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(1) Significant improvement in noise
cancellation
Figure 2 shows the internal block diagram of
the MDR-NC500D.
To describe the processing simply, first the
sound  that arrives at the user's ear is subtracted
from the audio source signal  to
extract the noise signal that leaks into the
inside of the headphone from the outside environment.
With this signal, a reverse phase
sound has been created and cancellation is
achieved by playing back this signal along
with the audio signal .
However, since there is a time lag between the
signal created for cancellation and the audio
source signal, if the reverse phase sound is
simply issued, feedback (a sound with a peak
due to resonance) will occur. Therefore a filter
circuit is used to remove the sound in the
high-frequency band that is the cause of this
feedback.
There were, however, problems with the filtering
precision when analog circuits were
used. When several types of filtering circuits
were combined to increase the precision, the
circuit was saddled with the problem that
inversely, noise increased. By replacing the filter
circuit with the Sony DNC (digital noise cancellation) software engine, Sony was able
to achieve precise filtering and increase performance
significantly.
(2) Achieving a balanced sound
What sort of sound do we perceive as "noise"?
For example, consider a bus driving over a
bump with a thud. The chassis is distorted,
pressure changes occur within the bus itself,
and the riders feel pressure and vibration.
Actually, this is the noise that should be cancelled.
While this is also the case when an airplane
experiences sudden pressure changes, these
changes in sound pressure (air pressure) in
the low-frequency band are 10 to 100 times
greater than the sound pressure level changes
in the audio signal itself.
Without the power required to push back
against these high-amplitude low-frequency
signals, that is, if the sound pressure cannot be
created, we could not hope for a pleasant listening
experience. Therefore, noise canceling
headphones are tuned to have high sensitivity
(electric to audio conversion efficiency) in
the low-frequency band. This is a major difference
compared to normal high-fidelity headphones.
There is, however, a problem. Since the sound
to be reproduced from the audio source does
contain these frequencies, there is a tendency for the low-frequency range to be emphasized.
This can result in an unbalanced sound
in which, for example, the vocals can be
pushed into the background and the bass
seems to be out in front. To resolve this issue,
Sony implemented a digital equalizer in the
first stage of the noise canceling block. This
results in a signal-to-noise ratio that would
not be possible with an analog equalizer and
provides precise equalization to create a natural
tonal balance. (See figure 3.)
(3) Handles a wide range of noise environments
– AI noise cancellation function
Sony also focused on the fact that the noise
cancellation effect differs depending on the
noise environment. Sony isolated representative
noise environments ((1) airplane cabins,
(2) trains and buses, and (3) offices) and,
through extensive research, succeeded in
developing three types of noise cancellation
optimized for each of these environments.
With just a press of a button, the MDR-NC500D
analyzes the noise in the current
environment and automatically selects the
optimal mode. |
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Implementation is Possible
Because it is Digital
Our Strength was Our
Company-Internal Resources
Vol.53