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First Use Of The M20d, Feedback, Autotrim

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#1 radamo

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Posted 20 October 2013 - 12:55 AM

Hi,

 

I am not a musician, and I bought the M20d for conferences with people talking into mic. I choose the M20d for the weight, size, possibilities, and feedback control.

I made my first job 2 days ago, in a large room (40x30m), with 4 x KR200 system sound, and 8 mic HF (sennheiser). i was a little stressed because I did not know how the mixer will play and did only 10 minutes test with 1 mic the day before (i know, i should be more prudent).

 

1) The good :

incredibly intuitive. All i wanted to do, i could do it !! In one day without reading a lot and in real condition, i could do it. I used a yamaha 01V and it much much hard to use, and bigger. So, the M20d is my first choice mixer now, for big or small  configurations.

 

 

2) What gave me some trouble :

-Feedback control

That is my nightmare. I used dbx224, behringer, Sabine, 31 eq,...All these tool should in theory work perfectly if you have enough time to make them work. In the real world it is no so clear. To be able to have feedback control on each input was new for me and a real "plus".

It works well with the M20d but you have to make so much noise that you need the room to be empty. And if you switch off the mixer, you lose every filters settings. That is a big issue for me. During the show, in the morning after the test, i start without any filters and could not make the larsen start because there where many people in the room. Another problem is that the room does not sound the same will people and without. So, doing test without any people is not the best way to do.

Please, make us possible to keep the filters in memory, because if we can't the feedback control is useless.

 

- Autotrim

People on scene are moving and changing (male, female, lound voice, low, etc.) and i tried to put the autotrim "in" but did not notice any ajustement. Can you explain me how it works ? How long do it take to make the change in trim from a large then a very small voice ?


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#2 gordwait

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Posted 20 October 2013 - 01:44 PM

Someone else mentioned this, (that you can't save feedback control) but then I started thinking - just as you mention, the room sound changes over time, so really do you want to save the feedback settings from an empty room for use with a room full of people?

 

I think the idea is this:

Step 1: Turn on feedback control for your microphones.

Step 2: Just use the PA system.

 

Any feedback that may start will be caught by the m20d very quickly, and no need to do anything, other than perhaps turn down the mic just a little..

 

On autotrim, 

I don't think it will turn back up the level for a quieter voice. I think it will only fix any clipping issues by turning down the input trim, and also turning up the mic volume control so the level sounds the same.

 

Sounds like you could use an iPad remote to adjust for different speakers from your seat..


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#3 RonMarton

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Posted 22 October 2013 - 12:47 AM

...I bought the M20d for conferences with people talking into mic. I choose the M20d for the weight, size, possibilities, and feedback control...

 

OK radamo...

 

I do a large amount of multi-mic convention and conference work, but still had to think for a while about the best way to post something that might help you.

 

It seems to me that it would be very important for us to be clear about two things:

  1. A "routine" we need to practice until we no longer need to "think about it" and
  2. The difference between "Auto Trim" and so-called "automatic level control".

The Routine.

 

I'm referring to "Copy/Paste Channel Settings", as per page 10.17 of the "Advanced Guide" that arrived with your M20d and/or can be downloaded from here: http://line6.com/sup...uals/stagescape.

 

You will better understand why it's important for us to practice this icon-based "preset saving" routine once I've explained MY technique for handling these "conference" jobs, but for now, I'm suggesting that you spend an hour or two actually doing that practice by tweaking an individual channel to extreme settings that are very easy to "see and hear", solely so you can practice the "instant" creation (and subsequent elimination) of new "presets" to be copied to all of the mic channels we're about to use for the individuals who are participating in our "chat session".

 

Auto Trim.

 

This wonderful feature is definitely NOT meant for "on the fly level riding" of the sort we'd find useful to make our different voices more "even" in the mix.

