A relation between subband and frequency-domain adaptive filtering

Modelling acoustic impulse responses with lengths up to 250 ms is required for high quality echo cancellation in strongly reverberating environments, leading to FIR adaptive filters with several thousands of taps. Classical LMS-based solutions then clearly fail as they exceed the computational capabilities of present-day DSPs. Cheaper alternative solutions have been proposed and are mainly based on either subband or frequency-domain techniques. Subband filter implementations have many interesting properties but their inherent delay and residual errors have made them unattractive for real-time applications up till now. On the other hand, frequency-domain adaptive filters do not suffer from these problems despite being (nearly) equivalent to subband adaptive filters. We specify 3 realisation conditions for DFT modulated subband schemes. Standard subband adaptive filters cannot fulfil all conditions. We explain the operation of the frequency-domain techniques in the `subband jargon´ and show that the realisation conditions can be fulfilled in this case