Telos ZephyrExpress user manual Coding

deciBels

90

60

40

20

0

Masking

Threshold

Threshold

in quiet

Inaudible

Signal

Masker

Masked Sound

.02	.05 .1	.2	.5	1	2	5	10 20
				kiloHertz

Masking effects in the frequency domain. A masking signal inhibits audibility of signals

adjacent in frequency and below the threshold.

To benefit from the masking effects, perceptual coders use a filterbank to divide the input audio into multiple bands for analysis and processing. The maximum masked noise level is calculated depending upon the spectral content, and the available bits are allocated so as to keep the quantization noise below the masking threshold at every point in the spectrum.

While coding efficiency increases with more bands and better frequency resolution, the time domain resolution decreases simultaneously owing to an inevitable side-effect of the band filtering process: higher frequency resolution requires a longer time window – which limits the time resolution. Happily, masking works also in the time domain. A short time before and a longer time after a tone is switched on and off, other signals below a threshold amplitude level are not noticeable. Filterbanks with higher frequency resolution naturally exploit the ear’s time-masking properties.

deciBels

90

60

40

20

0

Pre- Simultaneous- Post- Masking

-60 0 60 100 150 0 50 100 150 200 t (in ms.)

Masking effects in the time domain. Masking occurs both before and after the masking signal.

5-4 CODING