 

As gordwait has noted, it is simply a very elegant way to quickly adjust for differences in the "types" of inputs we might plug in, the idea being to "hold back" the louder ones (so that they do not "clip" or distort), but apply more gain to the quieter ones, (so that they don't yield a lot of "hiss" or "system noise") BEFORE presenting them to the "channel strip rotary encoder" knobs that allow us to combine and "balance" them when we create our "mix".

 

My Technique For Multi-Mic Conferences.

 

First off, I think it's absolutely vital for every participant to be miked as close to identically as it's possible to do. That means identical mics for all, with each positioned the same distance from both mouths and large reflecting surfaces such as floors and/or desks.

 

The basic high-pass and eq settings should also be identical for all of these mics.

 

We then set the input trims (whether via "auto trim" or manually) by first wearing just one of those mics and literally shouting at the loudest level we can expect from our participants, so that our loudest shouts are "held" below clipping.

 

That input trim setting is then copied to the channel strips of all of our participants' mics ...and (in practice) is generally left unchanged for all such jobs where we use that model of mic.

 

Our next job is feedback suppression, which we pre-configure

  1. Via "FBS Enable" in "Deep Tweak" mode,
  2. With NO compression enabled and
  3. As "early" as possible ahead of the gig so our "room" is as empty as it can be. 

Long practice has taught me that the best way is to have just one volunteer miked up and positioned in the "worst" feedback-creating position that one of our participants might adopt, but if that can't be arranged, "propping" the mic there on the best "dummy body" we can find (paying particular attention to both the mic's height and its distance from those large reflecting surfaces) is almost as effective.

 

We then increase that channel's gain to the point where feedback just starts to give the room's first fundamental "note" and back it off to where it just stops again ...and only NOW enable FBS and re-increase the gain until we hear the initial note be "grabbed and notched out" by FBS.

 

We subsequently repeat that "gain increase and notch out" for no more than two or (at most) three subsequent "notes", ...whereupon we save that as our FBS setting, ...a technique substantially different from that required for a vocalist who's typically dealing with FOH and monitors.

 

...you have to make so much noise that you need the room to be empty. And if you switch off the mixer, you lose every filters settings. That is a big issue for me...

 

You'll also see from all I've posted above that OUR main use for FBS is to "notch out" the "worst case fundamentals" imposed by a given acoustic space, something that we need to establish just once in advance of each individual "rig" or "set-up", the subsequent arrival of more "damping" (in human form) only serving to possibly reduce our reliance on the notch filtering that FBS imposes.

 

I'd next like to discuss "automatic level control", which we'll achieve with our compressor in Deep Tweak mode.

 

One main hallmark of too much compression in many jobs is "sounding fine" when participants are talking, (or singing) but feedback suddenly surprising us when the talking (or singing) stops. No amount of FBS (or anything else) will fix that.

 

What we want here is a "gentle gain rider" that will "hold back" louder voices but "open up" for the quieter ones ...and the key to achieving that is actually having our compressor "holding gain" for any voice that's louder than "normal" (as you've noted) with

  1. A low ratio, being around 1.5 to 1,
  2. A reasonably fast attack of around 100 to 250 µs,
  3. A relatively slow release at around 250 to 500ms, being a quarter to half a second or so and
  4. Its threshold set to the level of our "average" participant, often being in the range -40 to -25 dB. 

As with "gain trim", it's often easiest to first set up a "model" or "template" for such compression "at base" or "at home", by listening to and tweaking the above settings on an "average" voice, (very often listening via headphones) until we're reasonably happy with our "automatic level control", ...bearing in mind that the "pumping" of room noise that's often really objectionable when listening to our one "sample" mic will almost certainly be "masked" by other voices during an actual multi-mic session.

 

(I'll just repeat, however, that it's vital for NO such compression to be enabled until after FBS has been set for a given venue.)

 

Having now established and enabled all of those settings in our "sample" channel, we then copy and paste them to every one of our participants' channel strips ...and save that set-up as the "starting point" for our gig, with the result that only slight individual adjustments should then be needed during each session. 

 

Happy mixing ! 


