当然，信号处理问题并不限于处理一维信号。多数图像处理应用都需要采用二维信号处理技术。X射线透视片的图像增强、为发现森林火灾或农业灾害用的航空摄影照片的图像增强和分析、卫星气象照片的分析以及月球或深空探测器送来的电视图像的增强等等，都属于二维信号处理。石油勘探、地震测量和核试验监测等要求做的地震数据分析同样要用到多维信号处理技术。

　　直到现在，信号处理主要还是用模拟设备实现的。到了上世纪50年代出现了一些例外情况，在那些需要进行复杂信号处理的领域中尤为明显，例如，在分析某些地球物理数据时就是如此，这种数据可以事先记录在磁带上，然后在大型数字计算机上加以处理。这类问题是利用数字计算机进行信号处理的早期例子之一。这种信号处理方法一般不是实时的，例如仅仅几秒钟的数据，往往需要几分钟甚至几小时的机时才能处理完。即使是这样，数字计算机的灵活性仍然使这一方法十分引人注目。

　　就在同一时期，还出现了用计算机作信号处理的另一种方法。由于数字计算机的灵活性，人们常常在用模拟硬件实现信号处理系统之前，先在数字计算机上仿真。利用这种办法，一个新的信号处理算法或系统，在提供研制经费和器材把它造出来之前，可以先在灵活的实验环境下进行研究。林肯实验室和贝尔实验室做的声码器仿真就是这种仿真的一个典型的例子。在实验模拟信道声码器时，滤波器的特性常常会通过许多难以预知的途径影响得到的语言信号质量。而通过计算机仿真，可在模拟设备制作之前，调整这些滤波器的特性，并估计出整个系统的质量。 (未完待续)

　　Signal processing problems are not confined,of course,to one-dimensional signals.Many picture processing applications require the use of two-dimensional signal processing techniques.This is the case,in X-ray enhancement,the enhancement,and analysis of aerial photographs for detection of forest fires or crop damage,the analysis of satellite weather photos,and the enhancement of television transmissions from lunar and deep-space probes.Seismic data analysis as required in oil exploration,earthquake measurements and nuclear test monitoring also utilizes multidimensional signal processing techniques.

　　Until recently,signal processing has typically been carried out using analog equipment.Some exceptions to this were evident in the 1950s,particularly in areas where sophisticated signal processing was required.This was the case,for example,in the analysis of some geophysical data which could be recorded on magnetic tape for later processing on a large digital computer.This class of problems was one of the first examples of signal processing using digital computers.This type of signal processing could not generally be done in realtime;for example,minutes or even hours of computer time were often required to process only seconds of data.Even so,the flexibility of the digital computer made this alternative extremely inviting.

　　During this same period the use of digital computers in signal processing also arose in a different way.Because of the flexibility of digital computers,it was often useful to simulate a signal processing system on a digital computer before implementing it in analog hardware.In this way,a new signal processing algorithm,or system,could be studied in a flexible experimental environment before committing economic and engineering resources to constructing it.Typical examples of such simulations were the vocoder simulations carried out at Lincoln Laboratory and at Bell Laboratories.In the implementation of an analog channel vocoder,the filter characteristics often affect the quality of the resulting speech signal in unpredictable ways.Through computer simulations,these filter characteristics were adjusted and the quality of a system evaluated prior to construction of the analog equipment.（游米儿）