In the last article, Recording the Guitar, Part 6 - we covered the comb filtering fix before recording the direct box and guitar amp. OK, we could delay the direct box path as in the last article but even better would be to put it in the future to keep the performance feel in tact. If you're using a hard disk recorder format, or any digital recorder format that allows timing offsets, we can deal with the comb filtering after recording. Here we go.
The Comb Filtering Fix, After Recording to Two Tracks
Using a hard disk recorder with waveform track readouts: The easiest way to fix comb filtering requires a hard disk recorder format that allows sample accurate timing offsets. After recording the direct box and guitar amp mic on individual recorder tracks, the fix is to look at the first attack of both tracks and time shift the late track waveform to match the track with the first attack. Make sure to shift the total length of the track, from beginning to end.
In this case, the late track will be the guitar amp miked track. Simply shift this track into the future to match the direct box track. (Note that when recording the performance, you may want to monitor the direct track slightly louder since you will be basing the feel on this track which will be heard first (yeah, in micro-seconds, but any delay is part of the feel).
Using a stand-alone digital recorder that allows sample accurate track timing offsets: If you're using a stand-alone with accurate time offset capabilities, you will need to experiment using track delay timing offsets to start. More on this as things unfold.
If the direct box and mic sound are totally different, meaning the direct box signal is very dark sounding and the amp is very bright sounding, you may not notice a comb filtering effect in the upper frequencies since the two signals would produce waveforms that are not very similar. In any case, there would still be some comb filtering happening so its always best to fix it.
The deal is, no matter how close the mic is positioned to the speaker, the signal will arrive at the mixer later in comparison to the direct box signal. Physics of the speaker alone cause the delay in comparison to the electronic direct box path.
1. Reverse the phase on either recorder return mixer module #7 or #8. (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 a 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!!!) 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.)
2. Mute all other instrument/vocal tracks, meaning only listen to the two guitar tracks. Set both mixer modules #7 and #8 to around 10 dB on their respective fader throws. Also pan both mixer modules to center.
3. Major fact here!!! EQ the direct box signal to sound like the guitar amp sound for the test Going back and forth between mixer module #7 (direct box path and #8 (mic path), using their mixer mute button for each, EQ the direct box as to get in the EQ area of the guitar amp sound (the guitar amp will surely sound much brighter). Try adding approximately 6 or more dB at 3 kHz and maybe the same around 5 kHz. (If youre not clear on this, mute the direct box module and listen to the guitar amp mic module. Now mute the guitar amp mic module and un-mute the direct box module and try to match the frequencies using EQ.) This is important to get the frequencies in the same ball park when looking for cancellation
4. While listening to the playback, very slowly move mixer module #7 up and down to listen for some or total signal cancellation. You may hear some signal cancellation but probably not as of yet. No matter since its time to experiment with track delay offsets.
5. Start by delaying track #7 in five-sample steps. Again, very slowly move mixer module #7 up and down as to listen for some or total signal cancellation. Keep adding five-sample steps and when the signal cancellation is becoming more noticeable, go to single-sample steps.
6. Experiment with single-sample delay settings until you hear the most signal cancellation between the two tracks.
7. 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 for the following stuff.
Note: Let's assume that the recorder sample rate is 48 kHz per second. There are 1000 milliseconds in a second so every millisecond equals 48 samples. As mentioned, sound travels around 1 foot per millisecond. We are using a sound delay of .3 milliseconds for the example so we divide 48 by .3 and arrive at an offset if 16 samples.
So let's say track #7 was delayed by 16 samples in comparison to track #8. We want to get track #8 (guitar amp mic track) into the future by 16 samples to make up for the delay.
Now its time to put the late guitar track back to 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.
1. You have written down the track delay setting. (Our fictitious settings are noted above.)
2. Take the first ADAT recorder out of track delay mode.
3. Digitally record (bounce) tracks #8 to an open track on another ADAT recorder. It's best to use a blank tape on the 2nd ADAT recorder to be safe.
4. We need to digitally bounce this track back to its original track (ADAT recorder #1) offsetting the bounced ADAT recorder into 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 #8 in the future by 16 samples. Offset ADAT recorder #2 to minus 16 samples this allows the 2nd ADAT recorder to send the bounce 16 samples early correcting the delay problem!!!
5. 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.
6. 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.
7. Now take the ADAT recorder used for the offset out of machine offset mode.
8. Now switch the phase back to normal on the mixer module that was phase reversed.
Important: If you're using a digital recording format, it's always smart to back up the tape or hard disk data at least once per session. Yeah, 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!!! It was only a few hours of extra work but I was pissed off!!! Yeah, I always say no rules, but take this rap as a rule!!!