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#4 dboomer

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Posted 22 October 2013 - 04:48 AM

Hi radamo

Welcome to the StageScape club!

The feedback management system in StageScape is not like the others you mentioned I that you do not set it up before using it. It is completely dynamic and will set filters only if feedback actually occurs. There's really no reason to hold the filters because if a mic moves even a few inches the frequencies that can cause feedback change and the filters will move as necessary. You really don't want them to stay fixed.

#5 nickinfrance

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Posted 22 October 2013 - 07:07 AM

Hi radamo

Welcome to the StageScape club!

The feedback management system in StageScape is not like the others you mentioned I that you do not set it up before using it.

 

You have made an interesting post for me there Don.  Are you saying that its not worth trying to ring out feedback before a gig using FBS?  Must admit I have always tried to do that.  I know that this is a process often done when you are using an external 31 band EQ.


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#6 RonMarton

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Posted 22 October 2013 - 02:08 PM

...Are you saying that its not worth trying to ring out feedback before a gig using FBS?... 

 

That's exactly what we're saying, Nick...

 

...Albeit in totally different ways that arise from totally different applications.

 

An M20d's Feedback Suppression System is fundamentally different from earlier systems in that it's constantly "looking at" the "waveforms" that are entering the channel/s in which it's been enabled.

 

Line 6's brilliant "Modelling Gurus" seem to have developed bit-mapping techniques that allow it to "spot" the (mainly sinusoidal) hallmarks of frequencies that are responsible for feedback, as distinct from (for this discussion's sake, let's say for example "flute") music.

 

Once one of those is detected, it's "notched out" from the channel in question, as and when required.

 

It's just that, for our "conference" applications, I've found that "pre-educating" the system via my "worst case" or "worst location" routine seems to work better for me than switching it in once things are underway.

 

I'm happy to admit that there's every chance that this is only a "placebo effect", but it undoubtedly remains a very good way to ensure the "across the board" consistency we definitely need when simultaneously using a whole bunch of spoken word mics.

 

However, I should probably also add that my biggest "lifesaver" (by far) for this application is the "Talk" setting of the "Dynamic Filter" in my Line 6 XD-V wireless mic systems. With the masking provided by overlapping "chat", it pretty much gives me an "auto mix" that's unparalleled for this type of conference work.


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#7 dboomer

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Posted 22 October 2013 - 06:39 PM

Hi Nick

 

Correct.  Engaging the FBS before a show is likely counter-productive.  The FBS will only add filters if they are needed.  If you set them before hand you will likely be cutting material that may never cause any problem.  But if it ever does the FBS will jump in in about half a second.  It is completely automatic and runs completely unattended (once you turn it on ... which happen automatically in most presets).

 

This however is a different process than using the parametric or graphic EQs to generally set the tone of the speakers to the room. 



#8 radamo

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Posted 24 October 2013 - 02:42 AM

Whaou !!

 

This kind of threads always remember me how important these communitys are and how much i have to learn...

 

@ Ron : I read your post several time (english is quite hard for me) and print it for practice. Thank you a lot for sharing your tips !

 

@ Dboomer : in my last experience, i could ear larsen being ready to jump and I controled them by reducing the sound level on the m20d. I wanted to increase the sound and make it disappear once for all but the audience would not have been happy, even for half a second. Is it possible to make the control more sensible than with "vocal" mode ? (even if the sound is a little bit damage)

 

 

Olivier


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#9 radamo

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Posted 24 October 2013 - 04:16 AM

Another thing :

 

What about the feedback eliminator's tool on the Line 6 L3 ? is it a good complement ? Is it useless with the M20d ?


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#10 RonMarton

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Posted 24 October 2013 - 12:09 PM

...What about the feedback eliminator's tool on the Line 6 L3 ? is it a good complement ? Is it useless with the M20d ?

 

It's never "useless", Olivier,  :lol:  ...but it does work the same way as the ones in the M20d !

