Mackie | TT24 | Digital Mixing For The Analog Mind

Digital Mixing For The Analog Mind
A look around the Mackie TT24
By John Boudreau & Fraser Warne
O
ver the past several years
we’ve witnessed a digital
revolution at the top end of
the live sound arena. Digital consoles
have now proven themselves robust
and roadworthy, and as a result are
now common for both large tours and
installations.
We’re also seeing digital consoles in
smaller applications. However, getting
digital benefits like recall, flexible rout-
From the top down, the work surface of the new TT24 digital console.
Figure 1: The view from the rear – ample I/O and expandability.
82 Live Sound International
July 2004
ing, “super” channel strips with gates
and compressors as well as equalization (and don’t forget internal effects
processors!) can get pretty expensive.
Further, some of the smaller digital
boards were originally developed for
recording applications, with features
and interfaces often better suited to
the studio. And, limited input/output
(I/O) capacity forces the live user to
add card slot expanders just to access
the power of the on-board DSP.
Set against this backdrop, Mackie
sought to develop a mid-size digital
mixer for live sound applications, with
the result being the new TT24.
We kicked off the design process for
the TT24 by listing all potential features
and then eminating those not inherently related to live mixing. Our team
focused on two primary goals: Develop
a control surface that would allow
instant access to any primary mix function within two hand movements, and
provide a “digital edge” to the tools
needed for mixing live every day.
We interfaced with live sound companies and installers from around the
world; this input was invaluable in
terms of tightening up our list of
potential applications and desirable
features. Further, this group worked
with us throughout the development
process, helping to keep us on the
right track.
INTERNAL PATH
Architecturally, the TT24 is a 24-bit/
96 kHz-capable digital mixer with a
32-bit internal data path. A quick
look at the rear panel shows connections to all of the mixer’s internal
capabilities. (Figure 1) External I/O
cards are not needed, although two
slots are provided for optional addon cards that we’ll address later.
The TT24 also includes 24 low-noise
microphone preamps, similar to those
found in both our Digital 8-Bus and
new Onyx analog mixers. Each analog
input channel offers XLR and 1/4-inch
TRS connectors, plus an insert jack
(another 1/4-inch TRS). Eight additional
line inputs on TRS can be stereo linked
and used as four stereo channels, for a
total of 32 possible analog inputs. In
addition, a dedicated pair of two-trackinputs, plus a separate talkback input,
ensure that channel strips will not be
used up for these functions.
The TT24’s digital inputs include a
set of ADAT light pipe ports providing
24 channels of I/O at 48 kHz, or 12
channels at 96 kHz using SMUX format. (An optional expansion card for
those who need the full 24 channels
of ADAT I/O at 96 kHz is planned.)
Figure 2: Mic inputs offer four-band EQ,
comp/limiter and gate.
The built-in digital inputs can be used
to connect up to 24 additional analog
mic inputs via rack mounted eightchannel mic preamps.
Stereo AES/EBU or S/PDIF digital
inputs are also available. Sample rate
converters can be inserted into the
AES/EBU or S/PDIF input path if
required (when connecting a 44.1kHz
CD player with S/PDIF output to a
TT24 for example).
The TT24’s primary outputs (main
left, center and right, plus eight
group/matrix outputs) are available on
balanced XLR connectors. Twelve aux
sends are available on 1/4-inch TRS
jacks. There’s a mono monitor output
for monitor mixing applications, a
stereo monitor out for control room
situations, and headphone out on the
front of the mixer. Analog stereo outputs are available as well.
Direct outs for the 24 mic preamps
are available on three optical connectors for easy integration into any digital recording system. Finally, there’s
also a selectable AES/EBU and S/PDIF
output for CDR, DAT, etc.
With so many connections, one of
our challenges was figuring out the
most logical way to access and apply
the TT24’s digital signal processing in
a practical and fast way for live sound
applications.
As you’d expect from any digital
console, the 24 mic inputs all have fourband EQ, comp/limiter, and gate available, and the eight line inputs have the
four-band EQ as well. (Figure 2) This
lessens the need for additional outboard dynamics processing.
