Avionics MCQs – Pulse Code Modulation
1 - Question
Which of the following type of multiplexing uses pulse code modulation?
a) Frequency division multiplexing
b) Time division multiplexing
c) Code division multiplexing
d) Amplitude limited multiplexing
Explanation: The most popular form of TDM uses pulse-code modulation (PCM), in which multiple channels of digital data are transmitted in serial form. Each channel is assigned a time slot in which to transmit one binary word of data. The data streams from the various channels are interleaved and transmitted sequentially.
2 - Question
Which of the following statements is true with respect to PCM?
a) The parallel binary data is converted into serial before transmission
b) Analog data is transmitted directly
c) Analog signal is amplified before transmission
d) The analog signal is converted into parallel binary data before transmission
Explanation: The analog signal is converted into a digital signal. Since the converted digital signal is parallel bits of data, it has to be converted to serial before it can be multiplexed and transmitted.
3 - Question
Which of the following is false with respect to PCM?
c) Resistant to noise
d) Not easily recoverable
Explanation: When signals have been degraded because of noise, attenuation, or distortion, all the receiver has to do is to determine whether a pulse was transmitted. Amplitude, width, frequency, phase shape, and so on do not affect reception. Thus PCM signals are easily recovered and rejuvenated, no matter what the circumstances.
4 - Question
A special PCM system uses 16 channels of data, one whose purpose is identification (ID) and synchronization. Find (a) the number of available data channels.
Explanation: 16 (total no. of channels) – 1 (channel used for ID) = 15 (for data).
5 - Question
A special PCM system uses 16 channels of data, one whose purpose is an identification (ID) and synchronization. The word length is 6 bits. Find the number of bits per frame.
Explanation: Here, Data channels = 16 and word length is 6 bits. Bits per frame = 6 x 16 = 96.
6 - Question
A special PCM system uses 16 channels of data, one whose purpose is an identification (ID) and synchronization. The sampling rate is 3.5 kHz. The word length is 6 bits. Find the serial data rate.
Explanation: Serial data rate = sampling rate x no. bits/frame = 3.5 kHz x 96 = 336 kHz.
7 - Question
All local and long-distance connections are digital.
Explanation: All modern telephone systems use digital transmission via PCM and TDM. The only place where analog signals are still used is in the local loop—the connection between a telephone company’s central office (CO) and the subscriber’s telephone, known as the customer premises equipment (CPE). All local and long-distance connections are digital.
8 - Question
In a TDD one channel is sufficient for transmission.
Explanation: Time-division duplexing (TDD) means that signals are transmitted simultaneously on a single channel by interleaving them in different time slots. Each time slot may contain one data word, such as 1 byte from an A/D converter or a D/A converter. As long as the serial data rate is high enough, a user will never know the difference. The primary benefit of TDD is that only one channel is needed. It saves spectrum space and cost.