The versatility of the allpass digital filter in 1-D and 2-D

Digital filters have found their way into many products from everyday consumer items such as telephones to advanced maritime and military communications and avionics systems. Much filtering can now be carried out as one of many software routines implemented on a DSP chip although there are applications in which the speed offered by these devices is not sufficient. In the general area of real-time digital image processing, a large number of floating-point operations per second need to be performed and special purpose hardware must be designed. It is therefore prudent to search for hardware efficient building blocks that may be used for any of the applications above. This presentation introduces a DSP building block with many interesting and potentially useful features such as low component count, low roundoff noise, bounded output and so on. This block is the allpass filter and has been around since the early days of filters in its analogue form. We discuss its structure and properties and look at simple way to design filters made up from these blocks in both 1-D and 2-D. The presentation concludes with a look at some topical applications.