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Chapter 2: Signalling
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- 12.1: DIGITISING A SIGNAL.
- 2What does a signal do?Transfers information from one location to another.
- 3How are digitised signals possible?Information can be coded into a digital string of binary digits that are transmitted.
- 4What is different about analogue signals?Vary continuously from 1 value to the next without fixed values.
- 5What is a problem with analogue signals?Signal may need to be amplified as it becomes weaker, but if the signal becomes distorted/'noisy' then amplification boosts both the signal and the noise.
- 6What is noise in this context?The random variation on the signal.
- 7Why might it not be optimal to filter out the noise?Because this causes a loss of detail in the signal.
- 8Why is this not a problem with digital signals?Easy to detect binary on/off signals even when they are weak and noisy, so a perfect copy of the message can be regenerated.
- 9What is another advantage of digital signals over analogue signals?transmit information much faster.
- 10Give 3 advantages of using digital signals.1) Easier to replicate
2) Less errors
3) Quicker. - 11In physics, what is sampling?Sampling is the process in which the displacement of a continuous (analogue) signal is measured at small time intervals and turned into a digital string of binary numbers (samples).
- 12What is the principle of converting a varying analogue signal into a stream of numbers?Levels are created, each coded with a certain number of bits in a certain combination.
- 13How do you know how many levels you have, and how do you know how many bits you code each level with?Binary rules:
Number of levels is N (2^b)
Number of bits is b (log2(N) - 14What are the levels in this process known as?Quantisation levels
- 15What must quantisation levels cover?Range of signal (potential difference) values from the lowest to the highest.
- 16What happens when the signal does not always match exactly with a quantisation level?The system records to the nearest level to the signal.
- 17What is the difference between the signal value and the quantisation level known as?The quantisation error.
- 18What does increasing the number of quantisation levels do?Produces a better match to the original signal.
- 19What is the resolution of a sample?The smallest change in potential difference that can be determined.
- 20What is resolution given by?Resolution = Potential difference range of signal / Number of quantisation levels.
- 21What is a negative effect of increasing the number of quantisation levels?Increases demands on data storage and transmission.
- 22What is another factor that plays a part in how many quantisation levels you should use?Noise.
- 23How is noise a factor?Pointless to have a smaller gap between quantisation levels than the size of noise variation, because this would mean sampling the noise to great detail rather than simply ignoring it.
- 24What is the equation for the maximum number of useful quantisation levels?Maximum number of useful levels = Total noisy signal variation / Noise variation.
Maximum number of useful levels = Vtotal / Vnoise - 25What is the equation for the maximum number of useful bits needed? What must you also do?Number of bits = log2(Vtotal / Vnoise)
You must always round up this number to a whole number of bits. - 262.2: SAMPLING SOUNDS & SENDING A SIGNAL.
- 27What is the sampling rate/ sampling frequency?The number of samples taken each second.
- 28What must be true for a varying signal to be sampled accurately?Time interval between samples must be shorter than the time in which important changes in the signal occur.
- 29What will happen if the time interval is too large?The reconstructed signal will lose detail.
- 30What 2 conditions must be met to ensure that the original signal can be reconstructed accurately?1) The signal cannot contain frequencies above a certain maximum.
2) Minimum sampling rate > 2 x Highest frequency component. - 31What will happen if the signal contains frequencies above the maximum component? Why?Signal will not be reconstructed accurately, because lower frequency signals called aliases (not present in the original signal) will be generated.
- 32What 2 problems arise because of sampling too slowly?1) High frequency detail in the original signal is missed.
2) Spurious low frequency (aliases) is created. - 33Why must the sampling frequency for music be greater than 40kHz?Because the human ear cannot detect sound above frequencies of about 20kHz.
- 34Why is aliasing (the formation of aliases) not a problem in sampling music?Because filters remove frequencies above 20kHz in the original signal.
- 35What is the bit rate?The rate of transmission of digital information.
- 36How can bit rate be calculated?Bit rate = Samples per second x Bits per sample.
- 37What is the unit of Bit rate? When?Hz, if sampling rate is samples per second
- 38What is the equation for the total information transferred?Total information transferred = Bit rate x time.
- 39What can we also use the idea of bit rate to do?Calculate the length of time a signal carrying an amount information takes to arrive at its destination.
- 40What is the equation for this duration of signal?Duration of signal = Number of bits in signal / Bit rate
- 41What is the unit for the duration of signal using Hz for bit rate?Seconds, s.
- 42What is a disadvantage of digital signals?Digital signals are numbers and so can be changed or scrambled.
- 43What are 3 ethical questions that arise because of this possibility?1) Are online banking details completely secure?
2) Will cyber terrorism become an increasing problem?
3) How can or should the web be policed?
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- AS - Level Physics (AS) Fichas sobre Chapter 2: Signalling, creado por Matt Clancy el 03/04/2016.
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- Matt Clancy
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