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Old January 4th, 2006, 06:13 PM   #1
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automatic gain control and line level input?

Hey, I've got an NTG2, waiting for a boom pole, shockmount etc. I plan on attaching it to my mini dv cam with a 1/8 to xlr. My question is related to automatic gain control. After reading this article I'm looking for the best way to fix the shwooshing sound(so it says), or wondering if it's really that bad. I'm not doing a pro shoot yet, but understand the importance of quality audio. Also I'm not sure what it says by a line level input. Also should i purchase a small mixer, or use the mixer i plan on using for my recording studio Behringer UB1204? Or is there a good beachtek type adapter that i can use, but also use for the future because i plan on having something along the lines of a canon xl2 in the future.


Dealing with Automatic Gain Control
Most consumer-grade camcorders and some industrial models have an
Automatic Gain Control (AGC), which adjusts the audio level up or down as
necessary. The AGC circuit is designed to compensate for the fact that the sound
source is often far from the microphone on the camera. An external microphone is
usually placed much closer to the source, however, and therefore a much higher
signal level is fed into the camera’s input. The Automatic Gain Control responds by
rapidly reducing the audio level during the peaks of speech, and boosting the level
up very high during pauses. The sound of the AGC’s action is often described as
“pumping”, “breathing”, or “whooshing”, and is usually undesirable.
It is impossible to defeat or bypass the AGC on most camcorders.
The only other options are:
• Use the camcorder’s line-level input, if it has one. On most camcorders, the line
input is not affected by the AGC. But since a microphone signal is not strong
enough to drive the line input directly, the mic must be connected to a mixer or
microphone preamplifier that has a line level output. Using a mixer also allows
for more precise adjustment and metering of signal levels, in addition to its basic
function of combining the signals from multiple microphones into one feed.
• Keep the external mic signal level very low. This causes the AGC to relax
and let the audio level rise to maximum. This tends to amplify the hiss of the
camera’s audio circuitry, but reduces the pumping action. The
microphone’s output signal can be reduced by moving it farther away from
the source, or by using a device called an attenuator or pad, which
decreases the signal level by a fixed amount. Some attenuators offer a
choice of settings, such as -15 dB, -20 dB, and -25 dB.


I need some help figuring out this crazy audio stuff. How do I know i have a line level input? What should I do. I need some kind of set up for this mini dv cam now, but definitely going to be using a higher quality cam in the future.Thanks

Luke
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Old January 5th, 2006, 07:50 AM   #2
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First of all, what camera do you have now? Are you sure the AGC can't be defeated on it? Check the owner's manual, also check regarding a line level input - some cameras have audio inputs that are line level but are intended for dubbing and can't record while shoooting video.

Line level output is high level, the output from a preamplifier, a mixer, or similar devices. Mic level is low level, produced by, well, microphones.

The NTG2 is a balanced output microphone. A simple 1/8 to XLR adapter is not a good idea as most of them are not wired correctly. Either a Beachtek adapter of at least a transformer adapter like Shure's X96F (I think that's the number) XLR to camcorder adapter is going to give you better results.

The Canon XL2 has XLR mic inputs so no adapter would be needed to use the NTG2 mic with it.
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Old January 5th, 2006, 09:22 AM   #3
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And some automatic controls work differently than others. For example the DVX-100's auto control is a limiter only, it doesn't vary the recording level during the normal variance in you're subject's volume.
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Old January 5th, 2006, 02:13 PM   #4
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Hey,

i have a 1/8 mic jack(plug in power) it says this jack also accepts a "plug in power" microphone. Don't know what means exactly. intelligent acessory shoe, and audio/video jack, ( 1 VP--,75 ohms, unbalanced,sync negative 327 mV, (at output impedance more than 47 kilohms) Output impedance lwith less than 2.2 kilohms, input impedance more than 47 kilohms) it says you can dub audio with the audio/video jack. As regards to the simple 1/8 to xlr i thought I could get away with it after reading this

