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Chapter Seven: Digital Communication. Introduction. Many signals in modern communication systems are digital Additionally, analog signals are transmitted digitally Digitizing a signal results in reduced distortion and improvement in signal-to-noise ratios. Types of Signal Transmission.
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Introduction • Many signals in modern communication systems are digital • Additionally, analog signals are transmitted digitally • Digitizing a signal results in reduced distortion and improvement in signal-to-noise ratios
Channels and Information Capacity • All practical communication channels are band-limited • There are theoretical limits to the rate at which data may be transmitted • The relationship between time, information capacity, and channel bandwidth is given by Hartley’s Law:
Shannon-Hartley Theorem • There is a limit to the amount of data that can be sent in a given bandwidth:
Pulse Modulation • Nyquist showed that it is possible to reconstruct a band-limited signal from periodic samples, as long as the sampling rate is at least twice the frequency of the of highest frequency component of the signal • Several types of sampling are available for pulse modulation
Sampling Rate Errors • Sampling rates that are too low result in aliasing or foldover • The figures illustrate correct and incorrect sampling rates:
Sampling • Sampling alone is not a digital technique • The immediate result of sampling is a pulse-amplitude modulation (PAM) signal • PAM is an analog scheme in which the amplitude of the pulse is proportional to the amplitude of the signal at the instant of sampling • Another analog pulse-forming technique is known as pulse-duration modulation (PDM). This is also known as pulse-width modulation (PWM) • Pulse-position modulation is closely related to PDM
Pulse-Code Modulation • Pulse-Code Modulation (PCM) is the most commonly used digital modulation scheme • In PCM, the available range of signal voltages is divided into levels and each is assigned a binary number • Each sample is represented by a binary number and transmitted serially • The number of levels available depends upon the number of bits used to express the sample value • The number of levels is given by: N = 2m
Quantizing • The process of converting analog signals to PCM is called quantizing • Since the original signal can have an infinite number of signal levels, the quantizing process will produce errors called quantizing errors or quantizing noise • The dynamic range of a system is the ratio of the strongest possible signal that can be transmitted and the weakest discernible signal • In a linear PCM system, the maximum dynamic range is found by: DR = (1.76 + 6.02m) dB
Companding • Companding is used to improve dynamic range • Compression is used on the transmitting end and expanding is used on the receiving end, hence companding
Coding and Decoding • The process of converting an analog signal into PCM is called coding, the inverse operation is called decoding • Both procedures are accomplished in a CODEC
Delta Modulation • In Delta Modulation, only one bit is transmitted per sample • That bit is a one if the current sample is more positive than the previous sample, and a zero if it is more negative • Since so little information is transmitted, delta modulation requires higher sampling rates than PCM for equal quality of reproduction
Line Codes • Line codes are methods of converting binary numbers back into analog voltages or currents • The simplest line code is to use the presence or absence of a voltage/current to indicate the logic state • Unipolar NRZ (non-return-to-zero) means that there is no requirement for a signal to return to zero at the end of each element • RZ (return-to-zero) methods are used to eliminate low-frequency ac components and dc components
Time-Division Multiplexing • There are two basic types of multiplexing: • Frequency-division multiplexing (FDM • Time-division multiplexing • In TDM, each information signal is allowed to use all available bandwidth • In theory, it is possible to to divide the bandwidth or the time among the users of a channel • Continuously variable signals, such as analog, are not well adapted to TDM because the signal is present all the time
TDM in Telephony • TDM is used extensively in telephony • The most common standard is the DS-1 signal, which consists of 24 PCM voice channels, multiplexed using TDM • Each channel is sampled at 8 kHz with 8 bits per sample, which gives a bit rate of 64 kb/s for each voice channel • The samples must be transmitted at the rate they were obtained to be reconstructed • The overall bit rate is 1.544 Mb/s • The whole system is known as a T1 Carrier
Carrier Signal Voice Channels Bit Rate (Mb/s) Typical Medium T1 DS-1 24 1.544 Twisted-pair T1C DS-1C 48 3.152 Twisted-pair T2 DS-2 96 6.312 Low-capacitance twisted-pair microwave T3 DS-3 672 44.736 Coax, microwave T4 DS-4 4032 274.176 Coax, fiber-optic T5 DS-5 8064 560.16 Fiber optics Digital Signal Hierarchy
Data Compression • Data compression is a technique used to reduce the bandwidth to transmit an analog signal in a digital form • The exact bandwidth necessary is dependent upon the modulation scheme
Lossy and Lossless Compression • There are two main categories of data compression: • Lossless compression involves transmitting all of the data in the original signal but using fewer bits. Lossless compression generally looks for redundancies in the data • Lossy compression allows for some reduction in the quality of the transmitted signal. Lossy compression involves reducing the number of bits per sample or reducing the sampling rate
Vocoders • A vocoder (voice coder) is an example of lossy compression applied to human speech • A typical vocoder reduces the amount of data that needs to be transmitted by constructing a model of the human vocal system
Vocoder Types • There are two main ways of generating the excitation signal in a linear predictive vocoder: • Pulse Excited Linear Predictive (PELP) • Residual Excited Linear Predictive (RELP)
