2: Signalling

Question 1

why are digitised signals possible?

Answer 1

information can now be coded into binary digits which can be transmitted in multiple ways.

Question 2

What are digital signals represented by?

Answer 2

Binary numbers (digits)

Question 3

What are the advantages of digital over analogue signals

Answer 3

1) Digital signals can often be sent, received and reproduced more easily than analogue signals because they can only take a limited number of values
2) faster transmission than analogue signals
3) Digital files can be compressed to reduce their size, and manipulated easily for artistic effect
4) Noise resistant

Question 4

What are the disadvantages of digital over analogue signals

Answer 4

2) Because digital signals can be copied more easily, digital information (films etc.) can be reproduced illegally.
3) Confidential information, such as personal data and photos, may be stolen without the owner’s knowledge or consent, more easily by hackers, infected networks or malicious websites
4) digital signals easily scrambled

Question 5

Are Analogue signals limited in the values they can take?

Answer 5

No. They also vary continuously from one value to the next.

Question 6

how can a signal get weak and what can you do about it?

what is noise in the context of signals

When you transmit an electronic signal, it will pick up noise. From what does it pick up noise?

what is a problem with apmifying a signal with noise

Answer 6

1) if it’s distorted, you can amplify it
2) random variation on the signal
3) Electrical disturbances or other signals
4) the noise is amplified as well

Question 7

Why is it easier to reconstruct the original signal from the noisy signal with digital signals? Why is it important that it’s easy to reconstruct the signal

Answer 7

The number of values, a digital signal can take is limited

You need to get an accurate representation of what was sent

Question 8

Can Analogue signals be digitised?

what is the process called, define it

Answer 8

Yes

the process is sampling, its the process in which the displacement of a continuous analog signal is measured at small time intervals and converted into a string of digital binary numbers (samples)

Question 9

How do you digitise a signal, where each sample taken is coded with 3 bits, potential problem?

Answer 9

a sample with 3 bits means that there are N=2³ levels called quantisation levels to represent the signals value.

You take the value of the signal at regular time intervals then find the nearest quantisation level.

Each quantisation level is represented by a binary number so you can convert the analogue values to binary numbers

The digital signal you end up with won’t be exactly the same as the analogue signal if the nearest quantisation level doesn’t match the signal when a sample is taken

Question 10

What 2 factors affect how well a digitised signal matches the original?

Answer 10

The difference between the possible quantisation levels

the time from one sample to the next (sampling rate)

Question 11

What happens if a signal is digitised using only a few, widely spaced samples?

Answer 11

It’s likely that a lot of the analogue values sampled will be far from the nearest digital value.

Question 12

what is quantisation error?

Answer 12

the difference between the signal value and the closest quantisation level

Question 13

What is the advantage of increasing quantisation levels?

Answer 13

The more closely the digitised signal will match the original

Question 14

What is resolution?

equation for resolution

a signal is detected over a 12V range, 8 bit sample of this signal is produced, calculate resolution of this sample

Answer 14

the smallest change in potential difference that can be determined.

resolution = potential difference range / number of quantisation levels

12v range / 2⁸(256) = 0.047V

Question 15

What is the advantage of using a lower resolution?

Answer 15

reduces the demand on data storage and transmission speeds

Question 16

What limits the number of bits that can be used for sampling, and so limits the number of quantisation levels

Answer 16

Noise. If the original signal contains noise then you would sampling the noise to greater detail rather than ignoring it. you should not have a smaller gap between quantisation levels than the size in noise variation

Question 17

how to calculate maximum number of useful quantisation levels?

Answer 17

max useful levels = total pd of noisy signal variation / pd of noise variation

Vtotal / Vnoise

Question 18

number of bits required for maximum useful quantisation levels

Q: a signal has a max total variation of 200mV. the noise variation is 5mV. calcualte the largest number of bits per sample worth using to encode the variation

Answer 18

b = log₂ V_total / V_noise

log₂ 0.2V / 0.005V = 5.3, so this goes up to 6 bits

we go up because 5 bits gives 32 levels, whereas the V_total / V_noise gives 40 max useful quantisation levels, so we go one up

Question 19

2 conditions to be met for accurately reconstructing og signal

Answer 19

signal cannot contain any frequency above a certain max

minimum sampling rate must be twice the highest frequency component

Question 20

Minimum sampling rate =

Answer 20

Twice the highest frequency component

Question 21

Why does the sampling rate have to be high?

Answer 21

To record all the high frequency detail of the signal

To avoid the creation of aliases

Question 22

What are aliases?

Answer 22

Low frequency signals that are created from having a too low sampling rate

Question 23

what is the sampling rate for music, how do you ensure aliasing doesn’t occur?

Answer 23

the human ear can’t detect frequencies above 20KHz, so for music to be sampled accurately, it is sampled at a frequency of 44.1KHz. filters remove frequencies above 20kHz in the original signal so aliasing doesn’t occur

Question 24

what can be the result of not sampling frequently enough and not filtering out frequencies above a maximum

Answer 24

you lose high frequency details

you can also create false low frequencies which is known as aliasing from frequencies above the max limit

Question 25

define bit rate

Answer 25

rate of transmission of digital information

Question 26

equation for bit rate units

a cd uses 16 bits per sample at a sample rate of 44.1kHz for 2 channels. calculate combined bit rate for 2 channels

Answer 26

bit rate = samples per second (sample rate(Hz or KHz)) * bits per sample

(16 * 44100) * 2 = 1.4 * 10⁶ bits s^-1 = 1.4MHz

Question 27

How can you speed up the process of uploading images?

Answer 27

Compress your image to reduces the file size

Also uses less storage

Question 28

how to calculate duration of signal

a song occupies 28Mbytes of memory on a phone. if the bit rate is 1.4MHz for the system, calculate the duration of signal

Answer 28

number of bits in signal / bit rate

(28 * 10⁶) * 8 / 1.4 * 10⁶ = 160s = 2 mins 40 sec

Question 29

how to calculate number of bits required for storing a recording that lasts 3.5 minutes with a sample rate of 40KHz where there are 16 bits per sample

Answer 29

bits = number of seconds * sample rate (samples per second) * bits per each sample

seconds = 210 Seconds, there are 40000 samples per second each with 16 bits

210 * 40000 * 16 = 134400000

Question 30

how to find sample rate from a graph

Answer 30

find number of samples taken in a second

Question 31

what does it mean if an image has a 6 bit resolution

Answer 31

the image uses 6 bits per pixel

Question 32

what is meant by speed of transmission

Answer 32

speed at which the signal travels

Question 33

what is a signal

Answer 33

the variation carrying the useful information being transmitted