Information Sources And Signals
6.20 Encoding And Data Compression
We use the term data compression to refer to a technique that reduces the number of bits required to represent data. Data compression is especially relevant to a commun-ication system because reducing the number of bits used to represent data reduces the time required for transmission. That is, a communication system can be optimized by compressing data before transmission.
Chapter 29 considers compression in multimedia applications. At this point, we only need to understand the basic definitions of two types of compression:
Lossy — some information is lost during compression
Lossless — all information is retained in the compressed version.
Lossy compression is generally used with data that a human consumes, such as an image, a segment of video, or an audio file. The key idea is that the compression only needs to preserve details to the level of human perception. That is, a change is accept-able if humans cannot detect the change. We will see that well-known compression schemes such as JPEG (used for images) or MPEG-3 (abbreviated MP3 and used for audio recordings) employ lossy compression.
Lossless compression preserves the original data without any change. Thus, loss-less compression can be used for documents or in any situation where data must be preserved exactly. When used for communication, a sender compresses the data before transmission and the receiver decompresses the result. Because the compression is loss-less, arbitrary data can be compressed by a sender and decompressed by a receiver to recover an exact copy of the original.
Most lossless compression uses a dictionary approach. Compression finds strings that are repeated in the data, and forms a dictionary of the strings. To compress the data, each occurrence of a string is replaced by a reference to the dictionary. The sender must transmit the dictionary along with the compressed data. If the data con-tains strings that are repeated many times, the combination of the dictionary plus the compressed data is smaller than the original data.
110 Information Sources And Signals Chap. 6
6.21 Summary
An information source can deliver analog or digital data. An analog signal has the property of being aperiodic or periodic; a periodic signal has properties of amplitude, frequency, and phase. Fourier discovered that an arbitrary curve can be formed from a sum of sine waves; a single sine wave is classified as simple, and a signal that can be decomposed into multiple sine waves is classified as composite.
Engineers use two main representations of composite signals. A time domain representation shows how the signal varies over time. A frequency domain representa-tion shows the amplitude and frequency of each component in the signal. The bandwidth, which is the difference between the highest and lowest frequencies in a sig-nal is especially clear on a frequency domain graph.
The baud rate of a signal is the number of times the signal can change per second.
A digital signal that uses multiple signal levels can represent more than one bit per change, making the effective transmission rate the number of levels times the baud rate.
Although it has infinite bandwidth, a digital signal can be approximated with three sine waves.
Various line coding techniques exist. The Manchester Encoding, used with Ether-net Ether-networks, is especially important. Rather than using absolute signal levels to represent bits, the Manchester Encoding uses transitions in signal level. The Differen-tial Manchester Encoding uses relative transitions, and has the property that it works even if the two wires are reversed.
Pulse code modulation and delta modulation are used to convert an analog signal to digital. The PCM scheme used by the telephone system employs 8-bit quantization and takes 8,000 samples per second, which results in a rate of 64 Kbps.
Compression is lossy or lossless. Lossy compression is most appropriate for im-ages, audio, or video that will be viewed by humans because loss can be controlled to keep changes below the threshold of human perception. Lossless compression is most appropriate for documents or data that must be preserved exactly.
EXERCISES
6.1 Give three examples of information sources other than computers.
6.2 Name a common household device that emits an aperiodic signal.
6.3 Why are sine waves fundamental to data communications?
6.4 State and describe the four fundamental characteristics of a sine wave.
6.5 When shown a graph of a sine wave, what is the quickest way to determine whether the phase is zero?
6.6 When is a wave classified as simple?
Exercises 111
6.7 What does Fourier analysis of a composite wave produce?
6.8 On a frequency domain graph, what does the y-axis represent?
6.9 What is the analog bandwidth of a signal?
6.10 Is bandwidth easier to compute from a time domain or frequency domain representation?
Why?
6.11 Suppose an engineer increases the number of possible signal levels from two to four. How many more bits can be sent in the same amount of time? Explain.
6.12 What is the definition of baud?
6.13 Why is an analog signal used to approximate a digital signal?
6.14 What is the bandwidth of a digital signal? Explain.
6.15 What is a synchronization error?
6.16 Why do some coding techniques use multiple signal elements to represent a single bit?
6.17 What aspect of a signal does the Manchester Encoding use to represent a bit?
6.18 What is the chief advantage of a Differential Manchester Encoding?
6.19 When converting an analog signal to digital, what step follows sampling?
6.20 If the maximum frequency audible to a human ear is 20,000 Hz, at what rate must the ana-log signal from a microphone be sampled when converting it to digital?
6.21 What time elapses between samples for the PCM encoding used in the telephone system?
6.22 Describe the difference between lossy and lossless compressions, and tell when each might be used.