Pulse Modulation Techniques Questions and Answers

Q: A coder is specified as follows. ParameterValue Sampling rate8 kHzWord length8 bitsCompandingA-law Which figure does the companding chiefly affect?

Companding redistributes the eight bits so that small signals get finer steps, which improves the ratio on quiet passages without touching the bit rate, the bandwidth limit or the frame capacity - those follow from the sampling rate and word length alone. Reading a specification table means asking which line each other line depends on, and here the companding line stands apart from the two that set the rate.

Q: Which of the following does not affect a pulse code system's bit rate? The companding law chosen The sampling rate used The word length per sample The number of channels multiplexed

The companding law changes how the available levels are distributed without changing how many bits describe them, so the rate is untouched. The other three multiply together to give the aggregate rate exactly. That is what makes companding such an attractive improvement: it is the one quality lever in the list that is free in bandwidth terms, and it is why every telephony standard uses it.

Q: Compressing a signal before quantising and expanding it afterwards is called ______.

The technique is companding, a contraction of compressing and expanding, and it gives small signals finer effective resolution at no cost in bit rate. Aliasing is the folding of high frequencies by under-sampling. Regeneration is the reconstruction of pulses along a route. And multiplexing is the sharing of one path among channels. Each term belongs to a different stage of the chain, and placing them correctly is most of understanding a coder's block diagram.

Q: Filling the gaps between one channel's samples with other channels' pulses is ______ multiplexing.

This is time division multiplexing, which exploits the idle time sampling leaves behind. Frequency division stacks channels in spectrum instead, code division distinguishes them by orthogonal codes while they share both time and frequency, and wavelength division is the optical form of frequency division. Every multiplexing scheme is named after the resource it divides, which makes the family easy to keep straight. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Q: What is the Nyquist interval?

The Nyquist interval is the time between samples at the Nyquist rate, so it is simply that rate's reciprocal - a quarter of a millisecond for a four kilohertz signal. Frame intervals, hold times and guard bands are all separate quantities that happen to be measured in the same units. Where a rate has a standard name, its reciprocal usually does too, and keeping the pair straight avoids a factor that is easy to invert. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: What does a reconstruction filter's cut-off frequency have to satisfy?

The filter must pass the whole signal band while rejecting the first spectral repeat, so its cut-off sits between the signal's top frequency and half the sampling rate. That window is exactly what oversampling widens, which is why a higher rate makes the filter cheaper. Cutting off above half the rate would admit the repeat, and cutting below the signal band would discard part of the signal. A requirement bracketed from both sides always has a window whose width is itself a design variable.

Q: Why does oversampling simplify the anti-alias filter?

Sampling well above Nyquist moves half the sampling rate far above the signal band, so the filter has a wide transition region and need not be steep. Word length is a separate matter, quantisation noise per unit bandwidth is unchanged by the rate alone, and the signal's bandwidth is a property of the signal. Buying an easier filter with a faster clock is a trade that became attractive as digital circuitry got cheap and precision analogue filters did not.

Q: The same link develops sharp clicks that grow more frequent as the path degrades. What does this indicate?

Clicks that track path quality are bit errors: a corrupted bit changes a sample's value abruptly and the decoder reproduces that jump faithfully. Quantisation noise is a steady hiss unaffected by the path. Slope overload distorts loud, fast passages rather than producing isolated clicks. And aliasing produces spurious tones. Whether a symptom follows the path or follows the signal is the single most informative observation about a digital link. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

81.

Consider the following pairs of parameter and what it limits.

Sampling rate - Highest signal frequency carried

Word length - Signal-to-quantisation-noise ratio

Guard time - Tolerance to pulse spreading

Pulse amplitude - Number of channels multiplexed

How many of the pairs given above are correctly matched?

A.Only two
B.All four
C.Only three
D.Only one

Answer: C

The first three are correctly paired and the fourth is not, so three match. The number of channels is limited by the frame length and the slot width, both time quantities; pulse amplitude affects the received signal-to-noise ratio and nothing about channel capacity. Each design parameter governs one property, and pairing them correctly is what lets a specification be traced back to the choice that produced it.

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82.

A coder is specified as follows.

Parameter	Value
Sampling rate	8 kHz
Word length	8 bits
Companding	A-law

Which figure does the companding chiefly affect?

A.The bit rate the channel is required to carry
B.The highest signal frequency the coder can accept
C.The number of channels that can share one frame
D.The ratio achieved on quiet passages

Answer: D

Companding redistributes the eight bits so that small signals get finer steps, which improves the ratio on quiet passages without touching the bit rate, the bandwidth limit or the frame capacity - those follow from the sampling rate and word length alone. Reading a specification table means asking which line each other line depends on, and here the companding line stands apart from the two that set the rate.

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83.

Which of the following does not affect a pulse code system's bit rate?

