2: Signalling_checked Flashcards

1
Q

Why are digitised signals possible?

A

Analogue signals [information] can be coded into binary digits which can then be transmitted in multiple ways e.g. along an optical fibre or copper cable.

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2
Q

What are digital signals represented by?

A

Digital signal are represented by Binary numbers (digits)

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3
Q

What are the advantages of digital over analogue signals?

A
  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. Digital files can be compressed to reduce their size, and manipulated easily for artistic effect
  3. Digital signals are more resistant to the effects of Noise
  4. Computers can be used to easily process digital signals, since computers are digital devices too
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4
Q

What are the disadvantages of digital over analogue signals

A
  1. Digital signals can never reproduce analogue signals exactly - some information will always be lost
  2. Because digital signals can be copied more easily, digital information (films, music etc.) can be reproduced illegally unlimited number of times.
  3. Confidential information, such as personal data and photos, may be stolen and copied without the owner’s knowledge or consent, more easily by hackers, infected networks or malicious websites.
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5
Q

Are Analogue signals limited in the values they can take?

A

No. Analogue signals also vary continuously from one value to the next.

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6
Q
  1. How can a weak signal be strengthened?
  2. What is noise in the context of signals?
  3. When you transmit an electronic signal, it will pick up noise. From what does it pick up noise?
  4. What is a problem with amplifying a signal with noise
A
  1. A weak signal can be amplified.
  2. Noise appears as random variation on the signal
  3. Noise is picked up from Electrical disturbances or other signals
  4. The disadvantage is that noise is amplified as well
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7
Q

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

A

This is much easier with digital than analogue signals because the number of values a digital signal can take is limited

You need to get an accurate representation of what was sent

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8
Q

Can Analogue signals be digitised?

What is the process called, define it

A

Yes. Analogue signal can be digitised

The process is digitising, its the process in which the value of a continuous analog signal is taken at regular time intervals and converted into the nearest digital values.

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9
Q

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

Identify a potential problem/limitation?

A

A sample with 3 bits means that there are N=23 levels called quantisation levels [or alternative 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

Limitation :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

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10
Q

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

A
  1. The number of samples per second - this must be at least twice the highest frequency in the signal to ensure that all the frequencies within the signal are transmitted [and reconstructed] correctly
  2. The number of bits per sample - this must be high enough that the transmitted signal closely matches the original but not so high that it is negatively affected by noise.
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11
Q

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

A

A low sapling rate can create low frequency signals - called aliases - that were not in the original signal at all

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12
Q

What is quantisation error?

A

The quantisation error for a sample is the difference between the value of the input signal and the quantised signal. Quantisation errors can be reduced by increasing the number of levels; however, as the number of levels increases, so does the number of bits needed to represent each sample.

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13
Q

What is the advantage of increasing quantisation levels?

A

The more closely the digitised signal will match the original

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14
Q

What is resolution?

State an equation for resolution.

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

A

Resolution is determined by the number of bits in the binary number representing the digital values - the greater the number of bits the greater the resolution.

Resolution = potential difference range / number of quantisation levels

12V range / 28(256)

= 0.047V

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15
Q

What is the advantage of using a lower resolution?

A

Using a lower resolution reduces the demand on data storage and transmission speeds

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16
Q

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

A

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 [alternative levels] than the size in noise variation

17
Q

How do you calculate maximum number of useful quantisation levels?

A

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

= Vtotal / Vnoise

18
Q

How do you determine the 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

A

b = log2 [Vtotal / Vnoise ]

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

We go up to 6 bits because 5 bits gives an insufficient 32 levels,

The Vtotal / Vnoise gives 40 max and this will be covered by 6 bits.

19
Q

List 2 conditions that must be met for accurately reconstructing a signal

A
  1. The number of samples per second - this must be at least twice the highest frequency in the signal to ensure that all the frequencies within the signal are reconstructed accurately.
  2. The number of bits per sample - this must be high enough that the transmitted signal closely matches the original but not so high that it is negatively affected by noise.
20
Q

State the Nyquist Theorem:

Minimum sampling rate =

A

Minimum sampling rate = Twice the highest frequency component

Nyquist Theorem

21
Q

Why does the sampling rate have to be high?

A

To record all the high frequency detail of the signal

To avoid the creation of aliases

22
Q

What are aliases?

A

Low frequency signals that are created from having a too low sampling rate. These low frequency signals are not part o the original signal.

23
Q

What is the sampling rate for music?

How do you ensure aliasing doesn’t occur?

A

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

24
Q

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

A

By not sampling frequently enough, you lose high frequency details.

If frequencies above a maximum value are not filtered out as well, then the low sampling rate can also create false low frequencies which is known as aliasing from frequencies above the max limit

25
Q

Define bit rate.

A

The bit rate tells us how many bits of data are processed every second. Bit rates are usually measured in kilobits per second (kbps)

26
Q
  1. State an equation for bit rate and give a unit.
  2. A cd uses 16 bits per sample at a sample rate of 44.1kHz for 2 channels. calculate combined bit rate for 2 channels
A

Bit rate = samples per second

= (sample rate (Hz or KHz)) * bits per sample

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

= 1.4MHz

27
Q

How can you speed up the process of uploading images?

A

Compress your image to reduces the file size

This also uses less storage

28
Q

How do you 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

A

Time taken = number of bits in signal / bit rate

= (28 * 106) * 8 / 1.4 * 106

= 160s

= 2 mins 40 sec

29
Q

How do you calculate the 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

A

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

= 134.4 Mbits

30
Q

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

A

bit resolution means the image uses 6 bits per pixel

31
Q

What is meant by speed of transmission

A

Speed at which the signal travels