Chapter 3. Signalling. Lesson Objectives. Equations. Chapter 3.1.1. Sampling and digitising 1. Signals and Noise. Digital Signals are much less prone to interference and so I theory should produce higher quality signals e.g. Digital TV and Radio . Sampling 1.
Digital Signals are much less prone to interference and so I theory should produce higher quality signals e.g. Digital TV and Radio
Instead of storing a whole waveform a signal can be sampled (digitised) at regular intervals. As long as the signal is sampled at a high enough frequency the original signal can be reconstructed
If you sample at the wrong frequency then the original signal cannot be reconstructed accurately .
The optimum sampling frequency is:
2x the highest frequency in the original signal
e.g. a music CD need a highest frequency of 20kHz to be stored so music is sampled at 44.1kHz.
Another problem is that if you sample at too a low frequency then spurious frequencies called aliases can be created in the reconstructed signal
You have probably seen this effect on TV or the cinema when a car moving forwards appears to have wheels that are rotating backwards
Signals can be digitised by turning the sampled waveform into numbers.
Sampling is done using an analogue to digital converter (ADC)
A Digital to analogue converter (DAC) can reverse the process
The diagram on the left shows that with 3 bits of information up to 8 levels can be stored (3 bits = 8 possible binary numbers)
Using more bits means more levels and a greater resolution
A telephone uses 8 bits = 256 levels for each sample
A high quality CD uses 16 bits = 65536 levels for each sample
Chapter 3.1.2 – 3.1.3Sampling and digitising 2
A fax converts text and pictures into pixels one line at a time. This is slow and requires a lot of data to be transferred
Slow information transfer rate
E-mail sends less information per page because it can encode letters as numbers instead of pixel by pixel.
This means a faster information transmission time even if we use the same transmission rate (64000 bits per second) as the fax machine
Look at the size of TV aerials on the roofs of houses - they give you and idea of the wavelengths being received .
Usually the rods are half a wavelength long – a few cm
The rods are a few cm long indicating that it is designed to receive waves with wavelengths of a few cm
TV and Radio waves are polarised
To pick up a signal the receiving rods must be parallel to the electric field oscillations of the wave
Again looking at TV aerials will tell you the direction of polarisation
The short Transverse rods indicate that this ariel is designed to receive waves that are horizontally polarised
If you are a Jazz Maverick and Electro-pop superstar its important that you know how to process sounds and music…