Next |
Prev |
Top
|
JOS Index |
JOS Pubs |
JOS Home |
Search
Human Audio Perception: Masking
The human auditory system has some interesting properties, which are
exploited in perceptual audio coding. We have a dynamic frequency
range from about
20 to 20000 Hz, and we hear sounds with intensity varying over many magnitudes.
The hearing system may thus seem
to be a very wide-range instrument, which is not altogether
true. To obtain those characteristics , the hearing is very adaptive --
what we hear depends on what kind of audio environment we are in. In the
presence of a strong white noise, for example, many weaker sounds get
masked (see section 3.2), and thus we cannot hear them at
all. Some of these masking characteristics are due to the physical ear,
and some are due to the processing in the brain.
Using masking principles, experiments have been performed by others
where correctly shaped noise has been added to audio data without audible
effect down to an SNR of 25 dB. On the
other hand, deliberately ``wrongly'' shaped noise,
i.e noise with high energy in
sensitive areas can be audible up to an SNR of 90 dB.
I will now show some of the most important masking properties of
the ear, and the models of those.
The models are combined in the coder to produce a
masking threshold
curve every 256 samples (5.8 ms), which is used to quantize the audio
data. According to the model, noise under that threshold is completely
inaudible to the listener. See section 3.4 for a description how
the masking threshold is used in quantization.
Subsections
Next |
Prev |
Top
|
JOS Index |
JOS Pubs |
JOS Home |
Search
Download bosse.pdf