Making Room Correction and Bass Management Filters with REW

[hulkss]

I'm setting up a new XMC-2 Processor and I thought I would share the process in this thread as I work through it for my Home theater.

I will be using the Parametric EQ filter capability of the XMC-2 for room frequency response correction. REW allows the user both control and insight into the filter design. The IIR filters will have essentially no audio latency which is important to me.

I'll be using a Mac Studio computer, a Lynx Aurora (n) audio interface, and a iSEMcon EMX-7150 measurement microphone.

Let's start with a quick loopback measurement through the hardware to make sure all is well. The audio signal path is from the Mac to the XMC-2 over HDMI. The XMC-2 outputs are connected to the Aurora with XLR cables. The Aurora is connected to the Mac with Thunderbolt 3.

This is a sweep measurement over the full frequency range of the XMC-2 left front channel checking phase and frequency response. This looks really good with flat frequency and phase response plus great low frequency extension below 10 Hz. The signal was very clean with THD below .02%

[PaulBe]

Dec 1, 2023 3:10:20 GMT -5 hulkss said:

I'm setting up a new XMC-2 Processor and I thought I would share the process in this thread as I work through it for my Home theater.

I will be using the Parametric EQ filter capability of the XMC-2 for room frequency response correction. REW allows the user control and insight into the filter design. The IIR filters will have essentially no audio latency which is important to me.

I'll be using a Mac Studio computer, a Lynx Aurora (n) audio interface, and a iSEMcon EMX-7150 measurement microphone.

Let's start with a quick loopback measurement through the hardware to make sure all is well. The audio signal path is from the Mac to the XMC-2 over HDMI. The XMC-2 outputs are connected to the Aurora with XLR cables. The Aurora is connected to the Mac with Thunderbolt 3.

This is a sweep measurement over the full frequency range of the XMC-2 left front channel checking phase and frequency response. This looks really good with flat frequency and phase response plus great low frequency extension below 10 Hz. The signal was very clean with THD below .02%

Nice baseline for the XMC-2. I'm going to enjoy your journey.

[hulkss]

The first and extremely important step, is to measure loudspeaker in room response. I have done this many times, with many methods, and many tools.

My best results for a home theater have been achieved with the "moving mic method" as discussed in this AES paper: Moving Microphone Measurements for Room Response in Cinema

Here is a free white paper from Harman who endorse this method for pro theater: Harman White Paper. See pages 9 & 10.

I have used many software tools for measurement. The best have been SMAART and Room EQ Wizard. For the Emotiva, I will be using REW as it automates custom IIR filter sets and outputs filter parameter files for the XMC-2.

I like bass, so, a mic that is good down to 16 Hz is needed. That is the lowest "musical" note in my opinion. Yes, special effect sounds go even lower. The mic included from Emotiva is calibrated down to 30 hz. The mic I am using from iSEMcon is calibrated down to 10 Hz: EMX-7150.

The next problem is finding a proper microphone preamplifier for measurement. I use the Lynx LM-PRE4 Preamp Module which has excellent performance for room measurement.

Measurements in the next post.......

[hulkss]

I'm going to use the Left Front channel for my first filter development (that measured fine in the first post of this thread). I should also check the subwoofer channel which gets its signal from "bass managed" small speaker channels and the LFE input.

OOPS! a problem with my setup has appeared. I'm sending 7.1 audio over HDMI to the XMC-2. I've discovered that channels 3 & 4 (Center and LFE) are reversed. I determined that the channel order for consumer HDMI devices is per CTA-861 not SMPTE ST 2035. For some audio systems the side and rear surrounds may be reversed as well.  Just to be sure I plugged in my PS5 and played a Blu-ray test disc, the XMC-2 works perfectly.

The channel assignments going into the XMC-2 over HDMI are:
1. Left Front
2. Right Front
3. LFE
4. Center
5. Left Surround Side
6. Right Surround Side
7. Left Surround Rear
8. Right Surround Rear

Now let's measure the LFE Channel. Uggh! another issue. This channel has a low pass filter on it that cannot be removed as far as I can determine. For now, suffice it to say that the LFE input cannot be used to measure a subwoofer. I will discuss this more later. Here is what that LFE channel filter looks like:

[hulkss]

While I'm doing loopback measurements with REW, let's look at the bass management rerouting filters. The 12 or 24 dB/oct setting in the XMC-2 only affects the subwoofer low pass.  The LP filter is a 12 dB/oct Butterworth.  When 24 dB/oct is selected it's a Linkwitz-Riley (2 stacked 12 dB/oct Butterworth filters). The high pass is fixed at 12 dB/oct Butterworth.

