Removal of low frequency transient noise from old recordings using model-based signal separation techniques

This paper is concerned with the removal of low frequency transient noise from old gramophone recordings and film sound tracks. Low frequency transients occur as a result of large breakages or discontinuities in the recorded medium which excite a long-term resonance in the playback apparatus. We present a signal separation-based approach to this problem. Audio signals and noise transients are modelled as autoregressive (AR) processes which are additively superimposed to give the observed waveform. A maximum a posteriori method is presented for separation of the two processes. A modification of this scheme allows for modelling of the large discontinuity at the start of each noise transient and successful restorations are demonstrated. A more practical scheme is then developed which uses a Kalman filter to implement the separation. In order to avoid low frequency distortions to the audio signal, the excitation variance of the noise transient model is tapered exponentially to zero away from the discontinuity. The method is fully automated and more practical to implement than existing schemes for removal of such defects. Results indicate a high level of performance.