 

You will also understand that it is more likely to be noticeable when the StageSource speaker is carrying any mixed output, as the speaker's FBS cannot control single inputs to that mix in the way that the M20d does.

 

I can, however, see no harm in having it after an M20d as a "final guardian" ...and it could indeed prove to be an essential "lifesaver" if a StageSource "t" version's impressive array of inputs were also being used to "expand" the number of inputs available.    


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#11 gordwait

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Posted 24 October 2013 - 01:21 PM

One possible problem if both the m20d input FBS and the speaker FBS are enabled at the same time, is the possibility that they both "go off" at the same time for a given feedback event.

Both would then notch out the offending frequency, so you would end up with a doubly deep notch. Probably not a good thing..

I'm guessing one would turn off the speaker FBS and let the m20d do the work..


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#12 RonMarton

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Posted 24 October 2013 - 01:41 PM

One possible problem if both the m20d input FBS and the speaker FBS are enabled at the same time, is the possibility that they both "go off" at the same time for a given feedback event...

 

It's been my experience, gordwait...

 

That the sequence you describe seems confined to the "theoretical world" !  :)

 

In our "real world", the huge difference in overall spectral content between an individual input and the full mix that's "downstream" of it makes such identical "double notching" very, very unlikely.

 

It might also be worth bearing in mind that

  1. The possible operation of identical filters in series would effectively multiply their slope, whereby that slope variation would act to reduce the overall bandwidth that such a combination might affect ...and...
  2. The digital bit-mapping techniques Line 6 use to achieve such "notching" are mercifully free of the artefacts ("phasing", "frequency creep" etc.) that plague more conventional (analog) filtering techniques. 

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#13 dboomer

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Posted 24 October 2013 - 03:10 PM

Whaou !!
 Is it possible to make the control more sensible than with "vocal" mode ? (even if the sound is a little bit damage)
 
 
Olivier


Vocal mode is the most sensitive on the mixer and "red" is the most sensitive on the speakers. I would generally avoid using the FBS on the speakers at the same time as the mixer.

#14 RonMarton

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Posted 24 October 2013 - 03:52 PM

...I would generally avoid using the FBS on the speakers at the same time as the mixer.

 

...And I heartily agree that most of us feel that way, Don, ...but IMHO the speakers' FBS is always a handy option to have, particularly when those extra inputs are needed.

 

My main point was (and is) that spectral or other "damage" is very unlikely to be audible on the rare occasions that require both speaker and mixer to have their FBS enabled.


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#15 Jess24

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Posted 10 February 2014 - 12:56 PM

Please enable some way to save FBS frequencies!

 

In my experience, when we are doing a conference, the Stagescape is not fast enough, or sensitive enough, to filter out that very low level ringing that occurs before full blown feedback, especially in a noisy environment. I have much better luck notching those frequencies before the room is full, because, at least in my experience, that slight ring has to mature into a louder feedback before the Stagescape will catch it. To that end, it was very frustrating when we lost power right before an event started, and 10,000 people were already seated. Too late to find those frequencies again, and we subsequently had less available gain for the entire event. I wasn't willing to let a little ring develop into full blown feedback in order to let the Stagescape notch those frequencies again. I imagine for a live band performance, when you get a sudden, sharp feedback, it is ideal. But for my purposes, I would like the ability to retain the FBS points even under a power outage. As it stands now, I have to keep using a couple of Sabines, even though I hoped to eliminate them. 

 

Thanks for your consideration!

 

Jess


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#16 litesnsirens

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Posted 11 February 2014 - 07:02 AM

I'm in the camp that theory only loosely translates to real live use. I do rely on my understand of feedback, frequencies, room dynamics etc. as limited as it may be compared to some of the experts.  But I also rely on things that I have tried that worked and continued to work from venue to venue.  Even if I couldn't figure out a theoretical reason why it worked.  

