Digital Developments
A look inside the new Lake Contour processor’s unique design and filtering approaches
In 1997, I approached
Roy and Troy Clair with an idea for a new digital solution that I believed
could, if pursued, put Clair Bros ahead of their competition. One thing
I was bringing to the project was a history of developing products in
the lab combined with decades of live sound mixing experience. And, my
long relationship with Roy and Troy, dating back to 1970 when I sold my
sound company in Australia and came to the United States, helped convince
them that my concept was worth exploring.
The vision that evolved in discussions with Roy was to fulfill a dream
many of us have had, which, in a nutshell, is to have an ultimate remote
control for live audio. Monitor mixers have long wanted to stand in place
of the performer with all controls at their fingertips, while house engineers
dreamt of walking to any position in the house with full control of everything
in their racks.
Prior to approaching Clair Bros, I had begun working with David McGrath,
cofounder of Lake Technology in Australia. Before working with David,
I thought I knew a thing or two about digital audio, but David is amazing.
He’s a mathematical genius and also one of the best lateral thinkers
I have ever met. We also pulled in Ed Meitner (an electronics whiz) of
EMM Labs to work with us, so together we all formed Clair LLC with the
intention of developing cool new digital technology.
A short time later, we were joined by Stewart Bartlett, who focused on
hardware, and Marcus Altman, who served as software architect. A couple
of years into the project, Jim Meyer of Clair Bros became an integral
member of our small, tight-knit team. We started out with a very ambitious
set of goals, which required fundamentally different underlying digital
signal processing. The user interface is based on my experience developing
a computer game, and the desire for the unit to have a touch screen interface.
Also, I didn’t want it to resemble “typical” Windows-based
formats in appearance.
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Many of you may have heard about the results of this labor, a device called the Clair iO, which up until now has been used by Clair Bros and Showco, exclusively. Now, however, we are able to offer this same technology in a new device we’re calling the Lake Contour. Justin Baird, Lake Contour product manager, will pick up the ball from here, taking you on a tour of the unit and technology. |
WHAT IS A CONTOUR?
The Lake Contour is named for its unique sound reshaping capabilities,
a package of hardware and software tools for live sound applications.
Unlike other DSP-based loudspeaker controllers, Contour synthesizes filter
responses on the fly, which supplies greater precision and flexibility
in equalizing systems than found in current systems on the market. This
synthesis technology also supports an infinite number of EQ curves as
opposed to the finite number imposed by parametric EQ sections or graphic
EQ bands. It also permits the user to define the EQ curve they wish by
layering multiple curves on top of each other.
For example, one could begin with a hidden complex curve to correct a
loudspeaker’s phase response, and then a family of parametric and
graphic curves can be added to more precisely mold the system’s
response. The EQ’s total frequency response is a combination of
all of the overlays.
The Contour Processor is a two-input, six-output unit utilizing extended
precision 40-bit floating-point math to implement crossovers, dynamics,
delays and EQ. Every internal calculation is performed with 40-bit extended
precision. Multiple units can be grouped to provide multiple levels of
control, and communicate over a standard Ethernet network.
The front panel of the unit has been kept intentionally simple because
most functions are accessed via the Controller software. The front panel
does include a push button that identifies the unit on the network and
a fourcharacter alphanumeric display that allows the user to both label
the unit and scroll status information.
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The front panel of the unit has been kept intentionally simple because most functions are accessed via the Controller software. The front panel does include a push button that identifies the unit on the network and a fourcharacter alphanumeric display that allows the user to both label the unit and scroll status information. LED’s indi-cate external control, Ethernet activity and power, while meters indicate signal level, clipping and mute the state. Push buttons mute inputs and outputs, while the front panel Ethernet connection provides local PC access to the processor. |
On the back panel, there are the two XLR inputs and six XLR outputs,
joined by an RS232 port and two RJ45 Ethernet connectors. The rear panel
also includes a three-position groundlift switch that was expressly designed
to offer the benefit of transformer-coupled ground isolation while maintaining
clean, direct-coupled inputs and outputs. A/D and D/A converters are floating
(actually, galvanically isolated) and also are not connected to the main
ground. It’s a bit of an elaborate technique that requires high-speed
transformers and opto-isolators to create a barrier between the device
and any grounding aberrations (i.e., electrical), but it also allows hums
and buzzes to be quickly isolated and removed.
While the look and operation of the Contour’s Controller software
is decidedly different from normal Windows applications found in pro audio,
it does indeed run on Windows (98, 2000, ME and XP) platforms. The software
provides the means to adjust Contour’s parameters. A typical setup
configures one unit as two modules, each with one input and up to three
outputs, to implement either a two or three-way crossover. However, four-way
crossovers can also be configured using one Contour unit as a single module.
