Pulse Modulation Techniques Questions and Answers

Q: Which parameter of a multiplexer's switch matters most for crosstalk?

Crosstalk is signal appearing where it should not, so the switch's isolation in the open state is what limits it. On-resistance affects amplitude accuracy and settling rather than isolation, current rating is a power consideration, and packaging is mechanical. Whenever a specification is about unwanted transfer rather than wanted transfer, the parameter to look up is the one describing the device's off state.

Q: Why did digital telephony displace analogue pulse schemes on long routes?

On a long route the decisive property is that each regenerator rebuilds the pulses cleanly, so noise does not accumulate hop by hop - an analogue pulse train degrades steadily with every amplifier. Digital coding needs more bandwidth rather than less, its coders were initially dearer, and it demands far more synchronisation. A scheme can win decisively on one property while losing on three others, which is why the application has to be named before any comparison is meaningful.

Q: In what application does pulse width modulation remain dominant today?

Power conversion and motor drives use it constantly, because a duty cycle controls average power directly and a switching device is either fully on or fully off, which is where its efficiency comes from. Long-distance voice and broadcast audio went digital. And deep space telemetry uses coded digital schemes with heavy error correction. A modulation scheme displaced from communication can remain the right answer in an entirely different field, and width modulation is the clearest example.

Q: Which property makes pulse schemes attractive for telemetry from many sensors?

Time division lets one transmitter and one path serve dozens of sensors, each occupying its own slot, which is decisive when weight and power are limited. Identifying channels by amplitude would consume the parameter carrying the data. Synchronisation is required rather than dispensed with. And every channel needs anti-alias filtering whatever its bandwidth, since interference can be faster than the sensor. Sharing one expensive resource among many cheap sources is the economic argument behind all multiplexing.

Q: Why is an eight kilohertz sampling rate used for a 3.4 kilohertz telephone channel?

Nyquist would allow 6.8 kHz, but a filter cannot fall from passband to stopband instantly, so the extra 1.2 kHz gives its transition band somewhere to live. It is not exactly twice the limit, the frame rate follows from the sampling rate rather than setting it, and quantisation noise depends on word length rather than sampling rate. Standard values almost always sit a little clear of a theoretical bound, and the margin is usually there for a component that cannot be ideal.

Q: What is the effect of increasing the number of quantisation levels?

More levels means finer steps and so less quantisation noise, but each sample needs more bits and the rate rises accordingly - six decibels bought for one bit per sample. There is no way to have the improvement without the rate increase in a uniform coder, which is exactly why companding exists: it redistributes a fixed number of bits rather than adding more. Every improvement in a digital coder is paid for in bit rate, and the only question is what the bits buy.

Q: Which coder design choice improves quality without raising the bit rate?

Non-uniform spacing, which companding implements, puts fine steps where the signal spends most of its time and coarse steps where it rarely goes, improving the perceived ratio at no cost in bits. Doubling the levels and raising the sampling rate both increase the rate. And a regenerator addresses path noise rather than coder quality. Matching a quantiser to the signal's own statistics is free in bandwidth terms and is the first place to look for an improvement.

Q: Why does companding help speech in particular?

Speech has a large dynamic range but the amplitude distribution is heavily weighted towards small values, so fine steps down there are used far more often than fine steps at the top. Its bandwidth is modest, silence suppression is a separate technique, and no signal tolerates aliasing. A coder tuned to the statistics of the particular signal always beats a general-purpose one, and speech's amplitude statistics are unusually well suited to the trade.

Q: A delta modulator's step is halved while its clock is doubled. Its maximum followable slope

The slope limit is the product of step and clock rate:\[ \begin{aligned} \text{before} &: \delta f_{s} \\ \text{after} &: \tfrac{\delta}{2} \times 2f_{s} = \delta f_{s} \end{aligned} \]so it is unchanged, while the granular noise improves because the steps are smaller. That is precisely the trade a faster clock buys: the same tracking ability with finer resolution, at the cost of bit rate. Recognising which combination of parameters a limit depends on shows immediately which changes cancel.

71.

Which parameter of a multiplexer's switch matters most for crosstalk?

A.Its isolation when open
B.Its resistance when closed
C.Its rated continuous current
D.Its physical package size

Answer: A

Crosstalk is signal appearing where it should not, so the switch's isolation in the open state is what limits it. On-resistance affects amplitude accuracy and settling rather than isolation, current rating is a power consideration, and packaging is mechanical. Whenever a specification is about unwanted transfer rather than wanted transfer, the parameter to look up is the one describing the device's off state.

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72.

Why did digital telephony displace analogue pulse schemes on long routes?

A.It needs no synchronisation between the two ends of a link
B.Regeneration stops noise accumulating with distance
C.Its bandwidth requirement is lower per voice channel
D.Its coders and decoders are cheaper than analogue ones

Answer: B

On a long route the decisive property is that each regenerator rebuilds the pulses cleanly, so noise does not accumulate hop by hop - an analogue pulse train degrades steadily with every amplifier. Digital coding needs more bandwidth rather than less, its coders were initially dearer, and it demands far more synchronisation. A scheme can win decisively on one property while losing on three others, which is why the application has to be named before any comparison is meaningful.

Discuss in Forum

73.

In what application does pulse width modulation remain dominant today?

