22 . Digital speech coder with baseband residual coding. A digital speech coder of the baseband RELP-type (Residual-Excited Linear Prediction) comprises a transmitter (1) having an LPC-analyser (10), a first adaptive inverse filter (11), a decimation lowpass filter (26) for selecting the baseband prediction residue and an encoding-and-multiplexing circuit (17), and a receiver (2) having a demultiplexing-and-decoding circuit (21), an interpolator (27) and a first adaptive synthesizing filter (14). The occurrence of 'tonal noises' due to the spectral folding in interpolator (27) is effectively counteracted by arranging prior to the decimation lowpass filter (26) in the transmitter (1) a second adaptive inverse filter (28) which with the aid of an autocorrelator (31) removes possible periodicy from the speech band residue, and by including subsequent to the interpolator (27) in the receiver (2) a corresponding second adaptive synthesis filter (32), which reintroduces the desired periodicity in the excitation signal.
DIGITAL SPEECH CODER WITH BASEBAND RESIDUAL CODING