Pulse Modulation Techniques Questions and Answers

Q: How does a receiver know which slot belongs to which channel?

A frame synchronising pattern marks the start of each frame, and the receiver counts slots from there. Distinguishing channels by amplitude or width would consume the very parameter carrying the message. And connection order is known to the equipment but useless without a timing reference. Any scheme that multiplexes by position in a sequence needs a marker for where the sequence begins, and that marker is pure overhead worth paying. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Q: What is the consequence of losing frame synchronisation?

Losing synchronisation shifts the whole slot count, so every channel's samples are delivered to the wrong destination and the output is unintelligible across the board. The failure is not local to one channel. Amplitudes and the transmitter's rate are unaffected. This all-or-nothing behaviour is characteristic of position-based multiplexing and is why frame alignment recovery is designed to be fast and robust.

Q: Which multiplexing method suits a channel with a wide bandwidth but poor linearity?

Time division multiplexing keeps only one channel present at any instant, so non-linearity cannot mix the channels together - the intermodulation that plagues frequency division multiplexing simply has nothing to work on. Frequency division needs good linearity precisely because all channels coexist. Wavelength division is the optical analogue of frequency division. And code division shares both time and frequency. Whether channels coexist or take turns decides how much linearity a system demands.

Q: What is the main drawback of time division multiplexing compared with frequency division?

Time division's cost is a dependence on accurate timing and synchronisation from end to end, which is why its early adoption waited on cheap digital circuitry. Channel counts are large rather than limited. Linearity and intermodulation are frequency division's problems, not its own. Each multiplexing scheme trades one kind of engineering difficulty for another, and here the exchange is linearity for timing.

Q: What additional step distinguishes pulse code modulation from pulse amplitude modulation?

Quantisation is the step that makes the scheme digital: each sample is rounded to one of a finite set of levels and then encoded as a group of bits. Pre-filtering, multiplexing and synchronisation are all common to the analogue pulse schemes as well. That one step is what buys regeneration - a quantised sample can be recognised and rebuilt exactly, which no analogue pulse can.

Q: What is quantisation noise?

Quantisation noise is the rounding error between the true sample value and the discrete level chosen to represent it, and unlike channel noise it is created inside the coder and travels with the signal. Thermal noise, aliasing and timing jitter are separate and independent impairments. Quantisation noise is the price of the digital step, and it is the one impairment that regeneration cannot remove.

Q: Why is companding used in telephony coders?

Companding compresses before quantising and expands afterwards, so quiet passages get finer effective steps and the ratio holds up across a wide dynamic range. It does not reduce the bit rate, which stays at eight bits per sample. Aliasing is the filter's business and synchronisation is the frame's. Making the quantiser's resolution follow the signal's own statistics is a far better use of a fixed number of bits than spreading them uniformly. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: What is delta modulation?

Delta modulation sends a single bit per sample saying whether the signal has risen or fallen since the last one, which makes the coder and decoder extremely simple at the cost of a much higher sampling rate. It has nothing to do with differences between channels, second derivatives or sample reordering. Trading sample rate for word length is the whole idea, and it is worth taking whenever simplicity matters more than efficiency. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

21.

How does a receiver know which slot belongs to which channel?

A.From a synchronising pulse marking the frame start
B.From the amplitude assigned uniquely to each channel
C.From the width of the pulse each channel transmits
D.From the order in which the channels were connected

Answer: A

A frame synchronising pattern marks the start of each frame, and the receiver counts slots from there. Distinguishing channels by amplitude or width would consume the very parameter carrying the message. And connection order is known to the equipment but useless without a timing reference. Any scheme that multiplexes by position in a sequence needs a marker for where the sequence begins, and that marker is pure overhead worth paying. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

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22.

What is the consequence of losing frame synchronisation?

A.The sampling rate at the transmitter drifts out of tolerance
B.Every channel is decoded into the wrong one
C.Only the channel nearest the synchronising slot is affected
D.The pulse amplitudes fall below the detection threshold

Answer: B

Losing synchronisation shifts the whole slot count, so every channel's samples are delivered to the wrong destination and the output is unintelligible across the board. The failure is not local to one channel. Amplitudes and the transmitter's rate are unaffected. This all-or-nothing behaviour is characteristic of position-based multiplexing and is why frame alignment recovery is designed to be fast and robust.

Discuss in Forum

23.

Which multiplexing method suits a channel with a wide bandwidth but poor linearity?

