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Q1 What is the difference between DEQX-HD™ and DEQX Calibrated™?
DEQX-HD™ is the most advanced form of loudspeaker management available. It provides the ability to correct – directly from measurements – both phase (timing) errors and amplitude (frequency-response) errors at hundreds of frequency groups to within a fraction of a decibel – the limits of our hearing. It normally is applied to 3-way active or 2-way active plus subwoofer configurations, so that each driver can be individually compensated. The crossovers in DEQX-HD™ are true linear-phase, usually with steep slopes in the order of 48dB/octave to 60dB/octave. This minimise each driver’s dynamic and crossover distortion and usually also provides more natural dispersion and greater volume without compression or distortion. Once the speaker’s natural (anechoic) behaviour is corrected, DEQX-HD™ then provides room measurements and real-time correction facilities.
DEQX Calibrated™ processing provides the same measurement-based time and frequency-response correction (as DEQX-HD™) but provides this to passive speakers that already have their own internal crossovers. It also provides the ability to crossover to one or two subwoofers using steep crossovers, that not only reduced bass extension and allows room correction, but can also reduce distortion form the main speaker’s low-end driver by reducing its bass output.
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Q2 Which DEQX products are right for me?
The new PDC-2.6 HD is processor-only, for systems already using a quality pre-amp, and allows set-and-forget installation. It provides DEQX Calibrated™ automated correction from measurements (using notebook PC or Mac running Windows XP/Vista with DEQX set-up software) for two passive speakers with the option to integrate one or two subwoofers, then time-align these with the main speakers. You can then perform Room measurements that are displayed graphically below corresponding parametric EQ bands that you can set automatically and/or manually in real-time. Each EQ band’s bandwidth is adjustable from one-semitone to 4-octaves wide. For advanced users, the unit also provides 3-way active crossover outputs if you want to experiment going fully active to provide the ultimate DEQX-HD™ performance.
The new PDC-2.6P HD Preamp provides all of the features described above. However, its additional Remote-control allows you to select from two analogue and two digital inputs. It also provides post-DAC
analogue volume control to utilize the full dynamic range of DEQX’s audiophile DACs at low listening levels.
The new HDP3 is DEQX’s reference audiophile product that provides the full features of the PDC-2.6P HD Preamp described above. It also contains DEQX’s audiophile grade linear power supply, in a large 2U chassis that allows balanced and digital output options simultaneously (field upgradable).
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Q3 How does DEQX speaker correction work and what makes it unique?
Much has been written about DEQX’s patented DSP correction processing. The forums have been alive with some informed and misinformed discussions. Misinformation is easy to find a whenever anew technology threatens traditionally respected ‘experts’, whose status has understandably beenbuilt by extracting the most from traditional ways of doing things, such as designing great passive speakers—achallengeindeed.
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Q4 Do the letters in ‘DEQX’ stand for anything?
Yes, DEQX stands for – Digital EQ and X-over, and is usually combined with Calibrated or HD as in DEQX Calibrated™ meaning correction of typically full-range passive or powered active (pro) speakers. DEQX-HD™ implies a fully active implementation of DEQX’s speaker correction where each driver is individually amplified so it can also incorporate DEQX’s linear-phase crossovers to reduce crossover and dynamic distortion and provide more natural (even) dispersion in addition to individual driver correction.
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Q5 What are the new ‘HD’ models of PDC-2.6P and HDP-3 processors?
All of DEQX processors provide both DEQX-HD™ and DEQX Calibrated™ correction. The original
PDC-2.6 processor and PDC-2.6P pre-amp/processor pioneered a revolution in loudspeaker and room correction performance. With hundreds of systems installed in some of the worlds highest-end audiophile and professional mastering facilities. Now we have upgraded the power supply and analogue sections to provide the new PDC-2.6 HD processor, the PDC 2.6P HD preamp/processor, and the new HDP3 Audiophile preamp/processor with analogue power-supply in a larger, more expandable chassis.
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Q6 What is the difference between DEQX's Speaker correction and Room correction?
DEQX processors provide both speaker correction and room correction. Each require a different type of measurement and correction process because speaker correction means that the speaker must first have its errors measured anechoically (without room reflections) to allow its frequency-response errors and its frequency-related timing or phase errors (group-delay errors) to be determined. This is impossible to do from a room measurement.
DEQX calibration software provides the same type of anechoic measurement tools that professional speaker designers use to show them how well their speakers perform. In fact DEQX’s software goes much further by automatically creating the precise DSP correction template required to compensate for the speaker’s timing and frequency-response errors. Compensation, using DEQX’s patented Digital Signal Processing occurs in real-time for hundreds of frequency groups simultaneously. When the speaker is performing as accurately as possible, the room measurement is made, at least at the main listening position or multiple measurements can be made. Multi-band parametric EQ can be used to reduce bass resonances in the room and add personal listening preferences in real-time.
