Dbx | 3bX digital series | b7 Voice Coil review for December 2009.

b7 Voice Coil review for December 2009.
Spotlight
Active Loudspeakers and
DSP: A Simplified Path to
Perfection
By Steve Mowry
I
n the vast majority of loudspeakers, the low-level output
signal from a CD or DVD player is the input to a receiver
or preamplifier (control center), with the output from the preamplifier being the input to the power amplifier(s), ~1.0V. In
these cases, each loudspeaker is connected to the power amplifier’s output terminals using a single pair of “cables.” However,
between the power amplifier and the audio transducers, there
is a passive RLC network. The power amplifier sees the RLC
network and the audio transducers (Fig. 1).
To implement a reasonable crossover and any equalization or impedance compensation is a difficult task, to say
the least, that usually becomes an empirical process (build
and try) at some point. Degradation and variability of the
high-level audio signal is simply unavoidable. One alternative
is a full-range audio transducer that eliminates the need for
a crossover network but not for equalization or impedance
compensation.
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Another alternative is active loudspeakers. In these cases,
multiple power amplifiers are used and the crossover(s) and
equalization are implemented electronically at low-level and
typically between the preamplifier and the power amplifier with regard to the audio signal path. This is sometimes
referred to as “bi-amp” or “tri-amp” for active two-way and
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FIGURE 1: Block diagrams of passive (upper) and active (lower)
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December 2009 23
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three-way loudspeakers, respectively. The power amplifiers are
connected directly to the loudspeakers, again using cables but
in multiple pairs. Signal processing including but not limited
to crossover, equalization, and phase compensation can be
implemented in the analog (continuous time) domain, typically using RC networks and operational amplifiers or in the
digital (discrete time) domain using software.
Some companies have implemented operational amplifiers using discrete components or even valves to implement
low-level operational amplifiers. The fact is that unlike
in the analog domain, in the digital domain the signal
is a number that can be preserved or processed to a high
precision.
Although an analog active crossover can result in significant
loudspeaker performance gains, implementing digital signal
processing (DSP) essentially eliminates any degradation of
the low-level signal and adds levels of flexibility and capability
to signal processing that are just not possible or cumbersome
for analog implementations with the inherent degradation
of the analog signal as the complexity of signal processing is
increased.
With that in mind, what if the DSP setup was via userfriendly Windows software allowing implementations of relative gain, time delay, filters, limiters, and expanders with the
mouse on the user’s PC? And what if you could import loudspeaker frequency response LMS acquisitions or equivalent to
tailor filtering or equalization to obtain some target response,
typically flat? This can, and in the limit, should be the in-room
26
response or the in-room power response (an averaging of
acquisitions at predefined on- and off-axis microphone positions throughout the listening room).
Finally, you have the option to implement signal processing
that best suits drivers, amps, and enclosures in the listening
room. DSP could also be used to equalize, filter, or phase compensate a full-range (single transducer) loudspeaker, which
would then require only one power amplifier per channel.
In the limit and with respect to “less is more” but requiring a
very wide bandwidth transducer, the latter could result in very
high performance.
With regard to active loudspeakers, because the power
amplifiers do not see RLC networks and each amp is band
limited to the bandwidth of the respective transducer, the
amplifiers are used more efficiently and, frankly, more effectively with a reduction in intermodulation and harmonic
distortion. The passive RLC crossover network effectively
disconnects the transducer from the power amplifier for outof-band frequencies, leaving the transducers’ cones and domes
free to vibrate passively driven by the relative sound pressure.
Now if the amplifiers are compact and reasonable mass, then
you can mount the DSP electronics and the amps within the
loudspeaker enclosure. This simplifies and effectively reduces
the number of components within the audio system, et al.
All you need to complete the A/V system are the input
components such as FM tuner, CD/DVD player, TV/monitor, PC/MAC, and a preamplifier with low-level out (these
may include home theater DSP of its own, along with the
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active loudspeakers).
Active loudspeakers facilitate enclosure design and alignments that consider the electromechanical and acoustic
parameters. Figure 2 illustrates a 6th-order high-pass woofer
response. The transducer is 2nd-order high-pass, the ported
box is 2nd-order high-pass, and the active filter is also 2ndorder high-pass. The net result of the cascading of the electromechanical and acoustic filters is 6th-order high-pass response.
You can apply the same basic methodology to an equalized
FIGURE 2: Illustration of an actively equalized ported box woofer
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sealed box with the result being 4th-order high-pass response
without the tuning null in the displacement magnitude.
Movers and Shakers
It would seem convenient to use an efficient class of power
amplifier such as Class-D, which would limit power supply
size and heat. Although two or three amplifiers are required
for two- and three-way loudspeakers, respectively, a single
high performance switching power supply can be utilized
for the power amplifiers, and others. Having said that, one
company stands out in the development of high performance Class-D power amplifier modules—B&O ICEpower.
