Audio Signal Processing and Coding

5.7 EXAMPLE CODEC PERCEPTUAL MODEL: ISO/IEC 11172-3 (MPEG - 1) PSYCHOACOUSTIC MODEL 1

It is useful to consider an example of how the psychoacoustic principles described thus far are applied in actual coding algorithms. The ISO/IEC 11172-3 (MPEG-1, layer 1) psychoacoustic model 1 [ISOI92] determines the maximum allowable quantization noise energy in each critical band such that quantization noise remains inaudible. In one of its modes, the model uses a 512-point FFT for high-resolution spectral analysis (86.13 Hz), then estimates for each input frame individual simultaneous masking thresholds due to the presence of tone-like and noise-like maskers in the signal spectrum. A global masking threshold is then estimated for a subset of the original 256 frequency bins by (power) additive combination of the tonal and nontonal individual masking thresholds. The remainder of this section describes the step-by-step model operations. Sample results are given for one frame of CD-quality pop music sampled at 44.1 kHz/16-bits per sample. We note that although this model is suitable for any of the MPEG-1 coding layers I–III, the standard [ISOI92] recommends that model 1 be used with layers I and II, while model 2 is recommended for layer III (MP3). The five steps leading to computation of global masking thresholds are described in the following Sections.

5.7.1 Step 1: Spectral Analysis and SPL Normalization

Spectral analysis and normalization are performed first. The goal of this step is to obtain ...