THEORY OF OPERATION
The U.S. Audio / Whirlwind MPM4 is designed to present
the user with a simple and intuitive analog style interface
while employing state of the art digital signal processing for
implementation of the signal path. Audio interfacing and
user interaction is familiar and easy to use no computer is
required, and the digital processing assures precision,
consistency and the ability to perform functions that would
be difficult or impossible with analog circuitry alone. For
the following discussion, it may be useful to refer to the
“MPM SIGNAL FLOW” diagram.
Audio Inputs
The Line Level input connections combine the left and right
signals into a single mono channel. After buffering, signal
passes through a digitally controlled gain amplifier. This
provides gain or attenuation to allow normalizing the audio
level prior to the Analog to Digital conversion. This
assures that the input maintains a wide dynamic range
over a range of audio levels up to +24dB maximum. The
device used for Level Trims is made by Burr Brown and
features precision, very low distortion and minimal noise.
Priority Inputs
Priority inputs are universal, they can accept microphone
or line level signals, balanced or unbalanced up to a level
of +24dB. The balanced microphone preamp features a
discrete transistor front end with digitally adjustable gain.
This is followed by a digitally controlled gain amplifier for
precise level trim. A 20dB pad is provided to handle line
level signals, and the XLR input features 15V phantom
power.
Zone Outputs
After conversion from the Digital domain to the analog
domain, the output signals pass through a digitally
controlled gain amplifier, followed by output buffers. As
with the Audio Inputs, the device used for Level Trims is
made by Burr Brown. The XLR and RCA outputs use
separate buffers. This allows both outputs to be used
simultaneously without concern for loading.
Digital Signal Processor
The Whirlwind MPM4 features an Analog Devices SHARC
processor. This processor features very high performance
and allows the use of 32 bit floating point representation of
audio signals. This assures that there is no degradation of
audio signals as a result of digital processing. Any artifacts
are kept well below the noise levels already present in the
source signals.
DSP Metering and Clip detectors
Level detection is done within the DSP and provides
accurate and consistent level metering and perfect
matching between channels and measurement points.
Clip detectors use separate level detectors with very fast
response to allow clip detection that recognizes even the
shortest periods of clipping. Note that in normal mode, the
Zone clip detectors will display not only output clipping, but
also any input clipping that is routed to the Zone indicated.
This is especially useful during setup. Any clipping that
occurs will be displayed and, the operator may then select
Input Trim or Priority Input Trim modes to see if they are the
sources of clipping.
10
100-250VAC
25W
POWER
SUPPLY
REMOTE
CONTROLS
1-4
FRONT PANEL PROCESSOR
CO DISPLAYS
ZONE
OUTPUTS
1-4
+24dBV MAX
REAR PANEL DUCK
LEVEL SWITCHES
-20, 0, -70
ZONE
MAX
LEVEL
-60 to +20dB
D-A
METERS
+ CLIP
DETECT
METERS
+ CLIP
DETECT
4
DIGITAL SIGNAL PROCESSOR
ZONE
VOLUME
1
2
4 BAND EQ
METERS
+ CLIP
DETECT
METERS
+ CLIP
DETECT
4 BAND EQ
A-D
PRIORITY
TRIM
0 TO +60dB
A-D
4
4
PAD
PHANTOM POWER
PRIORITY
IN
1-4
+24dBV MAX
IN
1-4
+24dBV MAX
4
INPUT
TRIM
-20 TO +20dB
METERS
+ CLIP
DETECT
4 BAND EQ
4
PRIORITY
ROUTING
and
DUCKING
SOURCE
TO ZONE
ROUTING
MPM4 SIGNAL FLOW
ZONE VOLUME
PRIORITY OVERRIDE