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I recently received a request to lay out how to set the time delays on multi--amped (bi-amp, tri-amp, quadruple-amp, etc.) driver channels in a single loudspeaker using a DSP crossover--such as a Xilica XP or XP series (or any other DSP crossover for that matter). Below you'll find a step-by-step discussion of how I approach that task. Note that setting the delays is only one part of a group of actions and that to set the delays properly, you must also check the polarities of the drivers versus the type (Linkwitz-Riley, Butterworth, Bessel, etc.) and order (first...6 dB/octave, second...12 dB/octave, third...18 dB/octave, etc.) of the DSP crossover. The higher the order (steeper the slopes), the more you'll be dealing with polarity issues of the drivers as you go about dialing them in. So to start this process, I'll be using a little graphic called a QFD House of Quality (HOQ) to show you how each step of the process traces back to what the goals (WHATs) are: determine which crossover filter types and order that you will be using dial-in the initial delays as determined by REW measurements and plots, fix any polarity issues that crop up, and flatten SPL (amplitude) response of the setup to achieve smooth, flat response that's also time-aligned The "Hows" across the top of the QFD HOQ matrix show how to achieve each of the "Whats" that we want. The "Hows" also just happen to show the initial sequence that I use them, namely: 1. Flatten individual driver/horn SPLs and set relative channel gains: This step is relatively simple: run REW's EQ facility to flatten the frequency response of each driver in the loudspeaker (one driver/horn at a time). After you have accomplished this, each driver/horn in the loudspeaker will be ready to integrate together and to set their relative delays. After you've flattened the response of each driver (using mostly attenuating PEQs), then run one sweep with all channels on to see their relative channel gains. You can set the approximate crossover frequencies and crossover filter types/slopes to see the relative SPL of each driver without having excessive overlaps between channels. Having the driver channels at the same relative gain will facilitate reading the spectrograms to see the delays more clearly. 2. Set crossover filter type & slope: Now that you have the individual drivers flattened in SPL and relatively close in terms of overall channel gains, now you can choose the preferred crossover slopes. Look at the driver overlaps in the frequency bands where you intend to cross them over by taking individual sweeps of each driver channel and then looking at the combined frequency response plots within REW using the second button, called "All SPL" on the button bar just above the plot. The width of the these natural bands of overlap will help to determine how steep or shallow a set of crossover filters that you should use. In the case of drivers/horns that can just barely make it to the crossover frequency on each side of the crossover frequency, you might be forced to use a steeper filter to bridge the crossover frequency region--the "crossover interference band". In the case of two drivers ( and horns as the case may be), they can have significant natural response overlaps, such as the following plot of a Cornwall bass bin with an ESS Air Motion Transformer (AMT-1) on top, both showing their two-octave overlap in frequency response: For the above case, it is possible to use a very low order crossover filter set, (high pass for the higher frequency driver, low pass on the lower frequency driver) because of the width of the crossover interference band--approximately two octaves from ~600 Hz to 2400 Hz. It's usually wise to choose somewhere in the center of the interference band as the center crossover frequency. If you wish to use higher order filters for other reasons than just driver interference band overlap width, that's certainly at the discretion of the person dialing in the loudspeakers. I personally recommend also trying out first-order filters (where possible) if using higher order filters to hear the difference, and set up two presets on your DSP crossover to be able to switch back and forth between them to hear the differences (if any). I've already matched the relative gains of each driver channel so that the output frequency response is relatively flat across the full frequency spectrum of the drivers that you're crossing (~40-20,000 Hz in this case). The next step is to set the crossover filter types and slopes within your DSP crossover (using the XConsole application in the case of Xilica crossovers). Note that you don't have to worry about calculating relative delays at this point if you wish to just take a REW upsweep measurement and look at the results of the measurement to directly measure the needed delays. Many people will select something like 24 dB/octave Linkwitz-Riley filters (i.e., fourth order). The Xilica can also go up to 8th order (48 dB/octave) filters--but at the expense of degrading the resulting impulse response of the loudspeaker. 3. Take REW sweep with all drivers and chosen crossover filters: At this point, you've got all drivers relatively matched in gain, their frequency responses flattened, and the chosen crossover filters set, so it's time to run a full sweep using REW to see the relative delays of each driver/horn within the single loudspeaker. What you will see is uneven frequency response and phase curves within REW. This is normal and indicates that time alignment of channels is now needed. 4. Read spectrogram relative channel delays: From the full sweep measurement with all drivers and crossover filters, select "spectrogram" view from the plot menu bar just above the plot (the next-to-last button on the plot menu bar). After setting the plot preferences (frequency vertical, dark background, and scaling zoomed in to 0.5 ms horizontal divisions), you'll see something like this: The above spectrogram is from a 1979 Cornwall measurement, showing the time delay between the K-77 tweeter impulse energy (at the top of the plot), the K-55 midrange horn/driver at about 750 µs behind the tweeter, then the direct radiating woofer at about 250 µs behind the tweeter, and 500 µs in front of the midrange (to find this, look at 400 Hz at the peak energy plot line for this particular measurement). So the channel delays must be set relative to the latest arriving channel--which is the midrange in the Cornwall (and is usually the bass bin in other loudspeakers). So the tweeter channel will need to be delayed 750 µs and the woofer by 250 µs in this example. 