Which modulation technique is preferred for medium-speed communication range up to 1200 to 2400 bits per second?
- (a)Amplitude modulation
- (b)Frequency modulation
- (c)Phase modulation
- (d)None of these
Correct — B, Frequency modulation.
Modulation impresses a message on a carrier wave by varying one of its three parameters — amplitude, frequency or phase. When the message is digital, the same three become amplitude, frequency and phase shift keying.
The classification this question is set from ranks these techniques by data rate: the low band goes to amplitude modulation, the 1200 to 2400 bits per second band to frequency modulation, and the rates above it to phase modulation.
The idea to carry away: frequency shift keying puts the bits in the carrier's frequency, while noise and gain changes on a line act on its level — so frequency stays readable where amplitude has already been disturbed.
- (a)Amplitude modulation — Amplitude modulation varies the height of the carrier in step with the message; its digital form, amplitude shift keying, switches the carrier between fixed signal levels — two of them in the binary case.
It is the technique this same classification assigns to the low-speed slot, roughly up to 1200 bits per second.
Because the bits sit in the signal level, noise and gain variation on the line strike the information directly, which is what holds this scheme to the slowest band.
- (c)Phase modulation — Phase modulation shifts where the carrier sits in its cycle; its digital form, phase shift keying, sends bits as jumps between fixed phase states.
It is the technique the classification assigns to the high-speed slot, above 2400 bits per second.
Using four or eight phase states lets a single symbol carry two or three bits, which lifts the bit rate — but that places it one rung above the band the stem names.
- (d)None of these — "None of these" is the right answer to a stem whose correct technique has been left out of the list, and that is not the case here.
Option (b) is exactly what the classification puts against the 1200 to 2400 bits per second band, so a listed option already fits the stem and the escape option is not needed.
A carrier is a steady high-frequency wave with three properties that can be altered: how tall it is (amplitude), how fast it oscillates (frequency), and where in its cycle it sits (phase). Modulation alters one of them in step with the message being sent.
When the message is digital, the alteration is switched between fixed states rather than varied smoothly. That gives amplitude shift keying, frequency shift keying and phase shift keying, and each is treated as the preferred choice over a different range of data rates.
This sits in the communication-systems part of general science, alongside carrier waves, bandwidth and the modes by which radio waves travel.
This ladder is the classification used in the computer-fundamentals texts the Commission's key follows; it is a teaching convention for matching a technique to a data-rate band, not a standards document.
The rate a link has to carry decides how the bits are put onto the carrier, and that choice trades robustness on a noisy line against how many bits each symbol can be made to hold.
- Modulation impresses a message on a carrier wave by varying one of its three parameters — amplitude, frequency or phase.
- The digital forms of the three are amplitude shift keying (ASK), frequency shift keying (FSK) and phase shift keying (PSK).
- In frequency shift keying, binary 0 and binary 1 are sent as two different carrier frequencies, so the information sits in frequency rather than in signal level.
- Noise and gain variation on a transmission line act mainly on signal level, which is the weakness of amplitude-based keying.
- Phase-based schemes can encode more than one bit per symbol — four phase states carry two bits each — which supports higher bit rates.
- Bit rate counts bits per second and baud counts symbols per second; the two are equal only when one symbol carries one bit.
- A modem converts digital data into a modulated form an analog line can carry, and converts it back at the far end.
The ladder the key follows: the speed figure in the stem, 1200 to 2400 bits per second, lands on the frequency-modulation rung.
- Treating the stem as a general 'which modulation is best' ranking. Each of the three is the preferred choice for a different band, so the answer is fixed by the speed figure given, not by which technique is better overall.
- Reading '1200 to 2400 bits per second' as a bandwidth or a baud figure. It is a bit rate, and it equals the baud rate only when each symbol carries a single bit.
- Importing the FM-beats-AM habit from broadcast radio without checking the band. It happens to point at the same option here, but on a stem naming a rate above 2400 bits per second the same reflex gives the wrong answer.
- Reaching for 'None of these' because 'medium-speed' sounds vague. The three named techniques are the standard set for this classification, and the medium band is the one assigned to frequency modulation.
Be ready to run the match in either direction: a speed figure given and the technique asked for, as here, or a technique named and the band of data rates it is assigned to asked for.
Be able to answer it in its digital dress too — which of ASK, FSK and PSK varies which property of the carrier, and which of them holds up better against noise on the line.
And be able to do the arithmetic behind the rates: converting between baud and bits per second once a symbol carries more than one bit.
NDA_GAT_2019_II_Q1002019Same corner of physical science — the waves that carry communication. The NDA item asks what an electromagnetic wave is: it travels through vacuum, with mutually perpendicular electric and magnetic components. This item takes the carrier for granted and asks what is done to it to carry data at a stated rate. Nature of the wave there, choice of modulation scheme here.
HPSC_2021_PRE_GSII_Q672021Shared thread: a wave's parameters do not behave alike. The HPSC item turns on which of frequency, wavelength and speed change as ultrasound crosses into a new medium. Here the working point is that noise disturbs amplitude while frequency stays readable. Different physics though — a medium boundary versus channel noise, sound versus a radio carrier.
- practice — not a real PYQ
In frequency shift keying (FSK), the binary digits 0 and 1 are sent as:
- (a)Two different amplitudes of the carrier
- (b)Two different frequencies of the carrier
- (c)Two different phase shifts of the carrier
- (d)Two different propagation speeds of the carrier
Answerb — FSK assigns one carrier frequency to binary 0 and a second frequency to binary 1, and the receiver decides the bit by measuring which frequency arrived.(a) describes amplitude shift keying and (c) describes phase shift keying, which are the other two members of the same family. (d) fails because the speed of a radio wave is set by the medium it travels through, not by the transmitter's choice of encoding.
- practice — not a real PYQ
In the same classification of modulation techniques by data rate, the one preferred for high-speed data communication above 2400 bits per second is:
- (a)Amplitude modulation
- (b)Frequency modulation
- (c)Phase modulation
- (d)Pulse width modulation
Answerc — the ladder runs amplitude modulation at the low rates, frequency modulation across 1200 to 2400 bits per second, and phase modulation above that, where several phase states let one symbol carry more than one bit.(a) and (b) are the two lower rungs of the same ladder. (d) varies the duration of pulses in a baseband pulse train and is used in control and power applications such as motor-speed control and lamp dimming, not as the carrier scheme for this band.
- practice — not a real PYQ
A modem sends 2400 symbols per second and each symbol carries 2 bits. Its bit rate is:
- (a)1200 bits per second
- (b)2400 bits per second
- (c)4800 bits per second
- (d)9600 bits per second
Answerc — bit rate is the symbol rate multiplied by the bits carried per symbol, so 2400 x 2 = 4800 bits per second.(b) is the symbol rate itself, which would be the bit rate only if each symbol carried one bit. (a) divides where the question requires multiplication. (d) would need four bits per symbol, not two.
- practice — not a real PYQ
Frequency shift keying generally survives a noisy transmission line better than amplitude shift keying because:
- (a)It uses a lower carrier frequency
- (b)It carries the information in the carrier's frequency rather than in its signal level
- (c)It dispenses with a carrier wave altogether
- (d)It transmits every bit twice as a built-in check
Answerb — noise and gain variation on a line disturb the signal level, so a scheme that reads frequency is not reading the property that has been corrupted.(a) fails because the robustness comes from which property carries the bits, not from the carrier frequency chosen. (c) fails because FSK is a carrier-based scheme — it uses two carrier frequencies. (d) fails because repetition is a separate error-control measure and is not part of what FSK does.