DSP for the outputs and groups is
one area where we focused our efforts
in order to get it right for live needs.
The main outputs, auxiliaries and
groups have a six-band EQ: two bands
of high/low shelving EQ, two sweepable mids, plus a pair of “kill filters” –
anti-feedback filters with quick manual control. (Figure 3)
Tapping the rotary encoder for one
of these kill filters inserts a narrow -6
dB band filter at 1 kHz. Tap again and
it’s a sharper -12 dB filter. A third tap
produces a deep and narrow -18 dB
filter. Dial the knob to sweep this
notch filter to the feedback frequency.
(Comp/limiters to protect amplifier
inputs or in-ear wearers are also avail-
Figure 3: Main outputs, auxiliaries have a
six-band EQ.
able on all of the main outputs.)
Further, the 12 aux sends can be
stereo linked, providing six built-in
stereo in-ear mixes with EQ and limiting. The groups have 8 channels of
EQ and comp/limiting available. The
eight matrix outputs each offer up to
600 milliseconds of delay for typical
applications like delayed loudspeaker
systems. Last but not least, there are
four internal effects processors.
FUNCTION PER BUTTON
One of the biggest challenges in
designing digital live mixers is developing an intuitive and live-friendly
user interface. The comfort factor built
up over years of working with “one
knob, one function” analog boards,
for example, can be instantly gone.
With so many more features available
in the digital realm, one function per
button makes it difficult to maintain a
compact footprint.
As such, some digital desk designers have tended to undervalue the
importance of the live user experience,
jamming as many features into each
button as possible. As a result, we
were judicious in applying our experience in the analog realm to make the
TT24 as easy as possible to learn
while providing quick access to vital
live application functions.
For example, the console has 29
100 mm motorized faders, and there
are four group faders separate from
the 24 channel faders. These group
faders control groups 1 through 4, or
5 through 8.
We also considered tactile “feel and
touch,” a given on analog boards but
something that can be lost in the digital
domain. New backlit buttons use ultrabright LEDs for daylight applications
and have a rigid – not soft – feel, so the
user definitely knows if a button has
been pressed. These buttons are multifunctional. In the case of the group
faders, there are two rows of buttons
above those faders labeled 1 through 4
and 5 through 8. Tapping any one of
Figure 4: Two rows of buttons. Tapping any
one of them will bank between the corresponding faders.
Figure 5: Fat Channel speeds workflow by
using the digital control paradigm of “select
and adjust.”
84 Live Sound International
July 2004
the buttons will bank between 1- 4 and
5- 8. Further, the entire row will illuminate accordingly. (Figure 4)
Press and hold the Group 1 button
and channels can be added or subtracted from that group using the channel
select buttons. Routing inputs to groups
and mains is a simple, quick task.
Each channel fader also has a
Virtual Potentiometer (V-Pot), which
consists of a push-button rotary encoder and a 15-segment LED ring.
Angled for easy viewing whether sitting or standing, every V-Pot function
(aux sends 1 through 12, trim, pan,
high-pass filter, and meters) is available on a set of buttons just to the
right of the channel fader. Pressing
“meters” turns the V-Pot into a highresolution meter bridge.
The high-pass filter (HPF) is another example of an old school, analog
interface approach. The red LED at the
bottom of each V-Pot indicates that
the HPF is engaged. Press the encoder
to engage the filter, and turn the knob
to sweep it between 20 Hz and 400 Hz
to the desired frequency.
FAT AND QUICK
The upper right of the console is
called the “Fat Channel,” consisting of
a 5.7-inch touchscreen, 12 push-button rotary encoders and 10 “Quick
Mix” buttons. (Figure 5) The Fat
Channel is intended to speed the
workflow on the TT24 by using the
common digital control paradigm of
“select and adjust.” Press “select” on
any channel, group or output and the
Fat Channel presents that section’s
parameters for adjustment. This portion of the interface is flat, as opposed
to deep or nested – no “forward” or
“back” buttons are necessary.