I'll use the 24 dB/Oct option. What ultimately matters, is the acoustic response that is achieved. I will be targeting a matching 24 dB/oct acoustic response for both high pass and low pass. Subwoofer phase at the crossover point will be matched to the other speakers by adjusting relative "time alignment" using the XMC-2 loudspeaker distance settings.

Here's a look at those filters if set to 100 Hz:

[hulkss]

OK, I'm ready for a moving mic measurement (see papers linked in earlier post). I put the mike on a short boom from a microphone stand, keep the mike aimed towards the ceiling, and don't get yourself in between the speaker being measured and the mic. Do a few measurements as practice until you are comfortable with it and see repeatable results.

In REW, the Real Time Analyzer (RTA) and signal generator are used.
Per REW: "Periodic Noise (PN) sequences are ideally suited for use with spectrum and real time analysers (RTA's). They contain every frequency the analyser can resolve in a sequence length that matches the length of the analyser's FFT. Their great benefit is that they produce the desired spectrum shape without requiring any averaging or windowing, so the analyzer display reacts much more rapidly to changes in the system than it would if testing with pink or white random noise, making them ideal for live adjustment of EQ filters".

Here's what the proper settings are using periodic pink noise. Set the output level to what works for you:

[hulkss]

I'm measuring the front left loudspeaker and then the subwoofer. I'm using the left channel with speaker size in the XMC-2 set to large for both to avoid the LFE filter discovered earlier. Don't worry about matching volume levels, that is done later.

After you get some good data on the REW RTA, save it as a measurement. Apply variable smoothing. Per REW: "Variable smoothing applies 1/48 octave below 100 Hz, 1/3 octave above 10 kHz and varies between 1/48 and 1/3 octave from 100 Hz to 10 kHz, reaching 1/6 octave at 1 kHz. Variable smoothing is recommended for responses that are to be equalised."

Here are my measurements in the theater seats with variable smoothing:

[hulkss]

Now that we have some measurements, we need a room curve to target the EQ filters. This is VERY VERY important and totally affects how happy you will be with the result. There is no right or wrong here and there is no precise or accurate answer. If you want scientific advice, there are only a few good sources. I have attached them to the bottom of this post. The Harman and Geddes documents have been truncated to the material that pertains most to what we are doing here.

If you don't like the sound of your system, the room curve is where you go to make changes. There is no standard for the tonality of program material. The XMC-2 has excellent tone controls and adjustable subwoofer output and LFE channel level controls to "tweak" your system for different program material.

Blu-Ray movies (not concert videos) seem to adhere more to "best practice standards". You may want to make a separate EQ room curve for movies as there are two Speaker Presets available in the XMC-2. I just turn down the LFE channel in the XMC-2 (not the subwoofer level) for movies with thunderous special effects.

Here is my first room curve setting in the EQ panel of REW. 

Attachments:

toole paper.pdf (1.67 MB)

OptimalBassPlaybackinSmallRooms.pdf (1019.36 KB)

The Subjective and Objective Evaluation of ....pdf (964.77 KB)

[hulkss]

I mentioned earlier that I want to achieve a symmetrical 24 dB/oct acoustic crossover between the subwoofer and other speakers. I set all of my speakers to small in the XMC-2. I select a single bass re-routing crossover (XO) frequency that works for all of my speakers. The crossover slope is set to 24 dB/oct in the XMC-2.

If differing XO frequencies are used, the XO filters sending audio to your sub(s) will be different. This can cause significant bass cancellation if correlated (similar) bass signals are on multiple channels. This is caused by the phase shift of the filters which is frequency dependent. The same goes for mixing "large" and "small" speaker settings. The large speaker bass will not be passed through a filter while the small speaker bass is.

I'm describing what I have found to sound the best. You can mix-up the bass management settings as you prefer. In the end some final measurements and listening will guide you.

The bass management crossover acoustic target must be included in the room curve target response. The following REW settings do that for small loudspeakers and the subwoofer.

[hulkss]

We are now ready to set-up the filter tasks in the EQ panel. I'll do the left channel first. You may have noticed the EQ device set to Generic & Generic (not Emotiva). We'll use Emotiva later.

I prefer to not use very narrow (high Q) filters, so I do not check that in the filter tasks (go ahead and use it if you get much improved results). The tasks are set to correct frequencies from 40-20K Hz.

Click on the EQ Filters button at the top and set-up the filters as shown below. Note the crossover filter on the right. This must match the crossover filter setting in the XMC-2 (high pass, your XO Hz, B'worth, 12 dB/oct). There are 11 parametric filters available in the XMC-2 so highlight only 11 filters.

