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[loraan]

I recently used TrueRTA (http://www.trueaudio.com) to tune the equalizer on my laptop and my living room HTPC. The results were amazing, so I thought I'd share what I did with the rest of the group.

TrueRTA is a program that takes the input into your sound card and shows a frequency graph for that input. The X axis represents the frequency, the Y axis represents the amplitude of that frequency in the input. With TrueRTA (and a bit of additional equipment--more on that in a moment), you can identify which frequencies are louder or quieter than they should be in your sound system and then use an equalizer to fix them. For example, I found that my HTPC setup had a huge dropout between about 100 Hz and 250 Hz and that frequencies above about 10 kHz were significantly attenuated. The reason for the dropout is because I'm using Aiwa bookshelf speakers with a subwoofer. The Aiwas don't go low enough to cover frequencies down to where the subwoofer picks up. The rolloff at the high frequencies is probably caused by the same thing: cheap speakers don't have as wide a frequency response as more expensive speakers. To fix this, I first turned the subwoofer's crossover as high as it would go (I know, that's not ideal, but in this system, it will produce the best sound) and then used MC's equalizer plug-in to boost frequencies from 10k up and from about 100 to 250 Hz (I had to compromise here because MC's equalizer doesn't fit those bands exactly. The result is, literally, night and day. Although the music sounded good before, when I do an A/B comparison (by turning the EQ plugin on and off) it's clear that the new settings are a great improvement. To me, this reinforced that you can't tune a sound system by ear, by what "sounds good". There are a wide variety of things that will seem to sound good, but only a flat response curve represents what the authors of the music intended. Now, granted, not everybody may like a flat curve, but it's certainly the best place to start tweaking. You won't know what you're missing until you flatten out your system.

More in a subsequent post... I'm going to split this between several posts because it is likely to get long.

[loraan]

To use TrueRTA, you will need a microphone. The problem with this is that most microphones don't have a flat frequency response. The microphone itslef will enhance certain frequencies and attenuate others. This means that, if you were to use that microphone to tune your system, you wouldn't truly end up with a flat frequency response.

Unfortunately, microphones with flat frequency responses (also known as "measurement microphones") can be very expensive--hundreds of dollars and up. Fortunately for us, there is an inexpensive choice that is nearly as good as the more expensive ones, and plenty good for home theater usage. That choice is the Behringer ECM-8000. This mic runs about forty dollars (check http://www.froogle.com). This mic has a nearly flat response from 20 Hz to 20 kHz. In addition, TrueRTA comes with a microphone compensation file for the ECM-8000, which means that it can compensate for the small ways in which the ECM-8000 isn't perfectly flat.

To use the ECM-8000, you will need a microphone preamp. You can't just plug it into your sound card because it's got an XLR connector, which your sound card probably doesn't. Even if your sound card does have an XLR connector, or maybe your'e thinking of using an XLR-to-1/4" adapter, you can't just plug the mic into your sound card, because the mic needs phantom power. Phantom power means that the preamp puts a certain voltage on the lines between itself an the microphone. No computer sound card that I know of supports this, but if yours happens to support it, you're in luck!

So, you'll need to buy the ECM-8000 for about forty bucks, and a preamp, which will run about sixty to one hundred dollars.

(Side note: I was lucky in that I already owned an Edirol UA-5 preamp and D/A converter. The Edirol acts as a USB sound card and has two XLR jacks with phantom power. If you've got something like this, then you don't need to buy a separate preamp.)

[loraan]

So now, you own TrueRTA and an ECM-8000 mic with preamp. Sadly, the weakest link in the "sound chain" is probably your sound card! Unless you've got a high-end sound card like an Audigy, Revolution 7.1, or a high-end external card like the Edirol UA-5, your sound card probably doesn't have a very flat frequency response. Then again, I've been surprised before! My Dell Laptop's built in sound is actually nearly as flat as my Revolution 7.1 (although its noise floor is much higher).

Fortunately, TrueRTA has a solution for you. TrueRTA can measure the response of your sound card and cancel it out, the same way that it can cancel out the qualities of the ECM-8000 microphone. To do this, you plug the sound card's Line Out into its Line In and then run a compensation test in TrueRTA. This is literally as simple as clicking a menu option. TrueRTA sends a test tone through the sound card and measures how the card changes it. Voila! By default, TrueRTA will now compensate for your sound card's frequency response.

