【文章內(nèi)容簡介】 ineer’s Guide to Digital Signal Processing is written for those who want to use DSP as a tool, not a new career. The remainder of this chapter illustrates areas where DSP has produced revolutionary changes. As you go through each application, notice that DSP is very interdisciplinary, relying on the technical work in many adjacent fields. As Fig. 12 suggests, the borders between DSP and other technical disciplines are not sharp and well defined, but rather fuzzy and overlapping. If you want to specialize in DSP, these are the allied areas you will also need to study. Telemunications Telemunications is about transferring information from one location to another. This includes many forms of information: telephone conversations, television signals, puter files, and other types of data. To transfer the information, you need a channel between the two locations. This may be a wire pair, radio signal, optical fiber, etc. Telemunications panies receive payment for transferring their customer39。s information, while they must pay to establish and maintain the channel. The financial bottom line is simple: the more information they can pass through a single channel, the more money they make. DSP has revolutionized the telemunications industry in many areas: signaling tone generation and detection, frequency band shifting, filtering to remove power line hum, etc. Three specific examples from the telephone work will be discussed here: multiplexing, pression, and echo control. Multiplexing There are approximately one billion telephones in the world. At the press of a few buttons, switching works allow any one of these to be connected to any other in only a few seconds. The immensity of this task is mind boggling! Until the 1960s, a connection between two telephones required passing the analog voice signals through mechanical switches and amplifiers. One connection required one pair of wires. In parison, DSP converts audio signals into a stream of serial digital data. Since bits can be easily intertwined and later separated, many telephone conversations can be transmitted on a single channel. For example, a telephone standard known as the Tcarrier system can simultaneously transmit 24 voice signals. Each voice signal is sampled 8000 times per second using an 8 bit panded (logarithmic pressed) analogtodigital conversion. This results in each voice signal being represented as 64,000 bits/sec, and all 24 channels being contained in megabits/sec. This signal can be transmitted about 6000 feet using ordinary telephone lines of 22 gauge copper wire, a typical interconnection distance. The financial advantage of digital transmission is enormous. Wire and analog switches are expensive。 digital logic gates are cheap. Compression When a voice signal is digitized at 8000 samples/sec, most of the digital information is redundant. That is, the information carried by any one sample is largely duplicated by the neighboring samples. Dozens of DSP algorithms have been developed to convert digitized voice signals into data streams that require fewer bits/sec. These are called data pression algorithms. Matching unpression algorithms are used to restore the signal to its original form. These algorithms vary in the amount of pression achieved and the resulting sound quality. In general, reduce the data rate from 64 kilobits/sec to 32 kilobits/sec results in no loss of sound quality. When pressed to a data rate of 8 kilobits/sec, the sound is noticeably affected, but still usable for long distance telephone works. The highest achievable pression is about 2 kilobits/sec, resulting in sound that is highly distorted, but usable for some applications such as military and undersea munications. Echo control Echoes are a serious problem in long distance telephone connections. When you speak into a telephone, a signal representing your voice travels to the connecting receiver, where a portion of it returns as an echo. If the connection is within a few hundred miles, the elapsed time for receiving the echo is only a few milliseconds. The human ear is accustomed to hearing echoes with these small time delays, and the connection sounds quite normal. As the distance bees larger, the echo bees increasingly noticeable and irritating. The delay can be several hundred milliseconds for intercontinental munications, and is particularly objectionable. Digital Signal Processing attacks this type of problem by measuring the returned signal and generating an appropriate antisignal to cancel the offending echo. This same technique allows speakerphone users to hear and speak at the same time without fighting audio feedback (squealing). It can also be used to reduce environmental noise by canceling it with digitally generated antinoise. Audio Processing The two principal human senses are vision and hearing. Correspondingly, much of DSP is related to image and audio processing. People listen to both music and speech. DSP has made revolutionary changes in both these areas. Music The path leading from the musician39。s microphone to the audiophile39。s speaker is remarkably long. Digital data representation is important to prevent the degradation monly associated with analog storage and manipulation. This is very familiar to anyone who has pared the musical quality of cassette tapes with pact disks. In a typical scenario, a musical piece is recorded in a sound studio on multiple channels or tracks. In some cases, this even involves recording individual instruments and singers separately. This is done to give the sound engineer greater flexibility in creating the final product. The plex process of bining the individual tracks into a final product is called mix down. DSP can provide several important functions during mix down, including: filtering, signal addition and subtraction, signal edit