2: Signalling Flashcards
What are digital signals represented by?
Binary numbers
Are Analogue signals limited in the values they can take?
No. They also vary continuously
When you transmit an electronic signal will pick up noise. From what does it pick up noise?
Electrical disturbances or other signals
Why is it easier to reconstruct the original signal from the noisy signal with digital signals? Why do it?
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?
Yes
How do you digitise a signal?
You take the value of the signal at regular time intervals then find the nearest digital value
Each digital value 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
What 2 factors affect how well a digitised signal matches the original?
The difference between the possible digital values (resolution) and the time from one sample to the next (sampling rate)
What happens if a signal is digitised using only a few, widely spaced digital values?
It’s likely that a lot of the analogue values sampled will be far from the nearest digital value.
What is the advantage of a higher resolution? And what does a higher resolution mean?
Higher res: more possible digital values
Ad: The more closely the digitised signal will match the original
What is resolution determined by?
The number of bits in the binary numbers representing the digital values - the greater the no. of bits, the greater the resolution
What is the advantage of using a lower resolution?
Lower rate of transmission can be used to send the information
What are the advantages of digital over analogue signals (5)?
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) Noise is more of a problem for analogue signals than digital
4) A digital signal can be used to represent different kinds of information in the same way
5) Computers can be used to easily process digital signals as computers are also digital devices
What are the disadvantages of digital over analogue signals (3)?
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 etc.) can be reproduced illegally unlimited times
3) Confidential information, such as personal data and photos, may be stolen or copied without the owner’s knowledge or consent, more easily by hackers, infected networks or malicious websites
Most signals are made up of several [ ] all with different [ ]
waves
frequencies
When you have a signal made up of different waves, how do you work out the amplitude at a given point?
The amplitude of the final signal is the sum of the amplitudes of the individual waves at each point in time
What is the fundamental frequency?
How do you find it?
The lowest frequency wave that makes up a sound wave
Find the shortest repeating part of the sound wave and calculate the inverse of its period
What limits the number of bits that can be used for sampling?
Noise
If the original signal contains noise then a really fine resolution will reproduce all the little wiggles caused by the noise
Minimum sampling rate =
Twice the max. frequency
Why does the sampling rate have to be high?
To record all the high frequency detail of the signal
To avoid the creation of aliases
What are aliases?
Low frequency signals that are created from having a too low sampling rate
Why does music need a high sampling rate?
Recordings of music need to be able to accurately reproduce the original sounds
What sampling rate do CDs or MP3s use? Why?
44100 Hz
The max. frequency sound that can be recorded at this rate is about 20,000 Hz, which is roughly the highest frequency sound audible to the human ear
Do DVDs/TV/Blue-ray use a higher or lower sampling rate than CDs?
Higher
What does the rate of transmission of a digital signal depend on? Do these need to be high or low? (2)
1) No. of samples/second: At least 2 * max. freq, to ensure that all the frequencies within its spectrum are transmitted accurately
2) No. of bits per sample: Must be high enough that the transmitted signal closely matches the original, but not so high that it is negatively affected by noise
What is difference between these 2 units for Internet connection speed? Mb and MB (per second)
Mb = Mega BITS MB = Mega BYTES
How can you speed up the process of uploading images?
Compress your image to reduces the file size
Also uses less storage
