2: Signalling Flashcards
why are digitised signals possible?
information can now be coded into binary digits which can be transmitted in multiple ways.
What are digital signals represented by?
Binary numbers (digits)
What are the advantages of digital over analogue signals
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
What are the disadvantages of digital over analogue signals
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
Are Analogue signals limited in the values they can take?
No. They also vary continuously from one value to the next.
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
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
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
The number of values, a digital signal can take is limited
You need to get an accurate representation of what was sent
Can Analogue signals be digitised?
what is the process called, define it
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)
How do you digitise a signal, where each sample taken is coded with 3 bits, potential problem?
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
What 2 factors affect how well a digitised signal matches the original?
The difference between the possible quantisation levels
the time from one sample to the next (sampling rate)
What happens if a signal is digitised using only a few, widely spaced samples?
It’s likely that a lot of the analogue values sampled will be far from the nearest digital value.
what is quantisation error?
the difference between the signal value and the closest quantisation level
What is the advantage of increasing quantisation levels?
The more closely the digitised signal will match the original
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
the smallest change in potential difference that can be determined.
resolution = potential difference range / number of quantisation levels
12v range / 28(256) = 0.047V
What is the advantage of using a lower resolution?
reduces the demand on data storage and transmission speeds