WIRELESS CONTROL AND OTHER FACETS
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The wireless touch screen that Bruce alluded to earlier is indeed a reality. It allows access to the Controller software while moving to any location in the venue, letting the user listen and adjust any parameter of any Contour processor on the Ethernet network. Adjustments can be made instantly, providing the responsiveness of an analog system with the benefits afforded by next generation digital signal processing. EQ is accessed by clicking on tabs, labeled by the user that can be engaged or bypassed resulting in easy comparison of EQ settings. |
Other Controller adjustments include A/D input headroom, input and output
gains, input and output delays, output limiting and output soft-clamp
peak limiting. Displayed on the wireless interface screen, high-resolution,
fast-updating meters simultaneously show true RMS and peak levels, with
peak hold. Most parameters can be adjusted to within a hundredth of a
dB. Fine and course adjustment modes are available, so you can be certain
that large adjustments are not accidentally made during a show.
Before focusing on Contour’s unique filtering approach, let’s
look at a couple of other notable facets. For example, a “designer”
mode has been developed to provide loudspeaker system designers with an
optimal base configuration for each type of loudspeaker cabinet. The base
configuration includes an optimized signal flow, front-end EQ, crossover,
post-crossover EQ, gain and dynamics, and these settings can also be password
protected. In addition, a scrolling menu in the Controller software presents
quick access to an available library of base configurations.
The Contour provides all of the traditional cross-over types commonly
used in loudspeaker systems. However, it also introduces linear phase,
“brick-wall” crossovers, with slopes exceeding 100 dB per
octave. The introduction of these advanced crossovers affords significant
benefits when applied to various types of loudspeaker arrays. Off-axis
lobing and cancellation between drivers through crossover transition bands
can be eliminated. In most loudspeaker array designs, improvements of
up to 6 dB in acoustic output power may be expected. The linear phase
aspect of the brick-wall crossover design significantly improves the loudspeaker’s
impulse response, providing a time-coherent wavefront. This optimized
impulse response not only reduces interaction effects between loudspeaker
array elements, it also provides for seamless transitions between different
loudspeaker types within an integrated array system, such as main array
and downfill enclosures.
Dynamics have also been carefully scrutinized, with the Contour offering
true RMS and Softclamp limiters with threshold and corner adjustments.
The corner adjustment eases the transition into limiting. The true RMS
limiter, which is a digital model of the equivalent analog system, calculates
RMS level for each audio sample. The possibility of clipping is removed
with the Softclamp limiter, which gently removes audio signal peaks, which
restricts the signal’s output swing that can sometimes drive an
amplifier into distortion. Both limiters are fully adjustable, so you
can optimize their settings for your particular amplifier and speaker
setup.
Now let’s shift to look at Contour’s EQ approach and technology.
Before getting into specifics, it’s important to point out that
this facet has been developed to provide “groups” that deliver
appropriate settings to large, segmented sound systems. Loudspeakers can
be members of more than one group. In a system with several loudspeaker
groups - for example, left and right mains, side and front fills, and
delays - a chain of processing units is required with traditional processors
to enable overall stereo master and various submasters to individually
control EQ, dynamics and gain. Contour’s groups send global (system-wide)
adjustments to all PA areas while still independently controlling individual
system components.
RAISED COSINE EQUALIZATION
Contour utilizes a new approach to equalization, using raised cosine functions
instead of traditionally implemented equalization filters. Raised cosine
functions provide for higher selectivity.
See Figure 1, which shows the results of two SIA SMAARTLive 5 transfer
function measurements, illustrating the difference in selectivity between
a traditional parametric filter and a raised cosine filter. The red line
shows the measured response of a third-octave equalization filter, centered
at 1 kHz with a boost of 6 dB. The blue line shows the measured response
of a third-octave raised cosine filter, centered at 1 kHz with a boost
of 6 dB. The shaded area between the two curves shows the difference between
the two filters.
Figure 1
The frequency axis of Figure 1 is marked at third-octave intervals.
Note the improved selectivity of the raised cosine filter; it provides
more precise third-octave selectivity, whereas the parametric filter leaks
into neighboring thirdoctave bands. A closer look at this figure shows
that the parametric section leaks into more than three neighboring third-octave
bands. The raised cosine filter doesn’t leak into other bands at
all. The Contour’s raised cosine filters give you surgical precision
to adjust a frequency response.
The raised cosine filter not only provides improved selectivity, it also
allows for perfect summation between neighboring filters. This ability
results in a significant advancement in graphic equalizer technology.
Figure 2 shows a portion of the third-octave graphic equalizer interface
provided by the Contour Controller software. The neighboring filters sum
flat and third-octave selectivity is precise among the filters.
Figure 2
TRADITIONAL VS. CONTOUR EQUALIZATION
A graphic equalizer is comprised of a fractional octave filter bank. In
professional applications, third-octave graphic equalizers are typical.
To cover the audio spectrum, 32 third-octave bands are usually provided,
from 25 Hz to 20 kHz. Due to the implementation of these traditional filters,
they interact with each other. The sides, or “skirts,” of
these filters interfere with the neighboring filter bands. This interaction
results in nonintuitive frequency responses, since the graphic equalizer
controls do not reflect the actual response of the filter bank.
For example, let’s compare some different EQ shapes on a conventional
graphic equalizer with the Contour equalizer. We pulled up seven faders
on a graphic EQ, to boost a region from 500 Hz to 2 kHz by 6 dB, and then
did the same in the Contour Controller software. See Figure 3 for a comparison
of the results of the measured responses of these two equalizers. The
transfer function of both devices was measured, and the resulting traces
show the differences.
