Comb Filtering - Part 3

Comb Filtering - Part 3 Brought to you by: guitar.com

Just Press Play Logo Gif 68596IMPORTANT - You should read Part 2 for important details as both articles directly relate.

Here is the deal. This is based upon a three amp, three mic set up. The odds are good a comb filtering problem will not be in play if youre using only one mic on each of the three speaker bottoms. The reason being is that the middle amp is the dry non effected signal and the other two amps are used for effects such as reverb, pitch shifting, delay, etc. All of these effects delay or mess with the signal where comb filtering problems would not matter. The only exceptions I can think of are using the exact delay setting on both effect amps or the dry non-effected guitar signal is routed to two or three amps. Since that is a possibility, here we go.

 

Important! For all of the following, do not use any effects that alter pitch and tone, meaning pitch shifters, wah-wah, etc. If you will use such effects, plug them in after completing the comb filtering tests.

Comb filtering got its name because the waveform looks like a hair comb. This is caused by one sound source and two (or more) mics picking up the sound when the mics are not the exact same distance in relation to the sound source. In this set up we have three amps and three mics positioned in front of three speakers. By now, you get the drift regarding the math and the fix.

As usual, all three mics are closed miked in regard to their respective speakers. You know the drill by now as the rap is the same for all of the past comb filtering articles, so lets get into the fix.

The Comb Filtering fix changing mic positions

If you're using a digital recorder format, and if youre using three mics to three recorder tracks, the fix can most likely be fixed after recording. Even so, you may want to do the following test to avoid the time shifting later.

After setting the basic mic placements and you are happy with the sound, we will work with the non-effected (middle speaker bottom) and the left speaker bottom first. The concept is to move one of the mikes to equal the same distance from the sound source as the other mic.

The routing has been set up for each mic to go to its own recorder track. Mixer module #9, #10 and #11 are the mic sources and the recorder tracks are #6, #7 and #8. Pan mixer modules #6, #7 and #8 to center and set their faders to around 10 dB on their fader throws. Mute mixer module #11 for now since we are dealing with two mics first.

If you have set up the path in full and have gotten a good guitar sound, and if a compressor is in the chain, bypass it for now. Also switch out the EQ for the time being. We simply want the mic path unaltered.

You also need to mark mixer fader settings since we will change levels. Put a piece of artist or masking tape next to the mixer faders used in the mic path and recorder return path. Mark the tape (using a sharpie pen or any marker that can easily be seen) next to the little pointer on the fader to be able to return to those settings later.

If you have set guitar amp levels, tone, etc., we are going to move the settings for the test. Put a piece of artist or masking tape next to the controls and mark with a sharpie pen or any marker that can easily be seen. If the controls do not have a little pointer also put a piece of tape on top of the control and mark both pieces of tape with a little arrow pointing towards each other.

The guitar could be used as the sound source but since the signal dies off in sustain land fairly quickly, its better to use a sound source such as your favorite artist CD or cassette tape.

In this case, you are multing off the guitar signal to three amps in some fashion so use that input for the CD or cassette player output.

Since the CD or cassette player is a line level source, its most important to make sure the guitar amps volume are off to start. If the CD or cassette player has outputs that allow the volume control to be active, use it so as to not overload the guitar amp input. We do not care about the stereo output, meaning use either the left or right output only

Play the CD or cassette and slowly bring up both guitar amps volume. In any case, all we need is a volume level that is around an average listening level. You may want to adjust the guitar amp tone control settings if the sound is too dark but try to match the tone settings on each amp is possible. The idea is to make sure to hear a wide range of frequencies. After you have set the controls to taste, hit stop on the CD or cassette player for now.

Back to the mixer. The idea is to move one of the mics to the point of hearing signal cancellation. This is achieved by flipping the phase of one of the mic mixer paths and setting both mic levels (and recorder return levels if using two records tracks) to the exact same levels.

If your mixer modules have phase switches, you could switch the phase on either mixer module #9 or #10 since all we want to do is swap either. Lets reverse the phase on mixer module #9. Doing so will lead to signal cancellation when the levels of both mics are the same and the mics are positioned at the same distance from the sound source. Note that full cancellation may not happen for many reasons but the idea is to find the point of the most cancellation.

(Important: if your mixer modules do not allow for phase reversal, the fix is to wire up or alter a mic cable inverting the phase. If you're using a two wire mic cable, on one end of the cable at the connector, reverse the hot and ground wire. If you're using three wire mic cable, on one end of the cable at the connector, swap pins 2 and 3. Make sure to label the cable inverse phase!!!)