A.Broadcast distribution of high-quality audio programmes
B.Satellite telemetry from deep space missions
C.Switching power conversion and motor control
D.Long-distance voice transmission over trunk routes

Answer: C

Power conversion and motor drives use it constantly, because a duty cycle controls average power directly and a switching device is either fully on or fully off, which is where its efficiency comes from. Long-distance voice and broadcast audio went digital. And deep space telemetry uses coded digital schemes with heavy error correction. A modulation scheme displaced from communication can remain the right answer in an entirely different field, and width modulation is the clearest example.

Discuss in Forum

74.

Which property makes pulse schemes attractive for telemetry from many sensors?

A.Each channel may use a different pulse amplitude for identity
B.The pulses need no synchronisation reference at the receiver
C.Slow sensors can be sampled without any anti-alias filtering
D.Many channels share one transmitter by taking turns

Answer: D

Time division lets one transmitter and one path serve dozens of sensors, each occupying its own slot, which is decisive when weight and power are limited. Identifying channels by amplitude would consume the parameter carrying the data. Synchronisation is required rather than dispensed with. And every channel needs anti-alias filtering whatever its bandwidth, since interference can be faster than the sensor. Sharing one expensive resource among many cheap sources is the economic argument behind all multiplexing.

Discuss in Forum

75.

Why is an eight kilohertz sampling rate used for a 3.4 kilohertz telephone channel?

A.It leaves room for the anti-alias filter to roll off
B.It is exactly twice the channel's upper frequency limit
C.It matches the frame rate of the multiplex hierarchy
D.It minimises the quantisation noise the coder produces

Answer: A

Nyquist would allow 6.8 kHz, but a filter cannot fall from passband to stopband instantly, so the extra 1.2 kHz gives its transition band somewhere to live. It is not exactly twice the limit, the frame rate follows from the sampling rate rather than setting it, and quantisation noise depends on word length rather than sampling rate. Standard values almost always sit a little clear of a theoretical bound, and the margin is usually there for a component that cannot be ideal.

Discuss in Forum

76.

What is the effect of increasing the number of quantisation levels?

A.No change to either the noise or the bit rate required
B.Less quantisation noise but a higher bit rate
C.Less quantisation noise at an unchanged bit rate
D.A lower bit rate but greater sensitivity to channel noise

Answer: B

More levels means finer steps and so less quantisation noise, but each sample needs more bits and the rate rises accordingly - six decibels bought for one bit per sample. There is no way to have the improvement without the rate increase in a uniform coder, which is exactly why companding exists: it redistributes a fixed number of bits rather than adding more. Every improvement in a digital coder is paid for in bit rate, and the only question is what the bits buy.

Discuss in Forum

77.

Which coder design choice improves quality without raising the bit rate?

A.Raising the sampling rate above twice the signal bandwidth
B.Adding a second regenerator midway along the transmission path
C.Non-uniform spacing of the quantisation levels
D.Doubling the number of uniformly spaced levels available

Answer: C

Non-uniform spacing, which companding implements, puts fine steps where the signal spends most of its time and coarse steps where it rarely goes, improving the perceived ratio at no cost in bits. Doubling the levels and raising the sampling rate both increase the rate. And a regenerator addresses path noise rather than coder quality. Matching a quantiser to the signal's own statistics is free in bandwidth terms and is the first place to look for an improvement.

Discuss in Forum

78.

Why does companding help speech in particular?

A.Speech occupies a wider bandwidth than most signals do
B.Speech contains long silences that need no coding at all
C.Speech is more tolerant of aliasing than other signals are
D.Speech spends most of its time at low amplitudes

Answer: D

Speech has a large dynamic range but the amplitude distribution is heavily weighted towards small values, so fine steps down there are used far more often than fine steps at the top. Its bandwidth is modest, silence suppression is a separate technique, and no signal tolerates aliasing. A coder tuned to the statistics of the particular signal always beats a general-purpose one, and speech's amplitude statistics are unusually well suited to the trade.

Discuss in Forum

79.

A companded coder gives \( 38 \ \mathrm{dB} \) of signal-to-noise ratio over a \( 40 \ \mathrm{dB} \) input range. A uniform coder of the same word length would give \( 38 \ \mathrm{dB} \) only

A.At full amplitude
B.Over the same forty decibel range of input
C.At the lowest amplitude within that range
D.At no amplitude within the range at all

Answer: A

A uniform quantiser's step is fixed, so its ratio falls decibel for decibel as the signal falls:

\[ \begin{aligned} \text{at full scale} &: 38 \ \mathrm{dB} \\ \text{40 dB below} &: -2 \ \mathrm{dB} \end{aligned} \]

so it reaches 38 dB only at full amplitude. Companding's achievement is holding the ratio roughly constant across the range instead, which is what a listener actually notices.

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80.

A delta modulator's step is halved while its clock is doubled. Its maximum followable slope

A.Is quadrupled
B.Is unchanged
C.Is halved
D.Is doubled

Answer: B

The slope limit is the product of step and clock rate:

\[ \begin{aligned} \text{before} &: \delta f_{s} \\ \text{after} &: \tfrac{\delta}{2} \times 2f_{s} = \delta f_{s} \end{aligned} \]

so it is unchanged, while the granular noise improves because the steps are smaller. That is precisely the trade a faster clock buys: the same tracking ability with finer resolution, at the cost of bit rate. Recognising which combination of parameters a limit depends on shows immediately which changes cancel.

Discuss in Forum

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