1. Dr. Clincy Professor of CS
Chapter 4 Handout #4
Dr. Clincy
Professor of CS
Dr. Clincy
Lecture

2. ANALOG-TO-DIGITAL CONVERSION
We have seen in Chapter 3 that a digital signal is superior to an analog signal. The tendency today is to change an analog data to a digital signal. In this section we describe two techniques, pulse code modulation and delta modulation.
Dr. Clincy
Lecture

3. Components of PCM encoder
PCM – Pulse Code Modulation
1st: analog signal is sampled
2nd: sampled signal is quantized
3rd: quantized values are encoded as bit streams (or codes)
Analog signal is sampled every Ts seconds
Sample rate is fs = 1/Ts
Dr. Clincy
Lecture

4. Three different sampling methods for PCM
High-speed switch used – able to retain the shape of the signal
Ideal but complex
Sample-and-hold method that creates flat-top samples by using a circuit
Sampling process also called pulse amplitude modulation (PAM)
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Lecture

5. Recovery of a sampled sine wave for different sampling rates
Nyquist theorem states that the sampling rate must be at least 2 times the highest frequency of the signal
Catches the essence of the signal
Doesn’t improve the case
Doesn’t capture the essence of the signal
Dr. Clincy
Lecture

6. Quantization and encoding of a sampled signal
actual-amplitude/D
actual amplitude
Quantization steps:
Determine Vmin and Vmax
Divide range into L zones, each of height D
D = [Vmin - Vmax]/L
3. Assign quantized values of 0 to L-1 to midpoint of each zone
4. Map the sample value to a quantized value
Norm. Actual
Error between actual and nornalized
Quant. value for code
Code that represents the voltage level
Assume sample amplitudes between -20V and +20V
Let L = 8 (levels) – therefore, D = [ ]/8 = 5
Quantization error can contribute to Shannon’s SNR: SNRdB = 6.02nb where nb is bits per sample
Bit rate = sampling rate x # of bits per sample = fs x nb
Dr. Clincy
Lecture

7. Components of a PCM decoder
PCM decoder recovers the original signal
Smooths out the staircase signal
What is the minimum bandwidth of the filter the digitized signal will need ?
Bmin = c x nb x 2 x Banalog x 1/r (nb = # bits per sample)
If 1/r=1 and c=1/2, Bmin=nb x Banalog
If the data rate and number of signal levels are fixed, minimum bandwidth is
Bmin = N / [2 x log2 L]
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Lecture

8. The process of delta modulation
PCM is more complex than Delta modulation
PCM finds the amplitude of the signal; delta modulation simply finds the change in the signal from the previous sample
Delta modulation doesn’t use codes – bits are sent one after another
Positive changes are encoded as 1; negative changes are encoded as 0
Dr. Clincy
Lecture

9. TRANSMISSION MODES
The transmission of binary data across a link can be accomplished in either parallel or serial mode.
In parallel mode, multiple bits are sent with each clock tick.
In serial mode, 1 bit is sent with each clock tick.
While there is only one way to send parallel data, there are three subclasses of serial transmission: asynch, syn and iso approaches (asynchronous, synchronous, and isochronous.)
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Lecture

10. Parallel transmission
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Lecture

11. Serial transmission
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Lecture

12. Asynchronous transmission
In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each byte. There may be a gap between each byte.
Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized; their durations are the same.
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Lecture

13. Synchronous transmission
In synchronous transmission, we send bits one after another without start or stop bits or gaps. It is the responsibility of the receiver to group the bits.
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Lecture

14. Isochronous Transmission
For realtime audio and video, uneven delays between frames is not acceptable – so synchronous transmission doesn’t work well
The entire stream of bits must be synchronized – this is isochronous transmission
Isochronous transmission guarantees data at a fixed rate
Dr. Clincy
Lecture