Once you have loaded the ECM-8000's microphone profile and run the loopback test, your computer should now be essentially as accurate as a several thousand dollar audio analyzer--at least close enough for your purposes, right?

Ooops--there's one thing we forgot: the pre-amp itself might not have a flat frequency response. There are two ways to take care of this: 1) buy a pre-amp that is specially designed for measurements and that has a flat response, or 2) if possible, when you do the loopback test, plug the sound card's output into the input on the mic pre-amp (some of them can take a 1/4" plug in addition to an XLR connector) then plug the pre-amp's output into Line In on the sound card. Now, when you run the loopback test, it will measure and compensate for the combined response of both the sound card and the pre-amp.

[loraan]

Now that you have calibrated your measuring system, it's time to calibrate your sound system. For this, it is better if TrueRTA is running on a different machine than Media Center, but it can be done if they're running on the same machine.

You will need to get a WAV file containing a special kind of sound called "pink noise". Pink noise has a specific energy distribution so that when you play pink noise into an analyzer like TrueRTA, you should get a perfectly flat line. To find a pink noise wav file, do a google search for, "pink noise wav". You should find one out there somewhere.

Note: DO NOT USE AN MP3 OR WMA FILE OF PINK NOISE. Because MP3 and WMA compression change the frequency distribution of the sound, they will throw off the results. Only WAV will work. Okay--or APE, since it's lossless.

Interesting Thought: However, if you always play back WMA's or MP3's encoded in a certain way, you could encode the pink noise to a similarly-encoded MP3 or WMA, then tune your system with that noise. This would allow you to compensate somewhat for the changes in frequency distribution caused by the encoding.

Set up the microphone at your listening position. The microphone should be pointed straight up, not pointed at the speakers. It is best to do this with a mic. stand, but if you must hold it with your hand, you can. Play the pink noise in MC, with "repeat" turned on. Start TrueRTA. You will see the frequency distribution of the sound coming into the microphone. Now, if your system and room were perfectly flat, then you would see a flat line on the analyzer, but that's not going to happen. Your speakers will amplify certain frequencies, your subwoofer will be turned up too high, the drywall in your walls will reflect certain frequencies, etc... etc...

Your goal is to get that line as flat as possible. The best scenario is if you have a dedicated standalone equalizer, but many people won't have that. If your stereo or receiver has an EQ built in, you will have to decide if you want to use it. Typically, these EQs aren't very good, sometimes only having three bands (bass, mid, and treble). If that's the case, I recommend that you set the EQ in your receiver/stereo to "off" and tune within MC.

First, if you've got a dedicated subwoofer, balance that with your main speakers. Play with the subwoofer's level and crossover settings while watching the frequency response. If the crossover is set too low, there will be a "valley" between the higher frequencies of the main speakers and the lower frequencies of the sub. I recommend that you turn the sub's crossover all the way down, then turn it up until that valley "fills in" with the main speakers' frequencies. Then set the sub's level so that it is approximately equal to the main speakers' level (e.g. the lower frequencies are the same height on the graph as the higher frequencies).

Look for peaks or dips in the frequency response. On the X axis, you can see what frequency those peaks or dips are at. Find the closest slider in MC's equalizer plug-in and move it up or down. Keep doing this until you have achieved the flattest response possible.

Note: you've got two options, by the way--pull down the peaks or push up the valleys. Sometimes, a valley will be impossible to fix with MC's EQ, but pushing down the peaks to either side of the valley will work fine. And vice versa.

Listen to some music. Turn the EQ on and off. Smile at the amazing difference!

[loraan]

By the way, unless you've got a very precise EQ, you're never going to get that line perfectly flat. Your goal should be to get it as flat as possible. First remove large, wide-band differences, then try to focus on smaller dips and peaks. A good parametric EQ will do a better, more precise job than MC's built-in graphic EQ plugin, since you can directly set center frequency and bandwidth of a parametric EQ. Sadly, the DirectX plugin isn't very stable, so I wasn't able to use its parametric EQ effect.