Mirror filters with minimum delay responses for use in subband coders

The technique of subband coding (SBC) is discussed with particular regard to its potential usefulness in the public telephone network. It is hypothesised that significant signal delay, arising from the filtering operations, could cause problems during 2-way transmission. For this reason minimum delay systems are investigated, and criteria are found for filters that could be used in their realisation. A class of filters that meet these criteria is identified, and a method for their synthesis is developed. The distinctive properties of these filters are: that they are recursive networks, they occur in pairs with mirror amplitude symmetry, but, unlike most other SBC filter pairs, they are not constrained to have quadrature phase relationships. The performance of these recursive filters is discussed and compared with other filters commonly used in SBC systems.