For example, the four upper-most
Quick Mix buttons on the left side of
the Fat Channel represent the channel
strip. The “FAT” button provides an
overview of all elements of the channel, “EQ” accesses the detailed EQ control screen, “DYN” accesses the compressor/gate, and “AUX/GRP” accesses
all of the routing for the channel. To
adjust the low end on a vocal track,
select the vocal channel, press EQ in
the Fat Channel, then dial it in using
the corresponding rotary encoders.
The 12 rotary encoders always cor-
relate to knobs on the 5.7-inch touchscreen. Go straight from the “FAT”
screen to “EQ” by touching and
adjusting the EQ on the screen itself.
The four upper-most Quick Mix
buttons on the right configure some
of the mixer’s deeper functionality.
“AUX MSTR” puts all 12 aux master
sends on the 12 rotary encoders,
another way to avoid banking on the
channel faders. The push button provides selection of that aux master for
EQ and comp/limiting.
Meanwhile,
“SNAP”
provides
detailed control of the 99 snapshots
including naming, locking and filtering. Any or all elements can be filtered
out of a snapshot. For example, you
can remove the pulpit mic from a
snapshot and still retain manual control. The bottom two Quick Mix buttons on either side of the rotary
encoders allow easy scrolling through
adjacent channels without reaching
across the console.
Figure 7: How the audio routing works.
offering single-fader level control of
this stereo drum mix, plus group-level
WORKING WITH GROUPS
EQ and stereo dynamics processing.
The “digital edge” begins with the
Configure another group as a VCA,
TT24’s eight “Flex-Groups.” Each
and the fader will act like a VCA fader,
group can be configured individually
controlling level to all post-fader desas a mono, stereo, or VCA-style group.
tinations of the selected channels (this
is also a simple way to create a mute
The TT24 initializes with eight mono
group). With eight groups, the TT24
groups just like many analog consoles,
can be set up with two mono groups,
but any group can be configured
a pair of stereo groups, and four VCAs
instantly by using the touchscreen to
to fit a given application.
select the group type on the group
“AUX MODE” also helps enhance
“FAT” screen. (Figure 6)
A stereo group can be designated,
workflow efficiency. This button, the
with all drum tracks routed to it. The
only blue backlit one on the board,
input channel panning remains intact,
effectively turns the TT24 into a monitor mixer. In this mode,
the channel faders
effect the selected aux
mix and the group
faders control aux master sends. Another press
of the blue AUX MODE
button and it returns
the console to standard
front-of-house mode.
Matrix Plus, referred
to earlier as an 11 by 8
matrix, is really eight
11-line input/mono output mixers, into which
the user can plug whatever is wanted. It
Figure 6: The TT24 initializes with eight mono groups, with any defaults to the common
group configured instantly by using the touchscreen.
arrangement. Groups 1
through 8 as well as left, right, and
center/mono are the 11 inputs available to each of the eight matrices. For
a broadcast feed, replace a subgroup
with an announcer’s mic and mix it in
at the desired level.
In some cases, Matrix Plus may be
an easier way to create a more appropriate mix. Let’s take a vocalist’s monitor mix. Use the drum, bass, back-up
vocals, keyboards, and guitar subgroups, which are already feeding the
matrix channel. Insert the vocal mic,
the vocal reverb and delay returns,
and dial in a more accurate monitor
mix using far fewer controls than with
the typical aux send method.
BUILDING BLOCKS
The TT24 is composed of three primary building blocks: the DSP section,
the mix engine and the audio router.
Twelve Analog Devices 32-bit, floating-point SHARC DSP chips provide
the input and output signal-processing
power (EQ, compression, limiting,
gating, etc.). They also perform all of
the user interface functions: reading
switches, running the touch screen,
reading and moving the motorized
faders, and so on. A dedicated SHARC
chip handles effects such as reverbs,
chorus and delay.
