Auto Gain Nightmare at DVinfo.net

Peter Manojlovic · April 19th, 2010, 09:53 PM

I don't know if this is a Rode NTG2 issue, a Canon XHA1 issue, or the evils of using auto gain?
But i could use a tip or two from the Audio gurus here.

Recording a live event, i had a Rode NTG2 mic set to Ch1, and a Sennheiser G2 wireless set to Ch2.
Ch1 was used for general direction and backup (instruments, background vocals and stage mics), and Ch2 directed the main singers' mics to me via a mixer...
Whenever the singing from the mics came to a let off, the Ch1 audio would pump up, and then release again once Ch2 came back. Background audio exhibits a strange climb and drop in volume...I suspect this is because audio gain is doing a terrible job.
Likewise with Ch2..Although it was better controlled, it seemed that drop off of volume on Ch2, correlated with rise in volume on Ch1...
I monitored the audio with my headphones, but the music must have been loud enough, that i couldn't here the subtlties required to do a better job..I was in the middle of the floor, and it was pretty loud..

My on camera mic NEVER exhibited such fluctuations in volume controls. In fact, it's very reliable in that regard.
Is this expected for XLR mics??

Thanx...

Richard Crowley · April 19th, 2010, 10:32 PM

Now you know why the rule is to NEVER use auto-levels when recording MUSIC! Many purists extend the prohibition to EVERYTHING.

Your situation was even worse because you were stuck with two very different signals on the two channels, but the auto-level mechanism is linked together left-right so that the loudest signal controls everything. I can't think of a worse scenario. You are lucky you got anything recognizable. Did I mention that the rule is to NEVER use auto-level for music recording? EVER.

No, it is not the fault of the microphone or the camcorder (except the auto-level function of the camcorder). And there is nothing wrong with the auto-level, either (as strange as that may sound). The auto-level is simply doing it's job. But it was NEVER intended to be used for music and you have discovered why with a real-world experiment.

I suspect that you have never heard such poor performance from auto-level with the on-camera mics because: (1) The mics are generally too far away to get decent pickup of most anything; and (2) the two mics are so close together (especially relative to the distance to the sound source) that they are picking up virtually the same signal as each other. So you won't hear that kind of left-right fighting that you are complaining about. NEVER use auto-level for recording music! The auto-level built into camcorders is NOT suitable for recording music. Period. Full stop. End of story.

Jim Andrada · April 20th, 2010, 01:04 AM

Amen!!!!!!

Peter Manojlovic · April 20th, 2010, 02:05 PM

Quote:

Originally Posted by Richard Crowley

......, but the auto-level mechanism is linked together left-right so that the loudest signal controls everything.

I thought the two channels were independent... The two channels are actually working together in auto gain mode..
Lesson learned. I'm having to break out to mono in post, and salvage whatever is possible.

So it was written, so it shall be done..
Thanks....

Warren Kawamoto · April 20th, 2010, 04:05 PM

Peter,
I don't know if my problem is related to yours, I have a Sony EX1 and Rode NTG3. I shot a percussion performance, had my audio on MANUAL. My meters were not clipping during recording, but I couldn't monitor with headphones properly because the performance was so loud. When I got back home and played back my footage, I noticed that whenever a loud bass drum was hit, my audio would go soft as if a limiter kicked in, then go back up to normal. This "breathing" was apparent throughout the performance, and never happened before I used the NTG3. This same breathing was also apparent when I recorded dancing at a wedding reception. Every loud bass note automatically turned the input down, but I always shoot in manual mode. With my old cheapie Sony mic, everything was fine. I'm beginning to suspect that maybe a Rode microphone picks up the fast transients so well that it engages the limiter that overcompensates?

Peter Manojlovic · April 20th, 2010, 06:35 PM

Interesting Warren..
Perhaps i can start hunting down Rode users to see if this is common...
Again, my posting was open to camera, microphone and auto gain issues..So i'm at the mercy of other similair users.

Although my camera bars were fine, i found that the talent near the microphone was singing so hard, that i was getting distortion, regardless of staying well below the overs...Did you get the same symptoms?

Warren Kawamoto · April 21st, 2010, 04:04 AM

Nope, no distortion here. Audio was clean, but level was pumping up and down. The only way I could fix it in post was to engage a compression filter which I cranked up so that all levels were even.

Jay Massengill · April 21st, 2010, 06:45 AM

Peter, remember that the recording level indicators only show the level of the signal being recorded. There are other parts of the audio chain before that point where distortion can happen. The usual suspect is overloading the mic input with too strong a signal. If that happens, then adjusting the recording levels down just records a lower level of already-distorted audio.

Battle Vaughan · April 21st, 2010, 02:42 PM

What Jay said. Also, I have encountered seveal cases that lead me to believe the NGT2 is prone to overload at a somehwhat low level, giving the distortion that Jay mentions. If the mike is overloaded, it doesn't matter what you do with the level on the camera, the signal is already distorted. It is possible that the signal level from a hot mike is too much for the preamps and invoking a level pad (like in the Canon XHa1) or an in-line attenuator could help, but only if the mike is not distorting itself...

