User Manual
Table Of Contents
- Title Page
- Quick Start Guide
- 1248 Front Panel
- 1248 Rear Panel
- 8M Front Panel
- 8M Rear Panel
- 16A Rear Panel
- 16A Front Panel
- MOTU AVB Switch
- MOTU AVB Control Web App
- Overview
- It’s not on your hard drive
- Use your favorite web browser
- Control from multiple devices
- Run the installer, get the app
- Make hardware and network connections
- Launching the web app
- Device tab
- Device tab (continued)
- Routing tab
- Mixing tab
- Aux Mixing tab
- Mixer input channel strips
- Main Mix and Monitor channel strips
- Aux bus channel strips
- Group and Reverb channel strips
- 1 About Your MOTU AVB Audio Interface
- State-of-the-art A/D and D/A conversion
- Complementary I/O configurations
- 1248
- 8M
- 16A
- Network I/O
- Universal connectivity
- On-board DSP with mixing and effects
- 32-bit floating point processing
- Modeled vintage effects processing
- AVB system expansion and audio networking
- Matrix routing and multing
- 256 channels of network audio I/O for your host computer
- Web app control
- Stand-alone mixing with wireless control
- ADAT digital I/O
- S/PDIF digital I/O with SRC
- Word clock
- Comprehensive metering
- Headphone outputs
- Precision Digital Trim™
- Rack mount or desktop operation
- AudioDesk
- 2 Packing List and System Requirements
- 3 Software Installation
- 4 Hardware Installation
- Overview
- Rack installation and heat
- Thunderbolt audio interface setup
- USB audio interface setup
- Setup for two interfaces
- Setup for three to five interfaces
- Setup for a multi-switch network
- Setup for multiple Thunderbolt and USB interfaces
- Setup for web app control
- Audio connections
- A typical 1248 setup
- A typical 8M setup
- A typical 16A setup
- Synchronization
- Syncing S/PDIF devices
- Syncing word clock devices
- Syncing an AVB network
- 5 Presets
- 6 The Front Panel LCD
- 7 Working with Host Audio Software
- Overview
- Preparation
- Run the web app
- Sample rate
- Clock Mode
- Enabling and disabling input/output banks
- Specifying the number of computer channels
- Making inputs and outputs available to your host software
- Configuration presets
- Naming computer input and output channels
- Streaming computer audio to and from the onboard mixer
- Working with AVB network streams
- Mirroring computer channels to multiple outputs
- Combining multiple inputs to one output
- Routing grid tutorials
- Choosing the MOTU Audio driver
- Reducing monitoring latency
- Working with on-board mixing and effects
- Synchronization
- 8 Mixer Effects
- 9 Networking
- A Troubleshooting
- B Audio Specifications
- C Mixer Schematics
- D Updating Firmware
- E OSC Support
- Index
MIXER EFFECTS
72
LEVELER
The Leveler™ (Figure 8-4) provides an accurate
model of the legendary Teletronix™ LA-2A®
optical compressor, known for its unique and
highly sought-after Automatic Gain Control
(AGC) characteristics.
Figure 8-4The Leveler module.
The Leveler is available on the Main Mix bus and
all Group busses, including the Reverb bus.
A model of an optical compressor
An optical leveling amplifier works by shining a
light on a photoresistor. The intensity of the light
source is proportional to the audio signal, and the
resistance of the photoresistor is in turn inversely
proportional to the intensity of the light. Photore-
sistors respond quite quickly to increases in light
intensity, yet return to their dark resistance very
slowly. Thus, incorporation of the photoresistor
into an attenuator followed by an amplifier which
provides make-up gain produces a signal which
maintains a constant overall loudness.
Automatic gain control using light
The AGC circuit of the LA-2A uses a vintage opto-
coupler known by its model number T4. The T4
contains an electroluminescent-panel (ELP) and
photoresistor mounted so that the emission of the
panel modulates the resistance. An ELP consists of
a thin layer of phosphorescent material
sandwiched between two insulated electrodes to
form a capacitor. Making one of the electrodes
transparent allows the light to escape. These
devices are essentially glow-in-the-dark paint on a
piece of foil covered by metalized glass or plastic,
and are the same devices used in low-power night
lights. Unfortunately, these devices need high
voltages to operate, and are best driven by tube
circuits which can supply voltage swings of several
hundred volts.
Response characteristics
Once the light has faded away, the photoresistor
then decays back to its dark state. The shape of the
decay curve varies depending on how bright the
light was, and how long the light lasted. A general
rule of thumb is that the louder the program, the
slower the release. Typically, the release can take up
to and over one minute. One thing to keep in mind
when using these types of devices is that the typical
concepts of compression ratio, attack, release, and
threshold do not apply. The light intensity is
determined by the highly non-linear interactions
of the input signal, AGC circuit, and ELP, and thus
exhibit a strong program dependence that is
impossible to describe without the mind-numbing
mathematics of statistical mechanics. The actual
results, however, can be almost mystical: even
when you feed the same material (a loop perhaps)
through the Leveler twice, you’ll often see a new
response the second time through a loop, complete
with unique attack times, release times and
compression ratios. Furthermore, two different
input signals with the same RMS levels may be
leveled in a drastically different manner.
It is precisely this self-adjusting behavior that
makes optical compressors the tool of choice for
smoothing out vocals, bass guitar and full-
program mixes without destroying perceived
dynamics.