1.9 Digital Modulation Techniques Flashcards
Describe Pulse Amplitude Modulation
(tarnsmission of sampled analogue signal)
Pulse amplitude is changed according to the input signal.
Noise in the channel will act on the amplitude, which makes this mores susceptible to noise.
Describe Pulse Width Modulation
(digital transmission of sampled analogue signal)
The pulse width is directly proportional to the message signal’s amplitude.
What are the difficulties with PWM?
Receiver must be sensitive enough to detect the width of the incoming pulses.
Difficult to multiplex.
Describe Pulse Position Modulation (PPM)
(Digital transmission of sampled analogue signal)
The position of the pulses is directly proportional to the message signals amplitude. As the message amplitude increases, the pulse is delayed in time.
Describe Pulse Code Modulation
Samples the analogue data and converts it into a binary code.
What device is used to sample and quantize analogue signals into a digital code for transmission? Vice versa?
PCM Encoder and Decoders.
ADCs and DACs operate in parallel but transmitters operate in serial. This requires a Parallel to Serial and vice versa.
What device is used to do the actual modulation>?
MODEM (Modulator-Demodulator)
What does a LPF do with the signal?
Low Pass Filter smooths the signal.
After the signal is recreated, it has sharp transitions and staircase look to it. We want to smooth that out.
What is PCM Companding?
A method of maintaining a uniform SNR. It compresses the dynamic range at the encoder and expands at the decoder.
This keeps the SNR more consistent for some reason, but it requries a very sensitive receiver because of the reduced dynamic range.
What is parametric coding?
Compression technology for audio. The audio is represented by a set of parameters. These parameters are coded into a bitstream and the decoder rebuilds the signal from them. Like the DK soundtrack.
What is Delta Modulation?
It will transmit a 1 if the slope of the signal is increasing and a 0 if it is decreasing. This gives it a data rate advantage but it can sometimes have difficulty keeping up with rapidly changing values in the signal.
