Abstract: A hearing aid includes a filter in an electrical feedback path, the characteristics of which filter are calculated to model acoustic coupling between the receiver and microphone of the aid. A limiter is inserted in the main electrical pathway between the microphone and the receiver to provide stability in the presence of sudden sound bursts. A noise signal is injected continuously into the electrical circuit and is used to adapt the characteristics of the filter to accommodate changes in the acoustic coupling. The level of the noise signal can be varied to match changes in residual signal level to maintain signal to noise ratio and to optimize rate of adaption commensurate with satisfactory hearing function while the noise itself is unobtrusive to the user.

Abstract: A digital filter arrangement in which an input pulse stream is entered in consecutive non-overlapping groups into a first shift register. The contents of the first register are transferred in parallel into a plurality of shift registers. Bits are read-out from selected positions of the plurality of shift registers to form address words for individual read-only memories (ROM). Each ROM has stored therein weighted impulse response outputs which are read out in accordance with the address words. The outputs from the ROM's are added in logic circuitry and the combined output is fed into a latch. The contents of the latch are entered into a buffer from where they are read out serially. If the input clock rate is 28 MHz and the plurality of shift registers number eight, the plurality of shift registers, the ROM's and the logic circuitry operate at 3.5 MHz.