The AKG AS8 Automatic Microphone Mixer is an 8 channel, single rack space audio mixer. Using the unique Adaptive
Level Proportional mixing algorithm, the automatic action is inaudible and simple to set up. The AS8 offers remote control
of all channel levels and the master level. In addition, a sophisticated compressor/ leveller is included for dynamic range
control of the audio program material. The AS8 has an RS-232 port for remote setup or control by a computer or other RS232 compatible controller. More than one AS8 may be coupled together when applications call for more than 8 channels.
The AS8 uses a straightforward analog signal path to provide excellent audio performance. This is coupled with a
sophisticated microcontroller to implement both the automatic mixing and the compressor/leveller algorithms. The
Adaptive Level Proportional automatic mixing algorithm is used by the AS8. This algorithm uses the signal level pattern at
the microphones to derive a pattern of channel gains. For instance, if only one microphone channel is being spoken into,
its level is dominant and that channel gets most of the system gain, while all other channels are turned down. If more than
one microphone is active, they share system gain according to their relative levels. A “skewing” function gives some
advantage to microphones which are active over time, to minimize interruptions from transient noise at other microphones.
Since each channels’ gain is apportioned continuously based on its relative level, no abrupt gain changes are made. This
makes the automatic action virtually inaudible.
The Compressor/Leveller function uses a combination of the short-term and long-term signal dynamics to determine the
optimum amount of gain reduction in any situation. The effect of the function is to control both the average program level
(for consistent long-term sound system level) and to minimize any large short-term deviations. The gain reduction function
is gated, which means that there are no noise ups associated with program signal pauses.
Figure 1 shows the simplified block diagram of the AS8. The microphone preamp is a low-noise discrete design, which is
extensively RF protected. The preamp has three gain settings. The 0dB setting allows an input to accept line level signals,
while the +30dB and +50dB gain settings accommodate both low and high sensitivity microphones. 15V Phantom power
is provided for each channel, and is switchable. In addition, each channel may be switched into either the Direct (always
on) or Auto mode.
Following the Channel Level control, the audio signal passes through a high quality Voltage Controlled Amplifier (VCA).
This VCA, controlled by a signal from the system D/A converter, is used to implement gain control for the automatic mixing
function, as well as the remote level function. A Channel On LED indicates which channels are currently active.
Signals from the Expansion In and Expansion Out connectors allow multiple AS8 mixers to be combined if more than 8
channels are necessary for the application. In addition, a Mix Minus signal is available which allows easy interfacing to
teleconferencing hybrids.
An RS-232 port is available to allow many of the functions of the AS8 to be controlled either by a computer or a dedicated
control system (such as the AMX or Crestron systems).
Installing the AS8 properly is simple, and only requires attention to a few issues. The AS8 should be used in a grounded
metal rack. If only one AS8 is being used, it should be placed in the Master mode, using the rear panel switch. Erratic
performance will result if the unit is in the Slave mode, but no damage will be done to the unit. Proper operation will be
restored when the unit is switched to the Master mode. If multiple AS8s are used together, the first AS8 (i.e. the one with
Expansion In port used but not the Expansion Out port) should be switched to the Master mode, while all other AS8s should
be in the Slave mode. Figure 2 shows the correct interconnection for multiple AS8s.
Master
Switch in
MASTER Position
Slave
Switch in
SLAVE Position
Slave
Switch in
SLAVE Position
INTERCONNECTING MULTIPLE AS8s
Microphone connections should be made with good quality braid or foil shielded twisted pair. The shield should be used for
the ground (Pin 1) connection. If an unbalanced signal is used, the unbalanced signal lead should be connected to the “+”
input terminal, while the unbalanced ground lead should be connected to both the “-” and ground terminals of the input
connector.
Output connections to the Main Out connector can be made balanced or unbalanced. If an unbalanced connection is
desired, simply connect the unbalanced signal lead to the “+” output terminal, and the unbalanced ground lead to the
output ground terminal. Leave the “-” output terminal unconnected.
Access to the Remote Level Control port is accomplished using the DB-15 connector (supplied).
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FRONT PANEL DESCRIPTION
CHANNEL SECTION
CHANNEL LEVEL - Controls the relative signal level of each channel. Normal operation of the Channel Level control is
from about the 10 o’clock position to the 2 o’clock position.
CHANNEL ON LED - Gives an indication as to which channels are active.
LF - (TC version only) Adjusts the low frequency response of the channel.
HF - (TC version only) Adjusts the high frequency response of the channel.
MAIN SECTION
MAIN LEVEL - Controls the overall output level of the AS8. If multiple AS8s are used together, only the Main Level control
of the last (i.e. Master) AS8 will be operational.
COMPRESSOR/LEVELLER IN/OUT - Enables or disables the compressor/leveller function.
COMPRESSOR/LEVELLER THRESHOLD - Sets the signal level above which the Compressor/leveller is active. Note
that the Threshold level is measured
will not affect the amount of gain reduction.
before the Main Level control. This means that adjustments of the Main Level control
COMPRESSOR/LEVELLER GAIN REDUCTION METER - Shows the instantaneous amount of gain reduction applied
by the compressor/leveller system.
DIR OUT - Provides a line level signal from each individual channel. The direct output is a post-VCA signal. Any attenuation
from the auto mixing process, compressor/leveler system or remote attenuation will be reflected in the direct out signal.
This provides versatility for different mixing applications such as distributed overhead speaker systems. This 100 Ohm
output provides a +20dBu max signal into a high impedance load.
Note: The DIR OUT shares a ground with the MAIN OUT.
--Don’t use the CHANNEL IN grounds for the DIR OUT!--
When connecting to other equipment, the ground connection of the MAIN OUT will
also be adequate for all the direct outputs. A separate ground connection is not
needed for each direct out.
FUNCTION SELECTION SWITCHES 1-8 - Allow various functions of the input channels to be set.
Dip switch 1 chooses the channel mode. Auto mode (down position) sets operation to automatic, with channel gain
determined by its signal level relative to other microphones. When a channel is set to Direct, its signal will affect the levels
of other channels set to Auto, but it will not be affected by other channels (no NOM attenuation will occur on a channel
set to Direct). Note, however, that the Remote Level control feature functions regardless of the setting of the Auto/Direct
mode switch.
Dip switch 2 turns +15V phantom power on (down position) or off (up position).
Dip switches 3 and 4 set the gain of the input preamplifier. 0dB gain, for line level sources, is set when switches 3 and 4
are in the up position. 30dB gain, typically for high output (electret) microphones, is set when switch 3 is in the up position
and switch 4 is in the down position. 50dB gain, used for low output (dynamic) microphones, is set when both switches
are in the down position.
MAIN OUT - Provides a balanced system output. The Main Out may also be used in an unbalanced mode by connecting
the “+” terminal of the Main Out to the signal lead of the unbalanced device, and the ground terminal of the Main Out to
the ground of the unbalanced device. Do not connect the “-” terminal of the Main Out.
REMOTE LEVEL CONTROL - Allows remote control of both channel levels and the master level. Either 10K ohm linear
pots or an adjustable DC control voltage may be used. The volume control action is internally audio scaled in software. This
gives an “audio-taper” characteristic to a linear pot. Approximately 15dB of attenuation happens for the first half of pot
rotation, with the rest of the attenuation (to full off) happening in the second half of pot rotation. When using a DC control
voltage, the control constant is .167V/dB (or 6dB/V) from 0V to 5V. At 5V, the channel is turned completely off. The control
voltage should not exceed 5V. Below is a listing of the 15 pins on the Remote Level control port and their functions.
AS8 REAR PANEL
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