Play the CD or cassette player. Set both mixer modules #9 and #10 to around 10 dB on their respective fader throws. Very slowly move mixer module #9 up and down to listen for the cancellation spot. After finding the level setting with the most signal cancellation, its time to move one of the mics regarding the distance to the guitar amp speaker. Again, I typically move the mic that is positioned farther away.

If you are working by yourself and can get to the guitar amp wearing headphones monitoring the mixer signal, simply move one of the mics until you hear the most signal cancellation. Now go back to the mixer and slowly move mixer module 9s level up and down as to hear further signal cancellation. Repeat this over and over until finding the point of the most signal cancellation as possible.

If you are working with a 2nd engineer, or someone to help you, all the better. The 2nd engineer or helper needs to wear headphones hearing the mixer signal you are hearing. The 2nd engineer or helper will hear what to do, meaning hearing the point of cancellation or more build up when moving the mic (if the guitar amp is not too loud in the studio). While the 2nd engineer or helper is moving the mic slowly, keep moving the level on mixer module #9 very slightly up and down looking for the spot of the most signal cancellation. Use the mixer talk back to communicate with the 2nd engineer or helper. When hearing the most signal cancellation, ask the 2nd engineer or helper to stop moving the mic.

The mic needs to be secured as usual so refer to the Miking The Guitar Amp article (Part 3). Make sure the mic is not moved when doing so!

Now flip the phase on mixer module #9 back to normal. DO NOT MOVE THE LEVELS ON MIXER MODULES #9 AND #10 OR #6 AND #7 FOR NOW!

Now its time to add mixer module #11 into the equation. Un-mute and reverse the phase on module #11. Repeat the above procedure moving only mixer module #11s fader slowly up and down. After finding the most cancellation, flip the phase on mixer module #11 back to normal. Reset all guitar amp settings, mixer settings, etc. Good news! You have set mic placement to eliminate comb filtering problems achieving a great guitar sound!

Since the mics were slightly moved, you may want to revisit EQ settings and a slight level change with the mics.

Most engineers do not take the time to perform the test. I have blown it off myself when in a hurry and have ended up with a sound that could have been better. Ouch!

The Comb Filtering fix in Digital Recording Land

This fix is to be done after recording the guitar. This is only going to work if a delay line is set to the exact same setting on the left and right amp or if the signal for all three amps is the non-effected sound.

Using a hard disk recorder with waveform track readouts: The easiest way to fix comb filtering requires a hard disk recorder format with a waveform readout that allows sample accurate timing offsets. So if the non-effected signal was recorded on all three tracks, the fix is to look at the first attack of the three-guitar tracks and time shift the late tracks waveform to match the track with the first attack. Make sure to shift the total length of the tracks meaning from beginning to end.

If a delay line was used with the exact setting on both channels, look at the waveforms of the left and right amp (recorder tracks #6 and #8). Look for the first delay and if not the same in time, shift the late track as to match. Make sure to shift the total length of the tracks meaning from beginning to end.

Using a stand-alone digital recorder that allows sample accurate track timing offsets: In that case, you will need to experiment using track delay timing offsets as to start. More on this as things unfold.

The following is based upon three amps with the non-effect signal. (If youre just dealing with the same delay setting on the effect channels/left and right speaker bottom, perform the following with mixer modules #6 and #8 only and simply adapt.)

If the non-effected dry guitar signal was recorded on all three tracks, the fix is to find the track that sounds first (the track that had the mic positioned closet to its speaker) and then delay that track to time match one of the others. Then the other track (3rd track) is added into the equation using the same concept. I will clear this up as we get into details. Note: We assume that all mics were close-miked. Reverse the phase on either recorder return mixer module #6 or #7. (If your mixer modules do not have phase switches, see the above section and look for a paragraph that starts with, (Important: if your mixer modules do not allow for phase reversal) In this case, you will need to substitute a cable from the recorder track output that is wired reverse phase for one of the two recorder outputs.) Mute all other instrument/vocal tracks meaning only listen to the two guitar tracks. Set both mixer modules #6 and #7 to around 10 dB on their respective fader throws. Also pan both mixer modules to center. While listening to the playback, very slowly move mixer module #6 up and down to listen for some or total signal cancellation. If hearing some signal cancellation, or none, in either case, time to experiment with track delay offsets. Start by delaying track #6 in five-sample steps. Again, very slowly move mixer module #6 up and down to listen for some or total signal cancellation. When the signal cancellation is becoming more noticeable, go to single-sample steps. Experiment with single-sample delay settings until you hear the most signal cancellation between the two tracks. Go past the point of hearing the signal cancellation and then work in reverse (backwards) meaning use single-sample delay steps in reverse (less track delay). When finding the general area of cancellation, go back and forth with delay settings until finding the track delay setting with the most cancellation. Write down the amount of delay samples since we will need these numbers later. If you are not hearing any signal cancellation and the sound becomes two distinct attacks after using delay numbers that are in the thousands of samples, track #7 is the track that needs to be delayed. In this case, repeat the above steps track delaying track #7 instead. By now, you should have discovered the track that needs to be delayed and have found the best cancellation point. You have also noted the track delay setting.