April 19th, 2010, 09:53 PM	#1
Peter Manojlovic Major Player Join Date: Jun 2007 Location: Hamilton Ontario Posts: 769	Auto Gain Nightmare I don't know if this is a Rode NTG2 issue, a Canon XHA1 issue, or the evils of using auto gain? But i could use a tip or two from the Audio gurus here. Recording a live event, i had a Rode NTG2 mic set to Ch1, and a Sennheiser G2 wireless set to Ch2. Ch1 was used for general direction and backup (instruments, background vocals and stage mics), and Ch2 directed the main singers' mics to me via a mixer... Whenever the singing from the mics came to a let off, the Ch1 audio would pump up, and then release again once Ch2 came back. Background audio exhibits a strange climb and drop in volume...I suspect this is because audio gain is doing a terrible job. Likewise with Ch2..Although it was better controlled, it seemed that drop off of volume on Ch2, correlated with rise in volume on Ch1... I monitored the audio with my headphones, but the music must have been loud enough, that i couldn't here the subtlties required to do a better job..I was in the middle of the floor, and it was pretty loud.. My on camera mic NEVER exhibited such fluctuations in volume controls. In fact, it's very reliable in that regard. Is this expected for XLR mics?? Thanx...

April 19th, 2010, 10:32 PM	#2
Richard Crowley Trustee Join Date: Aug 2007 Location: Portland, Oregon Posts: 1,238	Now you know why the rule is to NEVER use auto-levels when recording MUSIC! Many purists extend the prohibition to EVERYTHING. Your situation was even worse because you were stuck with two very different signals on the two channels, but the auto-level mechanism is linked together left-right so that the loudest signal controls everything. I can't think of a worse scenario. You are lucky you got anything recognizable. Did I mention that the rule is to NEVER use auto-level for music recording? EVER. No, it is not the fault of the microphone or the camcorder (except the auto-level function of the camcorder). And there is nothing wrong with the auto-level, either (as strange as that may sound). The auto-level is simply doing it's job. But it was NEVER intended to be used for music and you have discovered why with a real-world experiment. I suspect that you have never heard such poor performance from auto-level with the on-camera mics because: (1) The mics are generally too far away to get decent pickup of most anything; and (2) the two mics are so close together (especially relative to the distance to the sound source) that they are picking up virtually the same signal as each other. So you won't hear that kind of left-right fighting that you are complaining about. NEVER use auto-level for recording music! The auto-level built into camcorders is NOT suitable for recording music. Period. Full stop. End of story.

April 20th, 2010, 01:04 AM	#3
Jim Andrada Inner Circle Join Date: Feb 2007 Location: Tucson AZ Posts: 2,211	Amen!!!!!!

April 20th, 2010, 04:05 PM	#5
Warren Kawamoto Trustee Join Date: Nov 2006 Location: Honolulu, HI Posts: 1,435	Peter, I don't know if my problem is related to yours, I have a Sony EX1 and Rode NTG3. I shot a percussion performance, had my audio on MANUAL. My meters were not clipping during recording, but I couldn't monitor with headphones properly because the performance was so loud. When I got back home and played back my footage, I noticed that whenever a loud bass drum was hit, my audio would go soft as if a limiter kicked in, then go back up to normal. This "breathing" was apparent throughout the performance, and never happened before I used the NTG3. This same breathing was also apparent when I recorded dancing at a wedding reception. Every loud bass note automatically turned the input down, but I always shoot in manual mode. With my old cheapie Sony mic, everything was fine. I'm beginning to suspect that maybe a Rode microphone picks up the fast transients so well that it engages the limiter that overcompensates?

April 20th, 2010, 06:35 PM	#6
Peter Manojlovic Major Player Join Date: Jun 2007 Location: Hamilton Ontario Posts: 769	Interesting Warren.. Perhaps i can start hunting down Rode users to see if this is common... Again, my posting was open to camera, microphone and auto gain issues..So i'm at the mercy of other similair users. Although my camera bars were fine, i found that the talent near the microphone was singing so hard, that i was getting distortion, regardless of staying well below the overs...Did you get the same symptoms?

April 21st, 2010, 04:04 AM	#7
Warren Kawamoto Trustee Join Date: Nov 2006 Location: Honolulu, HI Posts: 1,435	Nope, no distortion here. Audio was clean, but level was pumping up and down. The only way I could fix it in post was to engage a compression filter which I cranked up so that all levels were even.

April 21st, 2010, 06:45 AM	#8
Jay Massengill Trustee Join Date: Jul 2003 Location: Burlington Posts: 1,976	Peter, remember that the recording level indicators only show the level of the signal being recorded. There are other parts of the audio chain before that point where distortion can happen. The usual suspect is overloading the mic input with too strong a signal. If that happens, then adjusting the recording levels down just records a lower level of already-distorted audio.

April 21st, 2010, 02:42 PM	#9
Battle Vaughan Trustee Join Date: Nov 2008 Location: Miami, FL USA Posts: 1,505	What Jay said. Also, I have encountered seveal cases that lead me to believe the NGT2 is prone to overload at a somehwhat low level, giving the distortion that Jay mentions. If the mike is overloaded, it doesn't matter what you do with the level on the camera, the signal is already distorted. It is possible that the signal level from a hot mike is too much for the preamps and invoking a level pad (like in the Canon XHa1) or an in-line attenuator could help, but only if the mike is not distorting itself...