Storage of user snapshots is in 32
MB of Flash memory attached to one
July 2004 Live Sound International 85
Figure 8: A look at the configuration of a 96-channel dual console LCR system.
of the SHARC DSPs. The TT24’s system state is stored in a Ferro-electric
memory (FRAM), a non-volatile storage device that is very similar to a
standard static RAM (SRAM) device.
Unlike SRAM, however, FRAM does
not require battery power. Batterybacked SRAM is guaranteed to fail
sooner or later (that battery is going to
go flat eventually) but the TT24 will
always be able to recall its configuration after power down.
Further, the unit can boot up and
be ready to go in less than five seconds. This was one of the key design
requirements the TT24 engineering
team incorporated in order to make it
suitable for live applications. If a circuit breaker trips, or someone trips
over a power cable, there should be
no need to delay performance while
the console reboots.
The TT24 mixes 88 channel inputs
to 33 output buses, including eight
main mix buses, 12 aux sends, the
L/C/R mixes, the solos, etc. The heart of
the Mix Engine is a Field Programmable
Gate Array (FPGA) that performs almost
279 million (88 by 33 by 96) multiple/accumulate operations per second
at a 96 kHz sample rate. The data path
of the mix engine is 32 bits at its narrowest, and 56 bits at its widest.
All audio channels pass through
an audio router, a fully non-blocking
86 Live Sound International
July 2004
switch that provides connection of
any of 480 32-bit input channels to
any of 640 32-bit output channels.
(Figure 7, previous page) For example, it lets the expansion network
card insert its received signals into
the audio path. The audio router has
more outputs than inputs because
several audio channels are connected
to more than one destination (what
we refer to as a multicast).
The console, which ships with control software, connects to an external
computer via USB and provides a fullcolor, large view of the touchscreen.
In addition to a large view, the outboard PC-based graphic user interface
(GUI) offers high contrast graphics
with daylight usage in mind.
Black lines on white backgrounds
or bright yellow text on black background is used for this very reason
and tested extensively in the Seattle
summer (believe it or not, there’s
plenty of sun up here that time of the
year!). The same screens designed for
the LCD are implemented in the GUI.
In addition, the GUI is unique in
that is an integrated mixing interface.
The default setup is for the computer
GUI to mimic the LCD screen, but it
can also be set up to operate independently if a second engineer is
working. The PC software also allows
for file saving and firmware upgrades.
EXPANDING OPTIONS
As noted earlier, there are two expansion slots on the rear of the TT24 chassis. These accommodate optional
expansion cards that offer additional
functionality.
The U-100 console-linking card links
two TT24 for applications that require a
larger channel count. This link translates to 48 mic pres and 16 line inputs.
If the 24 channels of ADAT light pipe
were used with rack-mounted mic pres,
the complete system would have up to
96 mic pres. (Figure 8)
The second expansion option is a
UFXII card, which provides additional
DSP (EQ, gate, comp/limiter) for those
TOS-Link inputs, as well as 31-band
graphic EQ and additional effects processing. For users that chose to
expand a single console to 48 mic
channels by using outboard mic preamps with optical outputs, the UFXII
supplies horsepower for the additional channels’ DSP requirements. (The
UFXII card is also an open platform
and in the future will allow for third
party plug-ins.)
The third expansion option available is the LP48 loudspeaker processor
card designed by Lake Technology,
the company behind the Contour
processor. The LP48 facilitates Lake’s
“Ideal Graphic” EQ and parametric
“MESA” EQ.
Further, the db25 connector on
the card adds eight fully balanced
outputs from the 4 by 8 loudspeaker
processor, facilitating the driving of a
stereo four-way system from the
TT24 without going through an extra
set of D/A and A/D converters. And,
the Lake GUI integrates with that of
the TT24.
One more TT24 expansion option
is an ADAT I/O card that allows all
channels to be available when operating the board at 96kHz internally.
With this combination of features
and functionality at a price we
worked hard to make very competitive with comparable analog boards,
it’s our hope that this new console
causes even loyal analog users to
consider making the move to digital
technology.
John Boudreau is the TT24 product manager and Fraser
Warne is hardware manager for the new console.
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