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Comb Filtering

Discussion in 'Sound, Music, and Intercom' started by Schniapereli, Jan 4, 2007.

  1. Schniapereli

    Schniapereli Active Member

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    I just entered High School, and became sound guy, and am having a little trouble getting used to the better actors. They act a lot closer to eachother, thus causing comb filtering, and sounds bad. The play we did was Darn Yankees, which has a lot of different romances from Joe to Lola and Meg, where they get close, and talk, then walk away, then get close, and back and forth a lot. Our Applegate also liked to talk to people with his nose resting on their foreheads. (...not really)

    Anyways, what is the best way to deal with these phase cancellations? When they are close together for a long time, I can easily just turn one down, but what do you do if they keep moving around a lot? Do you just get better at moving one mic up and down accordingly? I have heard of just moving the gain instead, but that would mess up the level where I want it. Is there some ancient Chinese technique I have never heard of?
     
  2. BNBSound

    BNBSound Active Member

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    Without knowing your rig I'd say that understanding and judiciously using compressors (ie: on the groups) could be a help. But the key issue here is gain. You need to absolutely perfectly nail the gain structure on every channel, trim the EQ within an inch of its life, and then commit the show to memory and work the faders. The good news is, to avoid comb filtering you don't need to take one of the mics all the way out, just drop it down some.
     
  3. Schniapereli

    Schniapereli Active Member

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    I don't take the mics down all the way... but I have a hard time being able to bring the sliders back up to the unison without looking. I watch the actors to tell when to adjust, and my fingers don't move it as much as I think they do. I'm still getting used to the board...

    We have 1 compressor/expander with 2 channels. (Behringer autocom pro mdx 1400)
    We haven't had them hooked up yet, due to cable shortage, but we will the next play. We don't currently have insert cables, and might not get any, so it will probably be just a sound path from mixer to compressor, to graphic equalizer, to amps.
    One sound guy told me it's probably good to put boys on one channel, and girls on the other. But, how do you adjust the settings to eliminate the cancellations?

    I will also know the script better for the next play. Last play I was asked to do sound 1 week before opening, (at the begining of tech rehearsal), and then I got my script the night before opening day. (1 tech/dress rehearsal to mark all the added and deleted scenes) But, the next play will be better organized.
     
  4. kovacika

    kovacika Active Member

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    Really for compression its best to do one person per channel. Since you dont have too many compressors, you might just want to do leads.(joe, lola, devil)
    I also second BNB Sound on the Gain. Gain is everything in sound. Get gain right and your at least half way if not more to sounding good.
     
  5. mbenonis

    mbenonis Wireless Guy Administrator Premium Member

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    Compressors won't help specifically with comb filtering, but they do help with the overall sound of the show by preventing loud peaks.

    It sounds like you're doing exactly what you need to be doing - ducking one of the two actors when they get close. You'll get better at it as time goes on. It just takes practice, like everything else!

    What console are you using, by the way? Also, how are your wireless mics performing?
     
    Last edited: Jan 4, 2007
  6. SHARYNF

    SHARYNF Well-Known Member

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    Couple of thouhts
    What mics are you using? have you looked at mic placement, since you seem to be on a restricted budget, I am assuming you are using some sort of lavalier mic, so one thing to do is to turn the mic so that it is facing DOWN on the person instead of the usual up position, this will still pick up the speaker but may give you a bit more control.

    what mixer are you using, do you have the ability to assign the input to an aux?
    you could look at grouping the mics giving your self more control, but you would then need to re combine them to feed your amps.

    Here is an example where a digital mixer would allow for more control.;)

    Sharyn
     
  7. Schniapereli

    Schniapereli Active Member

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    We have MKE-2 Lavs (sweet) with Sony wireless wrt 805 transmitters, and we also have a few Shure SLX 2 transmitters (with their own SHURE 93 lavs). Our board is a Mackie SR 32-4 VLZ Pro.
    (EDIT= we mount all of our lavs on the face over the ear.)
    Should I just hook up the compressors to 2 different submasters so Or would this be better with the aux sends, and just taking the channel out of the main mix, thus sending it only through the aux sends to the compressor, then into aux returns (or another channel).

    Also, how does the compressor help me with comb filtering? How does it react differently from when they are close together, and when they are not? (if at all)
     
  8. CURLS

    CURLS Member

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    i remember when i was your age and asked myself this ?

    My solutions at the time were the same mentioned, just simply fade out one mic a lil bit. You don't necessarily have to take it all the way out of the mix usually 3 to 6 db is all you need since your audience will precieve a 3db boost between both the mics compared to comb filtering.