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Q7 Can I adjust the target curves for Speaker and Room Correction?
DEQX speaker correction always targets a flat (accurate), on-axis, frequency and time response within the best operating range of the speaker. The loudspeaker is the instrument making the sound, but should not add its own tonal colouring, which can easily be applied later if desired. However, usually the need for absolute accuracy is essential for the usually elusive being there realism that DEQX processing aims to achieve. If its on-axis response is not flat (accurate) it is rarely capable of achieving this aim, although its dispersed sound also reaches us via room reflections, which also
effect the overall balance. Room correction is another matter however because an accurate speaker i.e. with a flat on-axis frequency response, rarely provides a flat room response. If we take say 1,000Hz as our typical average volume reference point, then bass below 100Hz will tend to be louder by several decibels, whereas by the highest frequencies, say around 10KHz, will tend to be lower in volume by around 5 decibels.
This is expected psychoacoustically because what we hear at the listening position is a mix of direct sound from the speaker itself, and sound reflected from all the room’s surfaces (the totality being the power response). Since the higher frequencies are reflected from the room less efficiently than bass and midrange, the balanced of highs is less at the listening position. This effect is taken into account by mastering engineers, so except in very dead rooms or due to personal taste, need not be corrected.
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Q8 Why do some audiophiles say that flat speakers don’t sound any good?
It is extremely difficult to make traditional passive speakers that have an accurate (flat)
frequency-response. The best that can usually be achieved is within a range of 6dB (+/-3dB). This 6dB range represents the difference in volume we hear if we drive a perfectly accurate speaker at 25-watts and then 100-watts! As a consequence, many speaker designs have attempted greater accuracy by using very complex passive crossovers that include simple EQ to compensate for the worst frequency-response errors. Unfortunately, this often increases phase shift errors and related timing incoherence (group-delay errors) that can easily sound worse than the cure of frequency-response errors. Group-delay errors can be considerable even in the best passive designs. They are traditionally difficult to measure, often producing a veil and sense of incoherence. By contrast, DEQX correction not only does not introduce more group-delay errors, it actually corrects phase and group-delay
errors already present indirect relation to the accuracy of the (pseudo) anechoic measuremen ttaken.
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Q9 What slope filters does DEQX use?
When correcting passive speakers (using DEQX Calibrated™ processing), a DEQX processor does not introduce any crossover filers of its own but works with the passive filters already inside the speakers. If one or two subwoofers are added, DEQX provides shallow or steep high-pass filters to the main (passive) speaker that limits the excursion of its bass driver, reducing its distortion. Corresponding low-pass filters disallow the subwoofer/s from outputting low-resolution upper-bass and midrange frequencies so distortion is kept to a minimum. In this case, slopes of 24dB/octave to 60dB/octave are recommended.
In active mode, to provide DEQX-HD™ operation, DEQX uses linear-phase filters to allow exact timing (phase) across the crossover region. Here, the slope used between mid-range and tweeter driver for example is usually from 48dB/ocatave to about 60dB/octave.
DEQX processors provide up to 300dB/octave crossovers, but steeper slopes require increasingly good matching between driver types or results can sound ‘hard’ if one driver is incapable of cancelling the effects of filter pre-ringing by the other driver. Note that filter pre-ringing is cancelled by the design of the DEQX linear-phase filters. These are calculated from the actual acoustic phase of each driver measured at the crossover region.
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Q10 What is the point of going active and steep crossover filters?
Good speakers usually require 3 drivers to deliver bass, midrange and highs. The ten octaves we hear are divided over three frequency ‘decades’; where for the sake of simplicity we might describe bass as 20Hz to 200Hz, midrange as 200Hz to 2kHz and highs as 2kHz to 20kHz. So this represents roughly 3.3 octaves for each driver type, although speaker designers may vary these divisions depending on the capabilities of the drivers being used.
As a general rule, as a speaker’s diaphragm moves furthest from its centre (distance called x-max), it behaves increasingly ‘non-linearly’ (distorts). The degree to which this happens often depends on its price to a large degree! This is where its amplitude modulating higher harmonics become increasingly distorted as x-max is reached. In a typical 1” tweeter for example, if we remove the lowest octave from its traditional (passive crossover) responsibilities, its maximum displacement is reduced by roughly
a factor of 4!
This can dramatically reduce its intermodulation distortion, and allow greater volume output before obvious distortion sets in.