As you can see, Contour’s measured response shows a perfectly flat
6 dB boost from 500 Hz to 2 kHz. The traditional graphic EQ’s response
is not what we expected. Because of the interaction between the filters,
the overall boost is much higher than 6 dB, at some points the response
reaches 10 dB of boost. There isn’t a single point in the graphic
EQ’s response that is at the desired level of 6 dB. The graphic
EQ’s “skirts” leak for more than an octave in both directions
as well.
Let’s look at another example. Pull one filter up by 6 dB and another
down by 6 dB. We did this a few times to see what happens when neighboring
filters provide boost and cut functions. And again, we measured both devices
to compare their responses, shown in Figure 4.
Contour’s response is exactly what we expected, 6 dB boosts and
cuts at the desired frequencies. In the graphic EQ, due to the interaction
between the filters, the boosts and cuts never get to 6 dB. For the final
comparison, we tried to make a gentle transition across a few third-octave
bands. Starting with a 2 dB boost, we then increased the boost to 4 dB
and then 6 dB, coming back down the other side 4 dB and then 2 dB. We
then did the same with Contour and then measured both devices, with results
in Figure 5.
Figure 3: Measured response of the two devices, with graphic EQ
in red and Contour in blue.
Figure 4: Graphic EQ in red, Contour in blue. Note that the graphic’s
boosts and cuts never get to 6 dB.
Figure 5: Graphic EQ is red, Contour is blue. Which matches the
desired response?
The Contour’s trace matches the desired response exactly, whereas the graphic EQ does not. Again, the graphic EQ’s response provides too much boost. The graphic EQ’s function leaks for over an octave in both directions. The bottom line is that with Contour EQ, neighboring filters do not interact in the traditional way. Selecting a frequency range with multiple filters does not produce a rippled response, the filters sum flat. Desired selectivity is achieved. If you pull one filter to 6 dB boost and the neighboring filter to 6 dB cut, you will achieve the desired response intuitively.
THE MESA FILTER
Contour also introduces an entirely new filter type called the Mesa Filter,
which is intended to supersede the traditional parametric equalizer. Figure
6 shows that the Mesa Filter starts out looking like a traditional parametric
filter. However, Figure 7 depicts that the two sides of the function can
be controlled independently. One side may be pulled down in frequency,
while the other side remains at the same frequency. Finally, Figure 8
shows that each side’s bandwidth may also be adjusted independently
to produce asymmetrical functions. These examples translate into a device
that is well suited for rapidly optimizing the frequency response of loudspeakers.
Figure 6: Mesa starts out looking parametric.
Figure 7: But the two sides can be controlled independently.
Figure 8: Independent adjustment of each side.
A single Mesa filter can rapidly optimize high frequency roll off, without introducing out-of-band frequency emphasis. Again measuring with SMAART, let’s look at the response of a high-frequency transducer. Ideally, we’d like to flatten this response at the listening position. Let’s add a Mesa Filter to optimize the response (Figure 9) and then show its optimized response (Figure 10) in blue, resulting from the Mesa Filter.
Figure 9: Mesa response shown in pink
Figure 10: Optimized response in blue.
The Mesa Filter’s response is shown in pink. In order to achieve
the same result with traditional equalization, we attempted to match the
Mesa Filter’s response with a parametric filter bank. We used all
six available filters to approximate the Mesa Filter’s response.
As you can see, we can make an adequate match; however, it took all of
the available filters to do it (not to mention more than a half-hour tweaking
the knobs just right to do so).
Figure 11: Approximate match of Mesa Filter's response required
six parametric filters.
One could argue that one parametric section can perform a similar function.
This is true, but a symmetrical function will emphasize frequencies outside
of the pass band of the transducer. Such emphasis has deleterious effects
contributing to audible distortion artifacts, as well as reduced life
expectancy for the driver.
The Mesa Filter also provides benefit at low frequencies. Low-frequency
bass management can be applied using a non-symmetrical Mesa Filter function,
in order to increase output within the pass band of the loudspeaker while
minimizing over-excursion of the driver (Figure 12).
Figure 12: Shaping the low frequencies.
In this example, a Mesa Filter is used to add some low-frequency boost
within the subwoofer frequency range. A smooth transition is used at the
higher side of the function to create a seamless transition between the
main system and the subs. The subs are tuned to 40 Hz, so excursion increases
dramatically below the 40 Hz resonance. To avoid overexcursion at frequencies
lower than 40 Hz, a sharp transition is used on the low side of the Mesa
Filter. The Mesa Filter provides the desired low frequency boost, without
damaging the loudspeaker in the process.
Lake Contour provides new tools that allow the sound engineer to quickly
and intuitively make adjustments.
Justin Baird is product manager for Lake Contour and can be reached
at J.Baird@laketechnology.com,
while Bruce Jackson is senior vice president of research for Lake and
can be reached at B.Jackson@laketechnology.com.
The authors would like to thank Jamie Anderson and Calvert Dayton of SIA
Software, makers of SIA SMAART software.