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Analogue and Digital Signals

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Example 1.

The variation with time of the output produced by a microphone is shown in the graph below.

The output is processed by a four-bit analogue-to-digital converter (ADC) that samples the output every . The first sample is taken at time and is shown in the table below.

Complete the table for the next five samples.
Explain whether the sampling frequency is adequate to enable detail of the output V to be reproduced.

Example 2.

The figure below is a block diagram illustrating part of a mobile phone handset used for receiving a signal from a base station.

Complete the figure above by labelling each of the blocks.
Explain the role of the base station and the cellular exchange when a mobile phone is switched on and before a call is made or received.

In a mobile phone system, the area covered by the system is divided into a number of cells. For this system, explain why
- neighbouring cells use different carrier frequencies,
- each cell has a limited area, even in sparsely populated regions.
A mobile phone handset is left switched on. Explain why, although a call is not being made, the computer at the cellular exchange is still operating for this phone.

A recording is made of some music. For this recording, the music is sampled at a rate of and each sample consists of a 16-bit word.

Suggest the effect on the quality of the recording of
- sampling at a high frequency rather than a lower frequency,
- using a long word length rather than a shorter word length.
The recording lasts for a total time of 5 minutes 40 seconds. Calculate the number of bits generated during the recording

An incomplete simplified block diagram of the circuitry for a mobile-phone handset is shown in the figure below.

State the name of the block labelled
- X,
- Y.
Explain the purpose of
- the switch,
- the parallel-to-serial converter.

In a cellular phone network, a region is divided into a number of cells, each with its own base station.

Suggest and explain two reasons why a region is divided into a number of cells.
A passenger in a car is using a mobile phone as the car moves across several cells. Outline how it is ensured that the phone call is continuous.

The digital transmission of speech may be represented by the block diagram in the figure below.

State the purpose of the parallel-to-serial converter.
Part of the signal from the microphone is shown in the graph below.

The ADC (analogue-to-digital converter) samples the analogue signal at a frequency of . Each sample from the ADC is a four-bit digital number where the smallest bit represents . The first sample is taken at time zero. Use the graph above to determine the four-bit digital number produced by the ADC at times

A simplified block diagram of a mobile phone handset is shown in the figure below.

Name and state the function of
- block A,
- block B,
- block C,
- block D.
Give two reasons why communication between a mobile phone handset and the base station is conducted using UHF.

The signal from a microphone is to be transmitted in digital form. A block diagram of part of the transmission system is shown in the figure below.

Suggest two advantages of the transmission of a signal in digital form rather than in analogue form.
State the function of the parallel-to-serial converter.
In a particular telephone system, the sampling frequency is . In the manufacture of a compact disc, the sampling frequency is approximately . Suggest and explain why the sampling frequency is much higher for the compact disc.

10.

Data may be transmitted as an analogue signal or as a digital signal.
- Explain what is meant by
  - an analogue signal,
  - a digital signal.
- State two advantages of the transmission of data in digital form.
The block diagram in the figure below represents a system for the digital transmission of analogue data.
- Describe the function of the ADC (analogue-to-digital converter).
- Suggest why the transmission cable has a number of channels.

11.

A mobile phone handset is, at its simplest, a radio transmitter and receiver. Outline the role of base stations and the cellular exchange when a mobile phone is switched on and before a call is made.

12.

A block diagram representing part of a mobile phone network is shown in the figure below.

State what is represented by
- the blocks labelled X,
- the block labelled Y.
A user of a mobile phone is making a call. Explain the role of the components in the boxes labelled X and Y during the call.

13.

The digital transmission of speech may be illustrated using the block diagram in the figure below.

State what is meant by a digital signal.
State the names of the components labelled X and Y on the figure above.
Describe the function of the ADC.

14.

An analogue signal is to be transmitted digitally. A block diagram for part of the transmission system is shown in the figure below.
- Complete the figure above by labelling block X and block Y.
- State the purpose of the parallel-to-serial converter.
The original analogue signal is shown in figure I. The recovered signal after transmission is shown in figure II.

Suggest and explain two ways in which the reproduction of the input signal may be improved.

15.

State two advantages of the transmission of data in digital form rather than in analogue form.
An analogue signal is converted into a digital signal D using an analogue-to-digital converter (ADC). After transmission of the digital signal, it is converted back to an analogue signal using a digital-to-analogue converter (DAC), as illustrated in the figure below.
- Outline the process by which the ADC converts the analogue signal into the digital signal D.
- The ADC and the DAC operate with the same sampling rate and the same number of bits in each digital number. State the effect on the transmitted analogue signal when, for the ADC and the DAC,
  - the sampling rate is increased,
  - the number of bits in each digital number is increased.

16.

State and explain two advantages of the transmission of information in digital, rather than analogue, form.
Convert
- the decimal number 13 to a four-bit digital number
- the digital number 0101 to a decimal number.

17.

During the transmission of a signal, attenuation occurs and noise is picked up. State what is meant by noise.
By reference to (a), explain the advantage of the transmission of the signal in digital form rather than in analogue form.
Part of an analogue signal is shown in the graph below.

The signal is to be transmitted in digital form. The analogue signal is sampled at a frequency of using an analogue-to-digital converter (ADC). The ADC produces 4-bit numbers. The times at which the analogue signal is sampled are shown in the table below.

On the table above:
- for the digital number at time , underline the least significant bit (LSB).
- state the digital numbers corresponding to the sampling times between time and time .

18.

State two advantages of the transmission of data in digital form, rather than analogue form.
Optic fibres are used for the transmission of data. A signal in an optic fibre is carried by an electromagnetic wave of frequency . The speed of the wave in the fibre is . For this electromagnetic wave, determine the ratio:

19.

A digital signal is produced by sampling an analogue signal and passing the samples through an analogue-to-digital converter (ADC).
- State what is meant by a digital signal.
- State one change to the sampling or to the ADC that will improve the accuracy of reproduction of the original analogue signal.
The least significant bit of the four-bit digital number 1100 represents a signal voltage of . Determine the signal voltage, in , represented by this digital number.

20.

The digital transmission of speech may be represented using the block diagram in the figure below.

Part of the signal at point P on the figure above is shown in the graph below.

The analogue-to-digital converter (ADC) samples the signal at time intervals of . Each sample is converted into a four-bit number with the smallest bit representing . Use the graph below to determine the four-bit number produced by the ADC at time
The digital number is transmitted and then converted to an analogue form by the digital-toanalogue converter (DAC). Use data from the graph above to draw, on the axes of the graph below, the output level of the DAC for time to time . Assume that there is no time delay of the transmission of the signal between point P and the output of the DAC.