5. Set Xover delays and PEQs, rerun REW sweep: When those values of delay are inserted into the output channels: After those delays are set and another REW measurement upsweep is made, something like the following will be seen in the spectrogram plot: Note the gradually increasing lag of the peak energy time as the frequency decreases. This is the characteristic that is desired in order to get the best impulse response. The smoother the peak energy time curve, the better the impulse response. 6. Look at driver-to-driver crossover band FR & phase: After the delays have been set, take another upsweep using REW, then look at the SPL & Phase plot (the first selection button on the menu bar just above the plot). 7. Check step/phase/group delay plots & update delays/filters: Note the big nulls at ~125 Hz and ~215 Hz. These are caused from drivers not being time aligned. This may indicate that you need to flop the polarity of the lower-frequency driver channel using XConsole. This will depend on the crossover type and steepness that is chosen. If you change the crossover type of slope, this will need to be revisited and adjusted again. The phase curve also goes through a tight little zig-zag at these nulls. This also shows up on the group delay plots (the first derivative of phase with respect to frequency): These are indicators that something is still not dialed-in properly. Go back to step 5 and re-set the time delay on the leading drivers until the frequency response, phase, and group delay plots are smooth. The step response (the plot is found under the Impulse Response plot window) should look something like this: and not like this: _________________________________________________________________________________________________________________________ Once you've completed the adjustments in frequency response and delays for the first loudspeaker, now it's time to move to the other loudspeaker and complete steps 1-7. XConsole (the Xilica PC/MAC control application) provides a "Copy" function under the "Setup" menu to copy settings from one channel to another (user selectable). This saves a lot of time, and allows you to clone your settings to the other loudspeaker channels without keying mistakes. 8. Listen: This is self explanatory, but when everything is dialed in for all channels, you should have both conscious and subconscious improvements in the way the loudspeakers now sound. If you don't get a subconscious improvement in sound, try using first order crossover filters instead of the higher order filters that you might have been using and compare the listening results. When dialed-in properly and playing most reasonably hi-fi recordings (fully acoustic instrumentation is the best choice to hear this, avoiding any electronic or electrical amplification which scrambles the phase information in the recordings), this subjective sound improvement will be most apparent. Chris
Assuming that you have taken a measurement using REW (Room EQ Wizard freeware), you probably have a plot that looks something like this: What you have is a frequency response that's not quite as flat as you'd like. So for that particular plot, you'd like to find the digital crossover equalization filters to make it flat--but without a lot of cut-and-try and doing a bunch of measurements (upsweeps) along the way. So you look at the REW window and you see the function bar across the top of the window: The next-to-last button is "EQ", which you push, and the following window pops up: The window has two plots: the plot that the started with (frequency response) and a second plot area (blank) which can provide a waterfall, impulse, or Pole-zero plot. There are two arrows next to the left border of this window. If you punch the arrow pointed down, the bottom plot is replaced by the upper plot whose size now occupies the space that both windows did. (Do it now.) On the right top side of the window, there are 5 bars with text embedded in them, namely: Equalizer, Target Settings, Filter Tasks, Modal Analysis, and Resonances. If you click on the first bar, it opens a menu downward and shows you a list of equalizers that are supported directly by REW. If you don't see your equalizer model in that list, fear not. You can pick one that's close in terms of your options. For example, using the EV Dx38 or Yamaha SP2060, I used "miniDSP", and it works. If using Xilica, then select "XP2040", etc. If you sequentially choose each equalizer, and look at the PEQ filters (which we'll generate in a moment), you'll find one that give you the best fit to the features of your crossover. Once you select a crossover type, minimize the Equalizer sub-window by clicking on its top bar again. Now click on the next menu bar - Target Settings: The important settings here are "Full Range" if using you're equalizing a full range loudspeaker. REW can also help you with subwoofers and "bass limited", i.e., HT surround loudspeakers that are intended to cross above a certain subwoofer crossover frequency. LF slope indicates intended the slope of the roll-off of the low frequency end (12 or 24 dB/octave) corresponding to the crossover filter slope used between the loudspeaker and the subwoofer. If you look at the measurement plot that you are using to equalize, it will show you the lowest crossover frequency that you should use. For my Jubilees and K-402-MEH, that frequency is about 30 Hz. If you're using HT loudspeakers, it may be as high as 100 Hz. Set that LF roll off point to match your loudspeaker. The same settings are used for the high end. All of these settings are settable to new default values within the "Preferences" menu on the top bar of the REW main window. This is the place to put in your "house curve" if you choose to have non-flat loudspeaker response. You can boost or cut highs or lows using the above controls, and the resultant curve is visible, so you can see what your "goal curve" is. I use flat response everywhere since I unmaster my recordings, but you may choose to use a "one size fits all" house curve to compensate for non-flat EQ used on your recordings. In the context of home theaters, that's what a "house curve" is doing. The last item on this menu is extremely important. It tells REW what SPL (loudness) to aim for overall when it optimizes the equalization PEQs. You want to get this right, or, not only does REW yell at you, it also will produce too many PEQs to try to flatten the response. For the measurement above, 70 dB is the right answer. If you use a higher value, REW will try to boost all frequencies below your target level. If you use a setting that's too low, REW will attenuate everything to make that lower SPL. It's better to set this value 1 dB too low than 1 dB too high. It's better to attenuate using PEQs than to boost. Use the overall channel gain setting on your digital crossover to generally boost or attenuate one driver channel of your loudspeaker. It will be apparent when you need to change the gain of a channel. Next up: Creating your optimized PEQs using REW