2.1 Recording and Playback

Question 1

What is an analogue signal?

Answer 1

Analogue signals are those which vary continuously with time.

Question 2

What is a digital signal?

Answer 2

Has discreet values.
A digital signal refers to an electrical signal that is converted into a pattern of bits.
The precision of the signal is determined by how many samples are recorded per unit of time.

Question 3

What are the stages in sound recording?

Answer 3

Sound waves cause a membrane in a microphone to vibrate.
These mechanical vibrations are converted- weak electrical signals- voltage mimics the displacement of air molecules by sound.
This signal is amplified.
Amplified signal fed into record head (electromagnet that produces magnetic field.)
The intensity of the field varies in the same way as the current (and therefore the original sound).

Question 4

Why and when is a mixer used in sound recording?

Answer 4

To balance the relative strengths of the signal if several microphones, or a series of microphones and guitar pickups are to be recorded together.

Question 5

What happens during playback of sound?

Answer 5

The tape passes the playback head where the tape’s magnetic field induces currents in this head.
These currents are amplified and the electrical signal is then passed to a loudspeaker where it is converted into sound by the mechanical vibrations of the loudspeaker cone or headphones.

Question 6

How is an analogue signal sampled?

Answer 6

1. Signal voltage measured at regular time intervals.
2. Measurement converted to a binary number.
3. Digital number stored or transmitted to receiver.
4. When sound is played again, binary data must be reconverted to analogue signal to drive loudspeaker.

Question 7

How does a digital signal processor work?

Answer 7

1. Microphone produces an analogue signal, an ADC digitises it into binary.
2. Signal sent to DSP, digital signal analysed and encoded using compression algorithm.
3. Compressed data stored in memory.
4. On playback, data retrieved from memory and decompressed.
5. DAC changes digital value back to analogue voltage.
6. Voltage applied to amplifier and speaker.
7. Routine used in DSP is called codec.

Question 8

How is compressed data from a digital signal processor played back as sound?

Answer 8

Upon playback, the data is retrieved from memory and is decompressed.
A digital-to-analogue converter chages the digital values back to analogue voltage that is applied to the amplfier and speaker.
The routine used in the digital signal processor is called a codec for COmpression-DECompression.

Question 9

What is a common codec?

Answer 9

MP3.

Question 10

The more frequently an audio signal is sampled…

Answer 10

…the closer a regenerated wave will be to the original analogue waveform.

Question 11

What is the rule for setting out the minimum sampling rate of a regular signal?

Answer 11

It must be at least twice the frequency of the signal.

Question 12

What can happen if minimum sampling rate of a regular signal is not at least twice the frequency of the signal?

Answer 12

The regenerated signal can produce false sounds in what is called aliasing,

Question 13

What is quantisation?

Answer 13

The process of converting a continuous range of values into a finite range of discrete values.

Question 14

What is data compression?

Answer 14

A technique by which the same amount of data is transmitted by using a smaller number of bits.

Question 15

What is a digital filter?

Answer 15

A system that removes random noise from a signal or to extract useful parts from it.

Question 16

What are some advantages of digital signals?

Answer 16

Electromagnetic interference, signal loss and noise which effects signal quality, is virtually elliminated.
Digital data can be encrypted to provide some measure of protection against unwanted viewing/listening.
Digital data can be shared among many different devices.

Question 17

What does the faithfulness of a signal depend on?

Answer 17

The quantisation and compression used.

Question 18

What is one of the limitations to digital signalling?

Answer 18

Transducers usually generate analogue voltages which need to be sampled and then converted back to analogue at the end of the process.
Binary numbers cannot be heard from a speaker or seen on a number.

Question 19

Explain the effect that increased sampling rate and decreased quantisation levels has on the amount of storage required and the faithfulness of a digital signal.

Answer 19

Increased sampling rate leads to the amount of storage decreasing. The higher the sampling rate the more accurate the regenerated wave will be to the original sound but this means more storage is used.

Question 20

Suggest why a good compression routine will mean that a compressed signal will be practically indistinguishable from an uncompressed one.

Answer 20

It will have a smaller wavelength and a higher frequency so all the information is compacted into a smaller space so if there is a small error in the compression it will expand when you uncompress it- if there is little/no error the uncompressed wave will produce an accurate depiction.

Question 21

What is pulse code modulation?

Answer 21

changing analogue to digital
(analogue signal) is sampled
digital signal is (a sequence of) binary numbers

Question 22

Name and explain one type of compression technique.

Answer 22

lossy compression

loses physiologically unimportant info

Question 23

State an advantage of using compression when storing a digital video recording.

Answer 23

need to store less information
fewer bits
more films in same space

Question 24

State an advantage of recording music in an analogue form rather than a digital form.

Answer 24

Not losing any of the original signal detail.

Question 25

State an advantage of transmitting a signal using frequency modulation (FM) compared
with using amplitude modulation (AM).

Answer 25

Less susceptible to (e-m) noise/interference by e-m waves)
Easier to remove noise
Better quality since higher bandwidth/transmit more detail/higher range of frequencies in
signal
Less power wasted in carrier wave (more used in side band)

Question 26

State and explain advantages of digital data transmission compared with analogue.

Answer 26

Signal unaffected by noise
Multiplexing possible
More difficult to ‘tap’
Digital signals carry more information per second

Question 27

Describe and explain the purpose of modulation in a communication system.

Answer 27

Carrier wave
Frequency/amplitude modulation
By signal to be transmitted
Modulation enables more information to be transmitted.

Question 28

Explain why the use of a sampling frequency that is lower than the minimum sampling frequency could reduce the quality of the data received.

Answer 28

Higher frequencies will be lost.