Let's say that track #6 was the track that was delayed, canceling out the signal between tracks #6 and #7. (If track #7, use track #7 in the following instead of track #6) This proves track #6 arrived to the recorder first in time (the mic was positioned closer). We now need to check track #8 in comparison to track #6. Reverse the phase on either recorder return mixer module #6 or #8. (If your mixer modules do not have phase switches, see the above section and look for a paragraph that starts with, (Important: if your mixer modules do not allow for phase reversal) In this case, you will need to substitute a cable from the recorder track output that is wired reverse phase for one of the two recorder outputs.) Mute all other instrument/vocal tracks meaning only listen to the two guitar tracks. Set both mixer modules #6 and #8 to around 10 dB on their respective fader throws. Also pan both mixer modules to center. While listening to the playback, very slowly move mixer module #6 up and down to listen for some or total signal cancellation. If hearing some signal cancellation, or none, in either case, time to experiment with track delay offsets. Start by delaying track #6 in five-sample steps. Again, very slowly move mixer module #6 up and down as to listen for some or total signal cancellation. When the signal cancellation is becoming more noticeable, go to single-sample steps. Experiment with single-sample delay settings until you hear the most signal cancellation between the two tracks. Go past the point of hearing the signal cancellation and then work in reverse (backwards) meaning use single-sample delay steps in reverse (less track delay). When finding the general area of cancellation, go back and forth with delay settings until finding the track delay setting with the most cancellation. Write down the amount of delay samples since we will need these numbers later. If you are not hearing any signal cancellation and the sound becomes two distinct attacks after using delay numbers that are in the thousands of samples, track #8 is the track that needs to be delayed. In this case, repeat the above steps track delaying track #8 instead. By now, you have discovered the track that needs to be delayed and have found the best cancellation point. You have also noted the track delay setting. If not, the next paragraph explains.

So what happened? If track #6 needed to be delayed, we know that track #6 is sounding first. If track #8 needed to be delayed, we know that track #8 is sounding first. No matter what, you get the logic. Let's assume the following fictitious layout so as to set up the time shifting fix.

Track #6 was delayed 18 samples in comparison to track #7. We want to get track #7 in the future by 18 samples as to make up for the delay.

Track #6 was delayed 23 samples in comparison to track #8. We want to get track #8 in the future by 23 samples to make up for the delay.

Back to the Future

Now its time to put the two late guitar tracks in the future. This is cake if youre using an ADAT recorder format (or a similar format) using two or more recorders that have a sample accurate time lock. You have written down the track delay settings. (Our fictitious settings are noted above.) Take the first ADAT recorder out of track delaymode. Digitally record (bounce) tracks #7 and #8 to two open tracks on another ADAT recorder. It's best to use a blank tape on the 2nd ADAT recorder to be safe. We need to digitally bounce one track at a time back to its original track (ADAT recorder #1) offsetting the bounced ADAT recorder in the future. This is performed by putting the 2nd ADAT recorder in machine offset mode. In our example, we know that we need to get track #7 in the future by 18 samples. Offset ADAT recorder #2 to minus 18 samples this allows the 2nd ADAT recorder to send the bounce 18 samples early correcting the delay problem!!! The 2nd ADAT machine offset setting has been set so now digitally bounce it over to its original track on the first ADAT recorder. Check playback to make sure the bounce was recorded correctly. If you want to play things very safe, meaning not recording over the original track; record the offset track to an open track. Mark this track as the comb filtering fixed track and mark its original as do not use and also note the offset time used on the track sheet this allows you to go back and change offset times if needed. Now digitally bounce track #8 in the same fashion BUT setting ADAT recorder #2 offset set to minus 18 samples. If you want to play things very safe, meaning not recording over the original track; record the offset track to an open track. Mark this track as the comb filtering fixed track and mark its original as do not use and also note the offset time used on the track sheet this allows you to go back and change offset times if needed. Now take the ADAT recorder used for the offset out of machine offset mode.

Important: If you're using any digital recording format, its always smart to back up the tape or hard disk data at least once per session. Yes, this is a time burner but trust me on this if you do not back up your stuff often, the odds are very good you will lose recorded information at some point in time. This happened to me a few times when working with drum samples and I was backing up every few hours!!! The sinking sick feeling in my stomach will never be forgotten!!! Only a few hours of extra work but I was pissed off!!! Yes, I always say no rules but take this rap as a rule!!!

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