The companding law chosen

The sampling rate used

The word length per sample

The number of channels multiplexed

A.The companding law chosen
B.The sampling rate used
C.The word length per sample
D.The number of channels multiplexed

Answer: A

The companding law changes how the available levels are distributed without changing how many bits describe them, so the rate is untouched. The other three multiply together to give the aggregate rate exactly. That is what makes companding such an attractive improvement: it is the one quality lever in the list that is free in bandwidth terms, and it is why every telephony standard uses it.

Discuss in Forum

84.

Compressing a signal before quantising and expanding it afterwards is called ______.

A.multiplexing
B.companding
C.aliasing
D.regeneration

Answer: B

The technique is companding, a contraction of compressing and expanding, and it gives small signals finer effective resolution at no cost in bit rate. Aliasing is the folding of high frequencies by under-sampling. Regeneration is the reconstruction of pulses along a route. And multiplexing is the sharing of one path among channels. Each term belongs to a different stage of the chain, and placing them correctly is most of understanding a coder's block diagram.

Discuss in Forum

85.

Filling the gaps between one channel's samples with other channels' pulses is ______ multiplexing.

A.code division
B.wavelength division
C.time division
D.frequency division

Answer: C

This is time division multiplexing, which exploits the idle time sampling leaves behind. Frequency division stacks channels in spectrum instead, code division distinguishes them by orthogonal codes while they share both time and frequency, and wavelength division is the optical form of frequency division. Every multiplexing scheme is named after the resource it divides, which makes the family easy to keep straight. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Discuss in Forum

86.

What is the Nyquist interval?

A.The interval between two consecutive frame synchronising pulses
B.The time for which a sample-and-hold circuit retains its value
C.The width of the guard band separating two multiplexed channels
D.The reciprocal of the Nyquist rate

Answer: D

The Nyquist interval is the time between samples at the Nyquist rate, so it is simply that rate's reciprocal - a quarter of a millisecond for a four kilohertz signal. Frame intervals, hold times and guard bands are all separate quantities that happen to be measured in the same units. Where a rate has a standard name, its reciprocal usually does too, and keeping the pair straight avoids a factor that is easy to invert. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Discuss in Forum

87.

What does a reconstruction filter's cut-off frequency have to satisfy?

A.Above the signal band and below half the sampling rate
B.Above half the sampling rate but below the sampling rate
C.Exactly at the sampling rate used by the transmitter
D.Below the signal band, to remove residual pulse energy

Answer: A

The filter must pass the whole signal band while rejecting the first spectral repeat, so its cut-off sits between the signal's top frequency and half the sampling rate. That window is exactly what oversampling widens, which is why a higher rate makes the filter cheaper. Cutting off above half the rate would admit the repeat, and cutting below the signal band would discard part of the signal. A requirement bracketed from both sides always has a window whose width is itself a design variable.

Discuss in Forum

88.

Why does oversampling simplify the anti-alias filter?

A.The signal's own bandwidth is reduced by the higher rate
B.The transition band available becomes much wider
C.Fewer bits are then needed to reach the same quality
D.The quantisation noise falls in direct proportion to the rate

Answer: B

Sampling well above Nyquist moves half the sampling rate far above the signal band, so the filter has a wide transition region and need not be steep. Word length is a separate matter, quantisation noise per unit bandwidth is unchanged by the rate alone, and the signal's bandwidth is a property of the signal. Buying an easier filter with a faster clock is a trade that became attractive as digital circuitry got cheap and precision analogue filters did not.

Discuss in Forum

89.

A pulse code link produces intelligible speech with a persistent background hiss that does not vary with signal level. Which cause fits?

A.Excessive noise on the transmission path between regenerators
B.An anti-alias filter cutting off below the signal's top frequency
C.Too few quantisation levels in the coder
D.Loss of frame synchronisation at the receiving end

Answer: C

A hiss present at all levels and unaffected by them points at quantisation noise, which is generated inside the coder and rides along with the signal. Lost synchronisation would render the speech unintelligible rather than merely hissy. Path noise would be removed by the regenerators unless it were severe enough to cause errors, which sound like clicks. And a low filter cut-off would dull the speech rather than add noise. Asking whether an impairment varies with signal level separates coder faults from channel faults at once.

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90.

The same link develops sharp clicks that grow more frequent as the path degrades. What does this indicate?

A.Quantisation noise from too coarse a coder
B.Slope overload in the coder's tracking loop
C.Aliasing of components above half the sampling rate
D.Bit errors on the transmission path

Answer: D

Clicks that track path quality are bit errors: a corrupted bit changes a sample's value abruptly and the decoder reproduces that jump faithfully. Quantisation noise is a steady hiss unaffected by the path. Slope overload distorts loud, fast passages rather than producing isolated clicks. And aliasing produces spurious tones. Whether a symptom follows the path or follows the signal is the single most informative observation about a digital link. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

Discuss in Forum

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