【正文】 e most minor adjustments in a program of instructions, it was necessary to interrupt machining operations and make a new tape. It was also still necessary to run the tape thought the reader as many times as there were parts to be produced . Fortunately, puter technology bee a reality and soon solved the problems of NC, associated with punched paper and plastic tape.The development of a concept known as numerical control (DNC) solve the paper and plastic tape problems associated with numerical control by simply eliminating tape as the medium for carrying the programmed instructions . In direct numerical control, machine tools are tied, via a data transmission link, to a host puter and fed to the machine tool as needed via the data transmission linkage. Direct numerical control represented a major step forward over punched tape and plastic tape. However ,it is subject to the same limitation as all technologies that depend on a host puter. When the host puter goes down , the machine tools also experience down time . This problem led to the development of puter numerical control.The development of the microprocessor allowed for the development of programmable logic controllers (PLC) and microputers . These two technologies allowed for the development of puter numerical control (CNC).With CNC , each machine tool has a PLC or a microputer that serves the same purpose. This allows programs to be input and stored at each individual machine tool. CNC solved the problems associated downtime of the host puter , but it introduced another problem known as data management . The same program might be loaded on ten different microputers with no munication among them. This problem is in the process of being solved by local area networks that connectDigital Signal ProcessorsThere are numerous situations where analog signals to be processed in many ways, like filtering and spectral analysis , Designing analog hardware to perform these functions is possible but has bee less and practical, due to increased performance requirements, flexibility needs , and the need to cut down on development/testing time .It is in other words difficult pm design analog hardware analysis of signals.The act of sampling an signal into thehat are specialised for embedded signal processing operations , and such a processor is called a DSP, which stands for Digital Signal Processor . Today there are hundreds of DSP families from as many manufacturers, each one designed for a particular price/performance/usage group. Many of the largest manufacturers, like Texas Instruments and Motorola, offer both specialised DSP’s for certain fields like motorcontrol or modems ,and general highperformance DSP’s that can perform broad ranges of processing tasks. Development kits an` software are also available , and there are panies making software development tools for DSP’s that allows the programmer to implement plex processing algorithms using simple “drag ‘n’ drop” methodologies.DSP’s more or less fall into two categories depending on the underlying architecturefixedpoint and floatingpoint. The fixedpoint devices generally operate on 16bit words, while the floatingpoint devices operate on 3240 bits floatingpoint words. Needless to say , the fixedpoint devices are generally cheaper . Another important architectural difference is that fixedpoint processors tend to have an accumulator architecture, with only one “general purpose” register , making them quite tricky to program and more importantly ,making Cpilers inherently inefficient. Floatingpoint DSP’s behave more like mon generalpurpose CPU’s ,with registerfiles.There are thousands of different DSP’s on the market, and it is difficult task finding the most suitable DSP for a project. The best way is probably to set up a constraint and wishlist, and try to pare the processors from the biggest manufacturers against it.The “big four” manufacturers of DSPs: Texas Instruments, Motorola, ATamp。T and Analog Devices.Digitaltoanalog conversionIn the case of MPEGAudio decoding , digital pressed data is fed into the DSP which performs the decoding , then the decoded samples have to be converted back into the analog domain , and the resulting signal fed an amplifier or similar audio equipment . This digital to analog conversion (DCA) is performed by a circuit with the same name amp。 but id the doors or windows are opened or closed intermittently during the operating period, the final temperature inside the ho