A.Wavelength division multiplexing
B.Code division multiplexing
C.Time division multiplexing
D.Frequency division multiplexing

Answer: C

Time division multiplexing keeps only one channel present at any instant, so non-linearity cannot mix the channels together - the intermodulation that plagues frequency division multiplexing simply has nothing to work on. Frequency division needs good linearity precisely because all channels coexist. Wavelength division is the optical analogue of frequency division. And code division shares both time and frequency. Whether channels coexist or take turns decides how much linearity a system demands.

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24.

What is the main drawback of time division multiplexing compared with frequency division?

A.It cannot carry more than a handful of channels at once
B.It requires far greater linearity in every amplifier stage
C.It suffers intermodulation between channels sharing the path
D.It needs accurate timing throughout the system

Answer: D

Time division's cost is a dependence on accurate timing and synchronisation from end to end, which is why its early adoption waited on cheap digital circuitry. Channel counts are large rather than limited. Linearity and intermodulation are frequency division's problems, not its own. Each multiplexing scheme trades one kind of engineering difficulty for another, and here the exchange is linearity for timing.

Discuss in Forum

25.

What additional step distinguishes pulse code modulation from pulse amplitude modulation?

A.Quantisation of each sample to a discrete level
B.Filtering of the signal before it reaches the sampler
C.Multiplexing of several channels onto one pulse train
D.Synchronisation of the receiver to the frame structure

Answer: A

Quantisation is the step that makes the scheme digital: each sample is rounded to one of a finite set of levels and then encoded as a group of bits. Pre-filtering, multiplexing and synchronisation are all common to the analogue pulse schemes as well. That one step is what buys regeneration - a quantised sample can be recognised and rebuilt exactly, which no analogue pulse can.

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26.

What is quantisation noise?

A.The jitter in the timing of the sampling instants themselves
B.The error between a sample and its assigned level
C.The thermal noise added by the sampler's own circuitry
D.The aliasing produced when the sampling rate is too low

Answer: B

Quantisation noise is the rounding error between the true sample value and the discrete level chosen to represent it, and unlike channel noise it is created inside the coder and travels with the signal. Thermal noise, aliasing and timing jitter are separate and independent impairments. Quantisation noise is the price of the digital step, and it is the one impairment that regeneration cannot remove.

Discuss in Forum

27.

By how much does the signal-to-quantisation-noise ratio improve per extra bit?

A.About 12 dB
B.About 1.5 dB
C.About 6 dB
D.About 3 dB

Answer: C

Each additional bit doubles the number of levels and so halves the step size, improving the ratio by about 6 dB - the familiar rule that a sixteen-bit coder betters an eight-bit one by some forty-eight decibels. 3 dB corresponds to a doubling of power rather than of amplitude resolution. And the remaining figures follow from neither. A rule expressed per bit makes the whole design trade between bit rate and quality immediately visible. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

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28.

Why is companding used in telephony coders?

A.To reduce the bit rate the channel has to carry
B.To prevent aliasing of components above half the sampling rate
C.To synchronise the coder and decoder at the two ends of the link
D.To give small signals finer effective resolution

Answer: D

Companding compresses before quantising and expands afterwards, so quiet passages get finer effective steps and the ratio holds up across a wide dynamic range. It does not reduce the bit rate, which stays at eight bits per sample. Aliasing is the filter's business and synchronisation is the frame's. Making the quantiser's resolution follow the signal's own statistics is a far better use of a fixed number of bits than spreading them uniformly. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Discuss in Forum

29.

Which impairment does regeneration in a pulse code system remove?

A.Noise accumulated on the transmission path
B.Quantisation error introduced in the coder
C.Aliasing caused by an inadequate sampling rate
D.Aperture distortion from the flat-top sampling process

Answer: A

A regenerator recognises each pulse and transmits a clean replacement, so path noise is discarded rather than accumulated - the decisive advantage of digital transmission over long routes. Quantisation error, aliasing and aperture distortion are all created before transmission and are carried along faithfully by every regenerator. Regeneration removes what the channel added and nothing that the coder built in, which is exactly why coder design matters so much.

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30.

What is delta modulation?

A.Transmitting the samples in a different order each frame
B.Transmitting one bit per sample for the direction of change
C.Transmitting the difference between two adjacent channels
D.Transmitting the second derivative of the sampled waveform

Answer: B

Delta modulation sends a single bit per sample saying whether the signal has risen or fallen since the last one, which makes the coder and decoder extremely simple at the cost of a much higher sampling rate. It has nothing to do with differences between channels, second derivatives or sample reordering. Trading sample rate for word length is the whole idea, and it is worth taking whenever simplicity matters more than efficiency. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Discuss in Forum

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