iSP Technologies HUM EXTRACTOR +DECIMATOR G Owner's Manual page 5

on your countries AC line frequency. The HUM EXTRACTOR +DECIMATOR G is a digital
processor and requires optimizing the input signal level for proper headroom in the digital
signal path. Start by turning the guitar all the way up play a HOT signal adjusting the INPUT
LEVEL control so that the 0db LED just flashes with maximum playing levels. Avoid
excessive peaking of the INPUT LEVEL meter with playing at maximum signal level. Next
adjust the HUM EXTRACTOR Threshold until any background HUM is removed. Start with
the Threshold of the DECMATOR at minimum setting. For maximum transparency it is best to
adjust this threshold setting up and down slightly for the optimum transparency and removal
of the hum components. The HUM EXTRACTOR is a dynamic operation similar to the
DECIMATOR noise reduction. The HUM EXTRACTOR operates in multiple frequency bands
to selectively remove the HUM components which appear in the signal within each frequency
band. The operation is dynamic using precision level detection in each frequency band to
detect the presence of the HUM and audio signal and dynamically filter these components
within each frequency band of operation. Adjusting the HUM EXTRACTOR threshold adjusts
the dynamic operation of the HUM filtering in each band based on the level of the audio
signal. As noted above, it is best to adjust the HUM EXTRACTOR threshold first to remove
the HUM in the signal and then adjust the DECIMATOR to remove the remaining gain noise.
The DECIMATOR level detection is done after the HUM EXTRACTOR which allows the
DECIMATOR to operate with more precision after any HUM is removed.
When properly used, the DECIMATOR should be completely transparent; it should have no
effect on the audio signal other than to remove the background noise. To maximize the
performance of the DECIMATOR, it is necessary to understand both the operation of the
controls and the principles of how the system operates. After this understanding, it will be
easier to set up the DECIMATOR to suit any application.
The DECIMATOR achieves noise reduction by employing two individual noise reduction
processes, which work cohesively together to attain superior results. These two processes
are: Dynamic Low Pass Filtering and Low Level Downward Expansion.
Dynamic Low Pass Filtering is done by use of a high quality voltage controlled sliding low
pass filter. A frequency sensitive audio level detection circuit incorporating ISP Technologies
patent pending Time Vector Processing circuit controls the dynamic filter. When the audio
input signal contains high frequency information the dynamic filter increases in bandwidth to
allow the audio signal to pass unaltered and shown in the simplified graph below.
When the high frequency information in the input signal decreases the dynamic filter
bandwidth will track the decrease in high frequency and eliminate high frequency noise that
remains in the input signal. The simplified graph below shows the dynamic filter response
when there is no high frequency audio above 1KHz.