1. Field of the Invention
This invention is in the field of non-recursive digital filters used for digital processing of signals such as speech and video.
2. Prior Art
Digital filters process information by performing a predetermined set of arithmetic operations on digitally coded samples of that information. In conventional digital filters the information to be processed is sampled at a constant rate and each sample converted to a digital word usually consisting of a number of binary digits. Signals representative of these digitally coded samples are then applied to the digital filter at the sampling rate, the reciprocal of which is the sampling interval. A general background discussion of digital filters and their applications is found in Gold and Rader, "Digital Processing of Signals", McGraw-Hill Book Co., 1969.
Among the advantages of digital filters as signal processing devices is the fact that one such filter can be used to process data from several sources or channels simultaneously. This is generally accomplished by applying samples from each of the sources to the filter in a predetermined sequence, for example by time division multiplexing (TDM). Each delay unit is extended to provide capacity for the simultaneous storage of one sample from each source. If the filter coefficients remain constant, data from all sources will be subjected to the same filter function. By providing several sets of filter coefficients, it is possible to process data from each source using a different transfer function. Other than extending the capacity of the delay units, no other changes in filter configuration are necessary or necessitated by multiplexing.
The complexity of the digital transfer function or functions to be obtained determines the complexity of the required filter network. Hence, the more complex the transfer functions, the more delay units and signal paths are required in the filter. Since such a filter will in general include two adders, each of which must be capable of forming the sum of a plurality of simultaneously applied digital words corresponding in number to the number of delay units in the filter, the complexity of these adders is directly dependent on the complexity of the filter functions to be realized.
One such class of digital filters is known as non-recursive filters, that is those with only feed forward signal paths or taps. Non-recursive filters are unique in that the phase and frequency responses of such filters can be independently specified. A typical conventional non-recursive filter is composed of a tapped delayed element which in general consists of cascaded registers, multipliers and an adder. At each tap location there is a multiplier which multiplies the tap output with a coefficient and delivers the product to the adder. The input signal which is a series of sampled values appearing at a constant sampling rate is applied to the output of the tapped delay element. The output is obtained at the successive adder output and is delivered elsewhere for successive processing at the same sampling rate. Coefficient values are selected by design to give the desired filter transfer function and correspond to the sample values of the impulse response of the filter.
Frequently, non-recursive filters must include a relatively large number of feed forward signal paths. Hence, although the first or feedback adder of the general filter configuration as discussed above is entirely absent from a non-recursive filter network, the remaining adder (i.e., the second or feed forward adder) must often be exceedingly complex being required to form a sum of from 30 to 50 simultaneously applied signal quantities. Since only one filter cycle can be allowed for formation of this sum, the complexity of the required adder may limit the speed at which the filter can be made to operate. For this reason, the conventional non-recursive digital filter requires very fast multipliers to accomodate the necessary designed speed. Additionally, the number of multiplications and additions must be equal to the order of the filter. In a system such as commercial TV signal processing, where the output must be produced at least every 100 nanoseconds, utilization of non-recursive filters has not been considered practical because of the limited speed of available multipliers. Also, because of the speed requirement, each multiplication would demand dedicated hardware and a filter could not be built within reasonable limits of hardware cost. Also, the most efficient adders are neither readily adaptable to a variety of uses nor can filters employing such adder configuration be constructed of standardized subunits or modules. Attempts to overcome these inherent deficiencies in non-recursive adders in order to make them commercially viable are known in the prior art. U.S. Pat. No. 3,665,171 is typical of one such attempt in which the delay units in a non-recursive digital filter are selectively altered and additional elements comprising alternating series of two-input adders and partial sum delay units are used to perform the required addition of weight signal sampling. The interposition of these devices eliminates the large adder which is a salient feature of prior art non-recursive filter units. However, this patent still relies heavily on the use of multipliers in the filter circuit. Similarly, the patent to Jackson, U.S. Pat. No. 3,537,015 discloses a method of reducing the number of multiplier circuits in a digital phase equalizer but not the concept of the elimination in toto of these elements. In the Jackson patent, a reordering of the summing and multiplying operations reduces the number of required multiplier circuits in the filter.