 

I don't want this post to appear to be in rebuttal to anything that's been posted, I will always defer to the experts.  In my case, my mics stay on stands and don't move once we set them up.  Half a second of feedback is half a second more than I would like in a show if I can help it.  And I can help it by ringing out the room prior to a show so I do. As of now if that means I have to leave the M20d on until show time, that's what I do.   I've never done lab tests with feedback but in all my years of playing I've never experienced new feedback creeping in after moving a mic a few inches although I 100% believe that under the right conditions this would occur.  Perhaps the system would have to be crazy loud for this to occur?  With a lead singer that is moving all over the stage definitely there can be positions that are more problematic than others but I've also been able to tame this prior to a show and have it hold when doing sound for other bands. 

 

The beauty of the FBS on the dreamstage products is that the notches are so thin that it barely affects the sound.  In reality the overall sound of the mix is not compromised at all. 

 

Bottom line, I've asked for FBS settings to be stored or maybe even create 2 modes, static and dynamic


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#17 tochiro

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Posted 14 February 2014 - 12:47 AM

IMO FBS should be available with all input models and not only microphones.  For instance I had problems with low freq feedback with my acoustic guitar and I realized much later that the FBS could not be enabled with the input model I had chosen.  Also, I noticed the FBS takes too long to catch problematic frequencies.


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#18 dboomer

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Posted 14 February 2014 - 11:14 AM

 I've asked for FBS settings to be stored or maybe even create 2 modes, static and dynamic

 

Fixing and holding notch filters for feedback while possible is usually problematic.  The frequencies that would be triggering feedback change if you move the mic even inches.  Also if you move your head back and forth or the room heats up or many other things.  That's why we have a completely dynamic system.  As soon as you start fixing filters then they would become unusable if the system called on them to move to new better spots.

 

What you might do during setup would be to set the system up normally as you intend to use it and then during soundcheck (and with the singers in place behind the mics and their fingers in their ears) bring the system up until it begins to set filters.  Then on the Channel input page in deep tweak mode you should be able where the filters are set  Note those frequencies and then in Monitor deep tweak mode set the PEQ accordingly and LIGHTLY notch some of your main problem frequencies as the feedback is usually from the monitors and not the mains speakers.  YMMV :)



#19 Mavril

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Posted 07 March 2014 - 07:55 AM

Man I love this forum, great discussions and points of view. The main reason I love this M20d is that it allows trained/ experienced live sound engineers and budding amatuers and sound creators/ musicians to discuss the art or sound.

 

I believe the main thing to remember about automatic feedback suppression is that it is performed by a computer (revelation, I know!). This means that there are a set of rules/ conditions to be followed to achieve a consistent result. Simply, this means that you will generally hear feedback before the mixer or speaker removes it from the output because your ear/brain is a finely tuned and trained device that technology is still unable to replace.

 

If you want to be able to walk away from the desk to go to the loo or the bar without fear of the whole system taking off when the singer points the mic at the horn of the monitor, the FBS system is an excellent safety net. One that I am grateful for!

 

If you are concerned that the punters are going to revolt if they hear another 0.5sec of 2.5kHz or that low ring, exercise your ears and learn how to use the wonderful parametric EQs (as dboomer posted). Train your ears to know certain frequencies so you can snap within a stones throw of a troublesome resonant frequency, sweep to the Hz quickly and notch it out. Being able to do this manually is a beautiful artform, fatiguing but beatiful (hence my apprecition for a mixer that high fives me when I want a beer!)

 

As for positioning making a difference, of course it does! Resonant frequencies have nodes and mics are directional (generally) which means they will affect a certain location (and mic pattern rejection) dependant on frequency. Lower frequency feedback is more persistent across a stage than high frequencies for a number of reasons.

 

Reading these discussions reinforces how much I enjoy sound and the tools I use to make it. Without channeling my dad, the box in front of you has the power of a room full of equipment from the 1980s, still it is not as good a boat anchor if you are going to fight it.

 

Oh and I agree that FBS would be good across all input types but lets remember that Line 6 has strong roots in guitarland and these are the only people on earth who enjoy feedback :)


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