2
Mackie Industrial White Paper
Noise Sensing
September 2000
1 Algorithm Overview
This section summarizes the Ambient Noise Sensing Algorithm
(see Figure 1) implemented in the SP-DSP1™. The algorithm is
used to increase the sensitivity to ambient noise in the room by
rejecting the source signal (music) that is broadcast into the room.
This allows the SP-DSP1™ to control the volume of the music
based on the room ambient noise. The better the rejection of the
music signal the more sensitive the gain control without runaway
gain problems.
The algorithm adapts to the room characteristics by comparing
the room response to the source signal. It computes its own
approximation of the room response in order to cancel the music
signal from the signal picked up by a room microphone [1-5,7].
Room size is the most important factor determining the effective-
ness of the algorithm. Small rooms (ofce size) can achieve as
much as 40dB music rejection, while larger rooms (nightclubs
or restaurants) might only achieve 10-20dB. Even 10dB rejection
allows noticeably better sensitivity to noise.
32-bit
Floating-Point
DSP
Stereo
DAC
Stereo
ADC
EPROM
SERIAL
EEPROM
Program Input
Ambient Mic
Input
Program
Output
RX
GND
`
TX
RS-232
TRANS.
RS-232 DSUB9
Figure 0: Hardware Block Diagram
FIR
Down
Sampler
Down
Sampler
Anti-Alias
Filter
Anti-Alias
Filter
G
RMS
Measure
Coefficient
Calculator
Compander
error
Y
-
+
Noise
Threshold
Override
nLMS Adaptive Filter
Figure 1: Algorithm Block Diagram