The ASP modules include 115/230V at 50/60Hz power
supply (http://www.icepower.bang-olufsen.com/en/solutions/
speaker/ASP_SERIES), while the A series modules come
without power supply but utilize the power supply from
the ASP module (http://www.icepower.bang-olufsen.com/en/
solutions/speaker/A_SERIES).
You can use other amplifiers, but the ICEpower modules
offer high power ratings with the highest performance and
very low output impedances and the ability to comfortably
drive a 2W load. This offers further flexibility in transducer
design and selection.
Using very high power amplifiers (mega-amps) with very
low output impedance (high damping factor) and low noise
and distortion connected directly to the loudspeaker transducers represents the best possible system topology. Then the
utilization of DSP for crossover and EQ bring the performance potential to the very highest levels.
One company that has been committed to active DSP
loudspeakers since 1991 and has emerged as a leader in
the development of active DSP Loudspeakers is Meridian.
Photo 1 shows the Meridian DSP8000 active DSP loudspeaker with a MSRP of $79,999/pair. When I interviewed
Neville Thiele (Jan. 2005), I asked him what company he
thought was performing the most important R&D, and
he replied, “This must be a very selective estimate but I
would say Bob Stuart, Peter Craven, and Rhonda Wilson
at Meridian on loudspeakers and lossless digital coding.”
Here’s a video of Bob Stuart discussing the benefits of Active
DSP Loudspeakers http://uk.cinenow.com/videos/318meridian-bob-stuart-ondigital-dsp-loudspeakers.
Other mostly professional audio companies such as
Genelec have done some
work in this area; however,
most companies have yet
to implement active DSP
loudspeakers.
KSC in San Diego,
Calif., (http://kscind.com/
home.html) has developed turnkey solutions to
active DSP loudspeaker
implementations. They
PHOTO 1: The Meridian DSP8000
have developed proprietary
active loudspeaker.
D-Pro Windows software
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and hardware DSP boards with A/D and D/A on board. The
following DIGMODA models with the respective ICEpower
modules are available (http://www.digmoda.com).
2-channel
DC-520, 500ASP + 250A
DDC-550, 500ASP + 500A
DDC-1050, 1000ASP + 500A
DDC-1100, 1000ASP + 1000A
3-channel
DDC-552, 500ASP + 500A + 250A
DDC-1150, 1000ASP + 1000A + 500A
Subwoofers
DDC-500SW, 500ASP
DDC-1000SW, 1000ASP
DSP boards with D-Pro software only
DP130, 1-channel in × 3 out
DP240, 2-channel in × 4 out
Photo 2 shows the DDC-552. The DIGMODA electronics are packaged with large integral heatsinks and a convenient
mounting flange. Custom models are also available in reasonable minimum order quantities, and the DSP boards and
D-Pro software are available separately, for those companies
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that wish to use their own amplifier designs.
A company can literally “almost catch up” with Meridian
by implementing DIGMODA into new product development of active loudspeakers including subwoofers and twoand three-way loudspeakers. The D-Pro software is friendly
and good support is available from KSC and their representatives. Pricing, which depends on quantity purchased and must
be quoted by KSC, is surprisingly low for such a high performance product. In Asia and Australia/New Zealand, support
for DIGMODA and
D-Pro is available from S.
M. Audio Engineering.
In addition to the performance aspects of an
active DSP loudspeaker,
the development of such
products is simplified
through the implementation of DIGMODA. So
much can be done with
the DSP with regard to
equalization and compensation that the results
are essentially assured.
Upgrades of transducers
and enclosures can be
implemented with only
changes to the software
PHOTO 2: DIGMODA DDC-552.
setup. Additionally, a
single DIGMODA model can be used with several loudspeakers with again only a change to the software setup. Finally,
products can be brought to market more quickly and with
lower development costs.
Installers of custom A/V systems should find DIGMODA
useful in implementing very high performance systems that
can meet a wide variety of user needs. This includes, but
is not limited to, in-wall and in-ceiling applications. These
applications seem to also suggest that DIGMODA would be
an excellent solution for kit loudspeakers from small manufacturers.
Furthermore, the flexibility and capability of DIGMODA
makes implementing other technologies such as new transducers or an actively equalized trick enclosure a reasonable task
and with available support should not present any significant
problems. The use of the limiter functions can save expensive
transducers and eardrums and reduce warranty claims and
FIGURE 3: Illustration of the configuration of ICEpower modules for threeway and two-way loudspeakers.
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30
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thus improve product reliability.
The typical audio specifications of the DIGMODA
DDC-552 are as follows: Dimensions of the DDC-552,
DDC-550, and DDC-520 are illustrated in Fig. 4. For those
who want to use amps other than the ICEpower, KSC’s DSP
boards DP130 and DP240 are available. The DP130 is one
channel in, three out, and the DP240 (Photo 3) is two channels in, four out.
PHOTO 3: KSC DP240 DSP board (two channel in, four out) with
A/D converter.
