies to the combined main channel, stereo subchannel, and any SCA (Subsidiary Communications Authorization) channels.
9. How are the left and right channels recovered from the (L+R) and (L-R) signals?
- A) By matrix addition and subtraction
- B) By frequency discrimination
- C) By envelope detection
- D) By phase comparison
Correct Answer: A
The left and right channels are recovered through a simple matrixing process:
Left channel = (L+R) + (L-R) = 2L
Right channel = (L+R) - (L-R) = 2R
This process requires both the (L+R) and demodulated (L-R) signals.
10. What is the purpose of the 67 kHz SCA (Subsidiary Communications Authorization) channel?
- A) To provide additional stereo information
- B) To transmit background music for businesses
- C) To improve frequency stability
- D) To reduce interference between stations
Correct Answer: B
The SCA channel at 67 kHz is used to transmit additional content such as background music for businesses, reading services for the visually impaired, or other subscription-based services. It is modulated onto a subcarrier and does not interfere with the main stereo program.