Analogue and digital signals

The short message service (SMS) on a mobile phone can send a maximum of 160 characters per message. Each character is represented by its own seven-bit binary code as it is converted to digital data.

The mobile phone transmits digital data at a rate of 8 kilobytes per second (kB s⁻¹) when using the SMS function.

Determine the minimum time required to send 160 characters.

Electrical noise can affect communication systems.

Describe one origin of electrical noise in a communication system.

Describe the effect that electrical noise can have:

• on the signal and

• on the communication system.

Figure 6 shows a noisy digital signal V_in which is applied to a circuit.

The circuit output V_out switches to:

• 5 V when the input voltage V_in falls below 1.8 V

• 0 V when the input voltage V_in rises above 3.2 V.

Draw, on Figure 7, the output signal V_out from the circuit.

Several telephone conversations need to be transmitted simultaneously between two locations.

Describe the process that allows these conversations to be carried by the same optical fibre.

In your answer you should explain:

• the importance of the sampling rate in the analogue-to-digital conversion

• the effect of resolution in this conversion

• the technique that is used to enable several conversations to be transmitted simultaneously.

You may use diagrams to help with your explanation.

Pulse code modulation (PCM) is used to encode live music as an uncompressed digital audio file. Sampling of the analogue signal is carried out at 44.1 kHz. A 16-bit system is used to encode each of the two channels that make up the stereo signal.

Explain why the sampling frequency used is suitable for this task.

Calculate the number of quantisation levels available on a 16-bit encoding system.

A recorded piece of stereo music lasts for 3.5 minutes.

Calculate the size, in megabytes, of the digital file needed to store this recording.

The music file is used by a call centre to play as background music while a phone call is on hold. However, the telephone network is designed to use a bandwidth of 0.3 kHz – 3.4 kHz.

Compare the quality of the music heard by the telephone caller with that of the original file heard when played directly from a compact disc.

A telephone company transmits 15 speech channels across a single transmission link. The analogue information is first subjected to pulse code modulation (PCM) before transmission using time division multiplexing (TDM). The transmission system incorporates regenerators which remove noise from the signal.

Explain the basic principles of:

• pulse code modulation (PCM)

• time division multiplexing (TDM)

• a regenerator.

In a recording studio the output from a microphone is an analogue signal. The equipment in the studio converts this analogue signal into a digital signal before storing it.

Discuss aspects of the analogue-to-digital conversion in this context.

In your answer you should include:

• what is meant by quantisation

• factors that affect the quality of the digital version of the analogue signal

• the advantages and disadvantages of digitising the analogue signal.

You may use diagrams to help make clear aspects of your answer.