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HUSHm
NOISE REDUCTION AND DOWNWARD EXPANSION GATING
High quality compact discs provide very wide range volume levels with low background hiss levels.
Unfortunately, the car is a very noisy electrical environment so vehicle noises (pops and alternator
whine) often creep into the audio signal somewhere in the installation. Although mid and high level
music tends to mask background noises, vehicle noises become quite audible, and annoying, during
quiet music passages. This problem has been known for some time in the professional audio field
so a device known as a “Noise Gate” has been used to mute the signal path during quiet passages.
The problem with this system is the “choppy” sound as the Gate opens and closes with music
which is known as “pumping”.
The
HUSHTM
noise reduction circuits avoid this “pumping” by the use of a downward expander
which effectively expands the dynamic range of the signal thus pushing down the background
noise in relation to the desired signal. Also, the
HUSHTM
circuits offer a continuously variable set
point that smoothly moves up and down so none of the dramatic “pumping” of the desired signal is
experienced as with a noise gate. An added benefit is the ability of the
HUSHTM
circuit to reject
noise on recorded music such as cd’s, cassettes, and even on the AM/FM car radio.
INPUT/OUTPUT ELECTRICAL TRANSFER FUNCTION
-80
-60
-40 -20
OdB
OUTPUT
SIGNAL
OUTPUT SIGNAL
OUTPUT SIGNAL
SIGNAL TO NOISE CHARACTERISTICS
VERY LOW
MUSIC SIGNAL
LOW LEVEL
MUSIC SIGNAL
MID
LEML
MUSIC SIGNAL
5