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Digital/Analog conversionPowerPoint Presentation

Digital/Analog conversion

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### Digital/Analog conversion

How do we convert analog signals to digital data and vice versa?

Analog-to-digital conversion Pulse Code Modulation (PCM) samples the absolute amplitude of the signal Delta Modulation (DM) samples the change in the amplitude of the signal

- Converting analog signals to digital data is relatively straightforward.
- Sampling is the process by which we measure the value of the analog signal at discrete points in time.
- The sampling rate is how frequently the signal is sampled. To accurately capture the analog signal, the sampling rate must be twice the highest component frequency of the signal.
- After sampling, the signal is quantized - which is to say the set of continuous possible values is converted to a set of discrete digital values.

Digital-to-analog conversion

- This is the trickier problem - how do we accurately represent digital data in an analog signal?
- In the simplest versions, we vary one of amplitude, frequency, or phase to capture our digital data.
- The carrier frequency is the base signal for the information carrying signal. Information is carried by modifying the carrier signal.

Amplitude Shift Keying (ASK)

- Just like it sounds, we shift the amplitude of the carrier signal
- Binary ASK uses only two amplitude levels - 0 and the level of the carrier signal.

Frequency Shift Keying (FSK)

- Here we manipulate the frequency of the carrier signal.
- Binary FSK essentially uses two carrier frequencies - a high and a low - to represent different data elements.
- Multilevel FSK uses more than two carrier frequencies, where each level represents multiple data elements.

Phase Shift Keying (PSK)

- Shift the phase of the carrier signal.
- Binary PSK: One element has phase of 0°, the other 180°.
- Quadrature PSK: Uses two BPSK modulations to pack two bits into each signal element. Essentially has four levels: 45°, 135°, -45°, and -135°.

Quadrature Amplitude Modulation (QAM)

- Why alter only one aspect of the carrier signal? Imagine how much efficiency we could generate by altering two!
- QAM essentially takes QPSK and adds to it the ability to also modulate the amplitude of the carrier signal, increasing the number of data elements that can be carried with each signal element.
- Some common versions of QAM used today have 16 data elements per signal element (16-QAM).

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