【正文】 ing, etc. One of the most interesting DSP applications in music preparation is artificial reverberation. If the individual channels are simply added together, the resulting piece sounds frail and diluted, much as if the musicians were playing outdoors. This is because listeners are greatly influenced by the echo or reverberation content of the music, which is usually minimized in the sound studio. DSP allows artificial echoes and reverberation to be added during mix down to simulate various ideal listening environments. Echoes with delays of a few hundred milliseconds give the impression of cathedral like locations. Adding echoes with delays of 1020 milliseconds provide the perception of more modest size listening rooms. Speech generation Speech generation and recognition are used to municate between humans and machines. Rather than using your hands and eyes, you use your mouth and ears. This is very convenient when your hands and eyes should be doing something else, such as: driving a car, performing surgery, or (unfortunately) firing your weapons at the enemy. Two approaches are used for puter generated speech: digital recording and vocal tract simulation. In digital recording, the voice of a human speaker is digitized and stored, usually in a pressed form. During playback, the stored data are unpressed and converted back into an analog signal. An entire hour of recorded speech requires only about three megabytes of storage, well within the capabilities of even small puter systems. This is the most mon method of digital speech generation used today. Vocal tract simulators are more plicated, trying to mimic the physical mechanisms by which humans create speech. The human vocal tract is an acoustic cavity with resonant frequencies determined by the size and shape of the chambers. Sound originates in the vocal tract in one of two basic ways, called voiced and fricative sounds. With voiced sounds, vocal cord vibration produces near periodic pulses of air into the vocal cavities. In parison, fricative sounds originate from the noisy air turbulence at narrow constrictions, such as the teeth and lips. Vocal tract simulators operate by generating digital signals that resemble these two types of excitation. The characteristics of the resonate chamber are simulated by passing the excitation signal through a digital filter with similar resonances. This approach was used in one of the very early DSP success stories, the Speak amp。 Spell, a widely sold electronic learning aid for children. Speech recognition The automated recognition of human speech is immensely more difficult than speech generation. Speech recognition is a classic example of things that the human brain does well, but digital puters do poorly. Digital puters can store and recall vast amounts of data, perform mathematical calculations at blazing speeds, and do repetitive tasks without being bored or inefficient. Unfortunately, present day puters perform very poorly when faced with raw sensory data. Teaching a puter to send you a monthly electric bill is easy. Teaching the same puter to understand your voice is a major undertaking. Digital Signal Processing generally approaches the problem of voice recognition in two steps: feature extraction followed by feature matching. Each word in the ining audio signal is isolated and then analyzed to identify the type of excitation and resonate frequencies. These parameters are then pared with previous examples of spoken words to identify the closest match. Often, these systems are limited to only a few hundred words。 can only accept speech with distinct pauses between words。 and must be retrained for each individual speaker. While this is adequate for many mercial applications, these limitations are humbling when pared to the abilities of human hearing. There is a great deal of work to be done in this area, with tremendous financial rewards for those that produce successful mercial products. Echo Location A mon method of obtaining information about a remote object is to bounce a wave off of it. For example, radar operates by transmitting pulses of radio waves, and examining the received signal for echoes from aircraft. In sonar, sound waves are transmitted through the water to detect submarines and other submerged objects. Geophysicists have long probed the earth by setting off explosions and listening for the echoes from deeply buried layers of rock. While these applications have a mon thread, each has its own specific problems and needs. Digital Signal Processing has produced revolutionary changes in all three areas. Radar Radar is an acronym for Radio Detection And Ranging. In the simplest radar system, a radio transmitter produces a pulse of radio frequency energy a few microseconds long. This pulse is fed into a highly directional antenna, where the resulting radio wave propagates away at the speed of light. Aircraft in the path of this wave will reflect a small portion of the energy back toward a receiving antenna, situated near the transmission site. The distance to the object is calculated from the elapsed time between the transmitted pulse and the received echo. The direction to the object is found more simply。 you know where you pointed the directional antenna when the echo was received. The operating range of a radar system is determined by two parameters: how much energy is in the initial pulse, and the noise level of the radio receiver. Unfortunately, increasing the energy in the pulse usually requires making the pulse longer. In turn, the longer pulse reduces the accuracy and precision of the elapsed time measurement. This results in a conflict between two important parameters: the ability to detect objects at long range, and the ability to accurately determine an object39。s distance. DSP has revolutionized radar in three a