1. Digital Communication

2. Recap – Analogue Communication
We have previously seen that there is more than one way to send an analogue signal through a transmission medium.
AM – Amplitude Modulation
This form modulates the amplitude. Can be affected by noise.
FM – Frequency Modulation
This form is modulated by frequency Is not as affected by noise.

3. Sending digital signals
Digital data is sent over a transmission medium in bits (i.e. 0 and 1 – high and low voltage signals). Each 0 or 1 is a bit.
Example
In a 3 bit system the number 2 would be sent as
The problem we have to overcome is how to convert music or other complex analogue data into a digital signal and then send it over a communication link.
0 1 0 4 2 1

4. Modulating Digital Signals
There are a variety of ways to PULSE MODULATE digital signals
P A M
P C M
P P M
P W M
pulse
AMPLITUDE
modulation
pulse
CODE
modulation
pulse
POSITION
modulation
pulse
WIDTH
modulation

5. PAM
With all of the techniques used on the following pages, a sample rate is decided upon before starting.
Pulse Amplitude Modulation uses the voltage levels sampled from the analogue signal at regular intervals (sample rate). These samples are then sent down the communication lines as steady voltage pulses.
PAM signals are subject to noise (similar to AM radio signals) since the information is dependant on the amplitude of the digital pulse.
Sample rate
Sampled voltage
levels

6. PCM
Pulse Code Modulation is now widely used in favour of PAM for the creation of music CD’s and many other forms of digital communication.
Pulse Amplitude Modulation is the first step before actually creating the digital codes that would be sent down the communication lines. 7 7 6 6 5 5 4 4
Quantization levels 3 3 2 2 1 1
Sample rate

7. Connected electronically to the clock
PCM
The digital codes are created by dividing up the signal amplitude range into equal parts. The number of parts is decided by how many BITS the system is using. For example a 3 bit system (as seen on the previous slide) can be split from 0 to 7.
After creating each quantization code they are sent in order over the communication line via a PISO. Both the sampling gate and the PISO work with a clock pulse to ensure a regular sampling / send rate.
Low pass filter
Sampling Gate
ADC
PISO
Clock
Digital signal
Analogue signal
4 kHz
x 2
8 kHz
Example
1MHz
(1 bit every s)
Clock pulse must be twice the filter cut off frequency.
Connected electronically to the clock

8. Low Pass Filter
A low pass filter is used to filter out unwanted high frequencies from the analogue input. This enables the quantization codes to be placed closer together than they would previously because they are not spread over such a a high range.
If we are dealing with audio frequencies then the higher end analogue frequencies are not required because they are inaudible.
The clock pulse then needs to be be twice the frequency of the cut off point of the low pass filter.
Note Sampling frequencies in speech communication links are kept to 8kHz and only use an 8 bit sampling method
The human ear can detect sounds from 20Hz to 20kHz and thus CD quality sampling is done up to this range and requires more quantization levels.

9. PWM
Pulse Width Modulation uses the amplitude of the analogue signal to influence the width of the digital pulses.
The heights of the pulses do not change.
Analogue input
Digital output
Time V

10. PPM
In the Pulse Position Modulation system the pulses are made to vary in accordance with the amplitude of the signal. The position of the pulse can be deflected either way depending on the polarity of the analogue signal. V
Analogue input
Sampling frequency
Time
0 V
position V
Digital output
Time

11. Time Division Multiplexing
Human speech communication happens at a range of 4 kHz. As we have discussed previously this should be sampled at 8 kHz (twice the frequency – once passed through a low pass filter). An 8 bit code is then used (PCM – 256 quantization levels including 0) to transmit each sample.
Time between sampling = s = s (0.125 ms) (125 μs)
If this system uses a 1MHz PISO clock then 1 bit can be sent out every 1 μs (1,000,000 bits per second). With an 8 bit system, the 8 bit code will take 8 μs to be sent. Our issue here is that the sampling occurs every 125 μs and the sample is sent in only 8 μs. Over a communications link this leaves us with 117 μs of idle time. This time is often used to transmit other signal sources.
Note. BT have a 30 channel system
8000
No. of Channels = PISO frequency
SG frequency x number of bits