    Unfortunately your particular console does not do this. Another common trick in the basket of goodies is phase reversing one of the mic channels as the actor comes closer to the other actor.

    One other quick thought is considering your using MKE2's HECK YEA. You might want to try figuring out which works better you use the same way I do. If you mount one on one side of an actors face and then another one on the other side of the actresses face sometimes the distance and facial features can reduce the cancellation of noticable frequencies.

    Hope this helps!
     
    Last edited: Jan 4, 2007
  9. mbenonis

    mbenonis Wireless Guy Administrator Premium Member

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    See my post above. :)
     
  10. Schniapereli

    Schniapereli Active Member

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    ...missed that...

    But, just wondering, what does phase reversing do to the signal? Is that what broadway does?

    Also what do you mean by "You need to absolutely perfectly nail the gain structure on every channel, trim the EQ within an inch of its life." -BNBSound
     
    Last edited: Jan 4, 2007
  11. SHARYNF

    SHARYNF Well-Known Member

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    those are great mics, but the placement of them can be an art based on the performance of the various actors.

    the problem with the cheek mounting is that when the two people face one another the opposite person is facing and talking directly in to the other mic.
    Since they are cheek placed it tends to be quite direct.

    These are omni mics also

    I would guess that this is more of the problem you are having than actual comb filtering . All these terms tend to be a bit misused, so rather than try to re explain, take a look at http://www.moultonlabs.com/index.php/more/about_comb_filtering_phase_shift_and_polarity_reversal/

    You might try different placements
    One is on the forehead pointing down, the other is higher up on the temple in the hair instead of on the cheek.

    You might also try to determine if it is just one actor, or a specific pair etc.

    In addition these mics are well know for getting problems from sweat, and after getting sweated on, and drying the performance tends to vary also. It is important also to make sure if you can that beads of sweat are not getting directly into the mic. Sennheiser made a gold dot version vs the red dot version to try to help in this area.

    Again these are great mics, but not properly used or formerly abused you can have all sorts of problems.

    This is where on a large format console VCA's come in handy, where you can assign the level control to a common slider so that you can group the inputs together but still leave the assigned to the same output

    Here are a few suggestion that you could do with your mackie

    One is make sure that you are running FOH in mono mode, with what you have stereo pa is not a good Idea. If need be you can take the INPUTS to the rest of your system amps etc, and get a y cable and connect it to the mono out of the mackie.

    Now on each input you have the ability to select sub 1-2 3-4 and l-r
    you can place a pic either on 1-2 or 3-4 of l-r based on the pan knob, so now you have 6 channels that you can sort of work with. Take the sub 1-4 and plug them into your 29 30 31 32 inputs and assign these just to l/r, now what you have is the ability to
    group your inputs into 6, L-r are for mics that you assign and typically do not need to alter as a group, the set and forget except for individual channel tweaking, the mics that you do need to control can then via the sub assigns and the pans be split into 4 with now 4 faders controlling these 4 groups. Set your master for your over all level, set the faders on 29-32 for the relative level to the rest of the board, and use the faders for subs as your grouping controls.

    This gives you in effect a second level of control on the board, where you can set individual levels, but at the same time move a single fader and control a group as part of the overall mix

    Probably sounds more complex that it is but if you sit infront of the mackie and think about it and look at the gain structure you will see what I mean

    Sharyn
     
  12. soundman1024

    soundman1024 Active Member

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    I fight this nearly daily at the TV station during interviews. Typically I take one mic down about 2dB and the person talking's mic up around 2-3dB. Maybe not quite that much. As soon as the person talking turns their head (or moves in your case) I can restore things to where I had them.
    Best thing you can do is ride the faders I would say.
     
  13. mbenonis

    mbenonis Wireless Guy Administrator Premium Member

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    I dealt with the same issue over the summer, when I worked at the local TV station doing sound for a few shows. They had WL93 mics (wired) and it was so difficult to get one person clearly without them leaking into the other mic - I ended up gating them very closely and riding the faders as necessary.
     
  14. audioslavematt

    audioslavematt Active Member

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    I won't go into all the details of the physics involved, there are plenty of books and articles to be found through google. Essentially, when you "flip the phase" you're really flipping the polarity (so pin 3 becomes hot instead of pin 2). Flipping the polarity simply rotates the phase 180 degrees, reversing it. If you need a visual, graph y = sin (x). This is your normal sine wave. To show the sine wave with the flipped polarity, graph y = -sin (x). Dave Rat himself does a better job explaining it than me. http://www.prosoundweb.com/live/articles/daverat/polarity.shtml
     
  15. Schniapereli

    Schniapereli Active Member

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    This phase switching thing still confuses me a little. I understand the articles, and how it could help with drums, and similar situations, but I don't see how it eliminates comb filtering. I attatched a drawing to show you how I understand it so far.

