Improvement of a parametric model for audio signal compression at low bit rates

The HILN (harmonic, individual lines, and noise) audio coder is included in the MPEG-4 audio standard for coding audio signals at very low bit rates (at and below 16 kbit/s). It uses a parametric model to efficiently represent audio signals under low bit rate constraints. In this paper we propose several improvements to the estimation and coding of the HILN model parameters. These include: estimation of the frequencies of closely spaced tones, estimation of multi-pitch periods, improved amplitude representation of harmonics, and better use of the underlying perceptual model. The proposed improvements result indeed in better audio quality, manifested in a 0.4 points improvement in EAQUAL score, used for evaluating the audio quality, as compared to HILN, at both 16 and 12 kbit/s.