The same principal applies to say a 5” bass/mid driver that might be expected to produce bass down to 50Hz and below. In this case, when we limit its bass responsibilities to above 100Hz with say a 60dB/ocatave high-pass filter (or even 200Hz if two subwoofers are used), its mid-range distortion is reduced, even at and its output volume capacity potentially increased. By contrast, the filter slopes in passive crossovers (that pass the power amplifier’s output) are limited to 6dB/octave to about 18dB/octave at best. As a consequence they have problems quarantining distortion-producing bass energy from midrange and tweeter drivers. Attempts at steeper passive crossovers, as mentioned above, tend to increase phase and group-delay errors causing a loss of resolution and time coherence.
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Q11 Can steep crossovers cause problems?
As mentioned in Q7 above, very steep passive filters can be problematic if not properly implemented. As a rule of thumb, consumer speakers have tended to use around 12dB/octave slopes, pro-audio digital (DSP) or analogue active crossovers speakers typically use 24dB/octave crossovers, and DEQX typically uses 48dB to 60dB/ocatve. Although DEQX is not the only DSP active crossover around, it is one of very few offering true linear-phase crossovers, and uniquely, the phase of its linear-phase crossover filters are created directly from the measurement of acoustic phase on-axis, for each driver in-situ, leaving little to chance and production variables.
However, to confirm that problems are not being introduced by steep crossovers using particular driver types, all DEQX processors offer traditional 6, 12, 18, 24, 36+dB/octave Butterworth and Linkwitz Riley crossovers that can be directly compared with DEQX’s steeper linear-phase crossovers for any given set-up. Four memory pre-sets allow entirely different types and slopes of crossovers, crossover frequencies, and correction filters (or no correction) to be instantly compared in A,B,C,D comparisons. This ensures that if particular driver combination is deemed not best suited to steeper crossovers with correction, then traditional active crossovers with shallow slopes can be used in addition to normal room measurement and correction facilities.
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Q12 Why not use great speaker designs and drivers that don’t need correction?
Compared to a quality audio signal arriving at a typical loudspeaker’s terminals, the measured output from the speaker will be between ten to one-hundred times worse in all relevant measurable categories, depending on the loudness and complexity of the music being played. Good passive speakers will hopefully only fall into the 10-times worse category, and even that is based on static measurements that can’t measure the dynamic degradation that occurs due to intermodulation distortion at higher volumes, especially from complex music such as orchestra.
We are subconsciously sensitive to less than 1dB of volume at different frequencies, yet a 6dB windwo is considered “good” for a passive speaker. We also detect timing inaccuracies in the order of microseconds, and these errors also ring the ‘unreal’ alarm bell – however unconsciously – as we listen to traditional loudspeakers. The way to achieve the elusive Being there experience is to minimise these subconscious cues that warn that what we are hearing is not real. Even the best speakers need active compensation to minimise the need for these psychoacoustic reality checks.
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Q13 I hate computers! Will I be able to install the DEQX?
The DEQX PDC and HDP3 processors are incredibly powerful tools and much of their development has focused on ease of use and speed of operation. For best results a familiarity with PCs, general acoustic principals, and an adventurous spirit will help. Alternatively you may have local DEQX dealer or someone that can help you set it up. The DEQX software is based on four Wizards that divide the process into logical operations that can each be performed independently:
1) The Speaker measurement Wizard measures passive, bi-amped active or tri-amped active main speakers and subwoofers.
2) The Calibration Wizard takes the measurements mentioned above and using many typical default settings, allows you to confirm the frequency zone you want to correct and set maximum and minimum degrees of correction in decibels. If you are making an active bi-amp or tri-amp filter set, this is also where you choose the desired linear-phase crossover frequencies and slopes that will be incorporated into each driver’s correction filter.
3) The Configuration wizard sets up the system for your particular speaker configuration. This might be for two main passive speakers with optional mono or stereo subwoofers, OR: 2-way active main speakers (bi-amped) with optional mono or stereo subwoofers, OR: 3-way active main speakers (tri-amped). You can then set timing delays (set in milliseconds or distance) to allow time main speaker time-alignment with subwoofers, and you can set high-pass and low pass filters to crossover the main speakers with subwoofers.
4) The Room Measurement Wizard allows you to measure the final in-room result from the main sweet-listening spot and other positions, although the sweet-spot is often sufficient. The room measurements are displayed for each speaker on the multi-band parametric EQ display. If frequency-response and group-delay issues have already been addressed using speaker correction, the degree or room correction necessary is usually limited to reducing three to six
ofthe main natural room resonances lying below about 400Hz
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