    The middle wave is the original person's voice picking up on another person's mic, and is slightly delayed. The top line is the original person's mic, and the bottom is the original person's mic inverted.

    I could see how this would work if it was exactly 180* off, but if they are just a little off, I don't see how phase reversing would help. They still look like they would cancel out.

    And mathematically speaking, it doesn't make any sense to me either. (that reversing it still keeps it a fraction of pi off from synchronization with the delayed version. Inverted is only in a sense moving it forward 1/2 pi, so the nubers are still ugly in between)

    What is wrong with my drawing and/or theory behind it?

    Is there maybe another site or another good article?

    (the wavelenghts are marked at the half to help with my crappy drawing.)
     

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  16. Peter

    Peter Well-Known Member

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    Although this isnt really an answer to your last question, it is another idea about how you can improve things.

    *If* your venue is setup with a stereo sound system you can try panning the two mics causing trouble to either side so they are separated. It doesn't have to be a complete pan, but play with various amounts, and make sure they can both still be heard from (nearly) everywhere in the house [you may have to sacrifice a handful of seats for the greater good of everyone in this case]. What you are doing in this case is greatly reducing the chance of comb filtering on the pickup (mic) end of things and just tweaking the comb filtering that is already happening to some degree on the output side of your system.

    For everyone doing bands, this technique also works really well for cymbal overheads which can be panned rather hard to either side in a good stereo PA system.
     
  17. CURLS

    CURLS Member

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    Ok so let me just say this there are very few systems in the world where you will not experience comb filtering to some degree. The least of these being outside where i like to work, instead of a theatre. And also a side note on trying to rid of comb filtering, I would be willing to note that most of your stationary seated audience do not even precieve what is happening with such said comb filtering, but that is audience psychology and not the topic at hand.

    Ok, so when I first made the comment about phase I did not intend for you to misunderstand that it could correct comb filtering. Comb filtering is usually corrected by time alignment.

    After I reviewed your initial question I noticed you were merely talking about two actors getting closer to each other thus the combining of two signals results in a positive 3db gain increase.
    Comb filtering results from 2 out of time signals processed by the ear. With that being said a delayed underbalcony fill system out of time with mains would have comb filtering that most people would agree to notice.

    So therefore two microphones that are not in time which is your example are then added to create a single wave that has an increase of 3db that has been shifted in phase based on the distance of the actors that will in turn go through your pa as one single signal. Or multiple delayed systems as still one single signal.

    Now to talk about the phase button that I talked about pressing. Your drawing was soewhat correct. But I'll do my bes to explain.
    Performer A emmits single sine wave 1k frequency at 3db
    Performer A also emits this into Performer B's mic.
    End resultant is a 3db increase through performer B's mic at 6db

    If Performer A does the same exact thing
    But, Performer B's mic is reversed in phase then 0db of gain is resulted and he remainsat 3db constant gain. This will only work if the reversed phase mic comes in consitent contact with a non reverse phasd mic. So in other words press it when you know two people are coming together.

    Or in super simplicity just fader juggle the two leads like the firs reponse ya got and you will be in businss. Happy MIXING!!
     
    Last edited: Jan 7, 2007
  18. Andy_Leviss

    Andy_Leviss Active Member Premium Member

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    The short answer is, it doesn't. It's an audio myth. If your actors emitted pure sine waves, and they were physically spaced exactly a wavelength apart, it would help keep the comb-filtering from happening, although you'd still experience a doubling in level.

    The way to solve the problem, as noted above, is to work with mic placement, blocking, and--this is the biggest key--actively mixing and choosing which of the mics will give you the best compromise at any one moment, but only having one up at a time. It's not easy, for sure, but it works.

    The other way, which is well beyond your means equipment-wise, is to use an A-B speaker system, and there's even some argument about whether that actually works well.
     
  19. Andy_Leviss

    Andy_Leviss Active Member Premium Member

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    To further clarify things, you reverse the polarity on things like bottom mics on drums to keep them in the same polarity as the top mics (or on the mic inside a kick drum, which is technically a bottom mic). Otherwise, for example. if you've double-miced a snare, when you hit the snare, the two mics will be pulling/pushing in opposite directions and will--to some extent--cancel each other out. It's not a matter of comb filtering in that case.

    Polarity reversals fix opposing polarities; delays fix phase issues (which manifest themselves as comb filters).
     
  20. Schniapereli

    Schniapereli Active Member

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    okidoki. :)
     

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