General Technical Description - AKG AS8 Operating Instructions Manual

G T
ENERAL ECHNICAL
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).
DIR OUT
DIRECT/AUTO
PHANTOM POWER
+5V
+15V
MIC IN +
MIC
PREAMP
MIC IN -
MIC GND
PREAMP
GAIN
0 dB (LINE)
+30 dB (MIC)
+50 dB (MIC)
Remote Level CH1
Remote Level CH2
Remote Level CH3
Remote Level CH4
A/D
Remote Level CH5
CONV
Remote Level CH6
Remote Level CH7
Remote Level CH8
Remote Level Master
Auto Mix Log In
D
ESCRIPTION
TO uP
CHAN
ON
AUDIO
CHANNEL LEVEL
VCA
SPEECH LOG
FILTER CONVERTER
From CH1 –
From CH2 –
From CH3 –
From CH4 –
From CH5 –
From CH6 –
Compressor/ From CH7 –
From CH8 –
Leveller
Threshold
AS8 BLOCK DIAGRAM
1 OF 8 INPUT CHANNELS
HI LO
Tone Control
AS8-TC Only
FROM
OTHER
CHANNELS
Σ
TO A/D
CONVERTER
To CH1 –
To CH2 –
To CH3 –
To CH4 – D/A
A/D
To CH5 – CONV
CONV
To CH6 –
To CH7 –
To CH8 –
SERIAL DATA LINK
3
MAIN IN 1
Σ
MIX MINUS OUT 2
AUTO MIX 3
LOG IN
MAIN OUT 1
MIX MINUS IN 2
AUTO MIX 3
LOG OUT
Σ
MAIN
LEVEL
Dir/Auto CH1 –
Dir/Auto CH2 –
Dir/Auto CH3 –
68HC705
Dir/Auto CH4 –
uC
Dir/Auto CH5 –
Dir/Auto CH6 –
Compressor/Leveller
Dir/Auto CH7 –
Gain Reduction Display
Dir/Auto CH8 –
12 dB
9 dB
6 dB
SERIAL
EEPROM
3 dB
EXPANSION
IN
EXPANSION
OUT
MAIN
OUTPUT
RS-232 OUT
RS-232 IN