[hulkss]

Hopefully you have been working with the measurement you want to make filters for highlighted (see image below). If not, select it now and make sure all the settings in the EQ panel are as discussed above. You can have all your speaker channel measurements here and set them up for EQ individually.

[ttocs]

hulkss I'm enjoying watching your process! Thanks!

[hulkss]

You may may have noticed that the boost levels in "filter tasks"are set to zero. In my opinion just forget about filters with positive gain. It is not a good idea to feed energy into acoustic nulls. I also want to be able to send a maximum signal (0 dBFS) to the XMC-2 and not worry about digital clipping. The XMC-2 software then only has to deal with its own tone controls, output level settings, and volume control. There is plenty of analog gain available. According to the manual, you can turn the volume up to ELEVEN.

"The Max Volume setting allows you to configure the maximum Volume Level which can be
selected from the front panel Volume knob or the Volume buttons on the remote control. This
setting controls the Volume setting in the Main Zone, and can be set from -30 dB to +11 dB in
1 dB increments."

[hulkss]

We need to set the Target Level in target settings. The selected measurement will be cut by the filters down to the target line as closely as possible (I have 1 dB set in the Flatness Target). Measurement areas that are below the target will not be acted on. Keep the target level as high as possible while allowing a good correction to be done.

[hulkss]

Time to make some optimized parametric EQ filters   Press the red highlighted command. If you get a warning about measured response being mostly above the target, that's OK, it's because we are using only negative gain filters. When this process is repeated for the subwoofer, the "Match Range" needs to be lowered if you want correction below 40 Hz. The crossover filter is also different in the EQ filters settings table (see small image below). Again, use your preferred crossover frequency that matches the XMC-2 setting.

[hulkss]

Filters for Left Channel and Subwoofer done! Let's look at the predicted response. I can tell you from experience, this is very good for a simple set of IIR parametric EQ filters. The left channel used 10 of the 11 available filters and the Subwoofer used only 4.

[hulkss]

Now let's look at the EQ filters combined with the crossover filters in the XMC-2 (the crossover filters were entered into the REW Generic EQ Filters pane). The colored area highlights the frequency profile of the filters + crossover showing how much signal is filtered out at any frequency.

Our next task will be to tweak the filter set-up in REW to be compatible with the XMC-2 and export the filter coefficients to files that will load directly into the XMC-2.

[hulkss]

With the filters looking good it's time to put them into the XMC-2. Select Emotiva as the EQ Manufacturer and RMC-1 as the Model (XMC-1 is missing some channel selections for the XMC-2).

The crossover filters will automatically be deleted in REW as they are in the XMC-2. The filter gains will be rounded off to the nearest 0.5 dB (an Emotiva limitation).

Click on "Save filter settings to file", choose the speaker name, and save the filter set to a folder named filtersPreset1. Duplicate the folder and name it filtersPreset2 if you want to populate both available presets with the same filters.

Use the following file names for the filter sets as needed:
center.emo
centerSubwoofer.emo
leftBack.emo
leftFront.emo
leftSubwoofer.emo
leftSurround.emo
rightBack.emo
rightFront.emo
rightSubwoofer.emo
rightSurround.emo

Copy the filter folders to a USB thumb drive.

Go to a Speaker Preset in the XMC-2 menu and do a "reset filters" command to set all the filters to a flat response. Now plug in the USB drive and import the filters. Repeat for the other preset if desired.

[danielb]

Dec 3, 2023 2:52:18 GMT -5 hulkss said:

The crossover filters will automatically be deleted in REW as they are in the XMC-2. The filter gains will be rounded off to the nearest 0.5 dB (an Emotiva limitation).

Not only the filter gains are rounded, but also the Q value is automatically set back to 10.00 if REW has calculated a value > 10 - which may lead to filters that are much too strong.

[hulkss]

Dec 3, 2023 7:46:53 GMT -5 danielb said:

Dec 3, 2023 2:52:18 GMT -5 hulkss said:

The crossover filters will automatically be deleted in REW as they are in the XMC-2. The filter gains will be rounded off to the nearest 0.5 dB (an Emotiva limitation).

Not only the filter gains are rounded, but also the Q value is automatically set back to 10.00 if REW has calculated a value > 10 - which may lead to filters that are much too strong.

Thanks for pointing that out.

In an earlier post, I recommended to not check "Allow narrow filters below 200 Hz". This limits the auto-generated filter Q values to a maximum of 5.

High Q filters may show good looking results on the computer screen, however, lower Q values yield the best sounding filters in my experience.