D-PRO SOFTWARE
FIGURE 4: Mechanical drawing of the DIGMODA DDC-552 with
dimensions.
D-Pro software utilizes virtual instruments that are intuitive
and easy to use. You wire the DSP by connecting the input
to the DSP channels and selecting the outputs with the PC
mouse.
The D-Pro software is typically downloaded from KSC’s
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December 2009 31
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FTP server. Once D-Pro is installed (Windows XP recommended), you connect the PC to DIGMODA with a common
USB printer like interconnect. There are two main screens that
require setup. The first screen is shown in Fig. 5. Here you wire
the inputs with respect to the DSP channels and outputs.
The second screen contains the DSP blocks as shown in
Fig. 6. Clicking on the respective DSP block opens a control
screen. The control screens for each block and sub-block are
shown in Figs. 7-13.
Figure 7 shows the user-selected FIR 2nd-order filter setup
screen. Up to 16 filters are available for each channel. The
filter types are selected by number code.
1 = Low-Pass
2 = High-Pass
FIGURE 5: Input to outputs routing configuration screen.
FIGURE 6: DSP block selection screen
FIGURE 7: 2nd-order filter implementation screen.
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3 = BandPass
4 = All-Pass
5 = Parametric EQ (peak or notch)
6 = Bass Shelf
7 = Treble Shelf
Then either enter gain, frequency, and Q or use the mouse to
drag the curve to the desired filter response.
In room frequency, response acquisitions can be utilized as
an interim evaluation as the appropriate filters are determined
and applied with the PC mouse to equalize the response with
respect to some target response curve. Figures 8 and 9 show
LMS response acquisitions before and after EQ and with
tweeter phase invert.
The gain control, shown in Fig. 10 allows tweaking the
FIGURE 8: Filters responses with LMS acquisitions overdrawn
before EQ with and without invert tweeter.
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FIGURE 9: Filters responses with LMS acquisitions overdrawn after EQ.
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relative levels of transducer input signals. Figure 11 shows
an interesting screen that reminds me of the old DBX 3BX
dynamic range expander that was sold from 1984-1986.
These were similar to open-ended noise reduction but also
allowed high power delivery to the loudspeaker on transient
peaks. They were based on analog compander chips.
Perhaps a more common use of these controls is to
implement a gated limiter such that the transducer(s) will
be protected dynamically, especially the tweeter. However,
the possibilities from this control screen seem limitless.
Figure 12 shows the screen for time adjustment that can
be adjusted with the PC mouse using onboard memory.
Each screen setup is independent of another. Once the
setup is completed, a binary file containing the setup can
be saved. D-Pro can read and write setup files. Suddenly,
passive networks seem primitive and analog electronics
seem cumbersome.
DIGMODA is an exciting and innovative product that
is “fun” to use. Additionally, DIGMODA brings the A/V
system closer to a convergence system that utilizes the
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FIGURE 11: Dynamic compression-expansion implementation screen.
FIGURE 12: Time delay control screen.
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voice coil
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FIGURE 13: 180° phase invert screen.
family PC as a powerful but easy-to-use signal processor
including effects and, along with the Internet, as a source
of program material. Fig. 13 shows the controls for relative
phasing of the transducers that can be implemented with
the PC mouse.
OTHER PERFORMANCE CONSIDERATIONS
With state-of-the-art electronics now available in
an easy-to-use turnkey solution, the requirements for
transducers must also be at a high standard to realize
the highest performance. Frankly, the transducers are
still the weak link in the performance chain of an A/V
system. S. M. Audio’s solutions are STEALLUS X woofer,
STEALLUS mid/bass, and STEALLUS Air Core tweeter.
However, there are several manufacturers and distributors of other high performance transducers.
High performance engineering materials can also play
a role in the quest for A/V perfection. Diaphragm and
cone materials such as beryllium foil, magonite (keronite
process applied to magnesium), and sandwich composites can reduce mass and eliminate frequency-related
bending. Carefully designed molded TEEE, treated
Acrylic, and Nomex surrounds and spiders can improve
suspension linearity.
Klippel GmbH is engaged in nonlinear system R&D
regarding a proprietary chip that would compensate for
nonlinearity in the large signal parameters, Bl(x), Le(x,i),
Kms(x) along with the thermal characteristics of the
loudspeakers’ transducers. First these large signal parameters are acquired with a KLIPPEL DA and then the chip
is “taught” these dynamic nonlinearites. A nonlinear correction or compensation signal is then convolved with
the low-level input signal. The result is dynamic reducDecember 2009 35
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tion in IM and THD at the root cause, the transducers’
motors and suspensions.
If readers have questions, concerns, interests, or comments regarding active DSP and high performance loudspeaker products and applications, feel free to contact me
at steve@s-m-audio.com.
VC
Steve Mowry is currently an independent researcher
and consultant in transducer, loudspeaker, headphone
design, and new product development, along with being
a frequent contributor to Voice Coil and MultiMedia
Manufacturer.
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December 2009 37
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