Is a Transformer Necessary?
If the length of cable between the microphone and the camcorder will be 20
feet or less, all that is necessary is to use a cable with the appropriate connectors
and wiring. If the cable will be longer than 20 feet, however, a transformer will be
required at the end of the cable nearest the camcorder. The camcorder-side of
the transformer is typically equipped with a very short cable and a 3.5mm mono
or stereo connector. The transformer provides two benefits:
• The transformer maintains a balanced connection between itself and
the microphone, minimizing pickup of hum and electromagnetic
interference and allowing for cable runs of up to 1,000 feet. (Longer
cable runs require a mixer or preamplifier to boost the microphone
signal up to Line level, about 1 volt.)
• The transformer can boost the signal level slightly, which may be useful
when connecting a low-output microphone (or one used at a
considerable distance from the sound source) to a camcorder whose
microphone input is not very sensitive. The amount of boost (called
“gain”) that the transformer provides depends on the ratio between its
input and output impedances, as well as the ratio between the
transformer’s output impedance and the camcorder’s input impedance.
Typical transformers provide between 6 dB and 12 dB of gain.

i will be using a cable that's 20 feet, so I thought I shouldn't need to worry about it. i know for sure that the AGC can't be disabled, it says nowhere in my instruction manual about disabling it. I just dpn't want to spend too much on a beachtek if i know i can't use it in the future on a nicer cam with xlr inputs, what you recomend I do, or invest in that i can use in the future, is there any adapters that can be used in the future, or should i buy some kind of mixer? If so what kind and what else will i need? Thanks for your replies so far.

Luke
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Old January 5th, 2006, 03:28 PM   #5
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You can use an XLR to 1/8 TRS adapter to connect the mic but there are several wrinkles as there are a variety of ways the adapter can be wired. The XLR connector has 3 pins - Hot(2), Cold(3), and Shield(1). The stereo mic input takes a TRS plug, left(tip), right(ring), and shield(sleeve). Then to top it off, some cameras put a 5 volt DC bias on the mic jack that can cause interference and they may be wired left and right(tip), 5vbias(ring), shield(sleeve). Unfortunately the typical XLR to TRS adapter is not intended to connect a balanced mono signal to an unbalanced stereo input. Instead it's designed to adapt the different connector styles so a balanced mono XLR can connect to a balanced mono TRS - for that connection it goes XLR2->TRS Tip, XLR3->TRS Ring, and XLR1->TRS Sleeve but that won't work for a stereo mic input as the left and right channels will be out of phase or will put 5 volt bias on XLR3 with a jack setup for a mono mic. Instead you have to connect XLR2 to both TRS tip AND Ring while XLR3 and XLR1 both connect to the TRS Sleeve. The Shure transformer I mentioned and the Beachtec adapters make all the correct connections and blocks the bias voltage if present but a simple plug adapter usually doesn't do that unless you wire it yourself.

20 feet of unbalanced extension cable could be subject to noise pickup - heck, in some environments even a foot or two is enough. The transformer is cheap enough while a Beach could always be resold on eBay later if you don't need it with a new camera.
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Old January 6th, 2006, 06:23 AM   #6
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Steve,

Doesn't that shure transformer also raise the impedance?

I know Glen Trew Audio (Nashville/Toronto) makes a properly wired cable. I'm not certain that it has blocking capacitors for the camera voltage. Contact them and fnd out.

http://www.trewaudio.com/

Regards,

Ty Ford
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Old January 6th, 2006, 07:55 AM   #7
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Quote:
Originally Posted by Ty Ford
Steve,

Doesn't that shure transformer also raise the impedance?

I know Glen Trew Audio (Nashville/Toronto) makes a properly wired cable. I'm not certain that it has blocking capacitors for the camera voltage. Contact them and fnd out.

http://www.trewaudio.com/

Regards,

Ty Ford
I may be wrong but I think this is relatively recent in the Shure line, #A96F, purpose-built for matching balanced mics to camcorder 3.5mm inputs. At least I don't recall seeing it until a couple of months ago. They call it a "Low- to medium-impedance microphone transformer with min-plug output connector" and it also blocks the DC bias. The specs rate it for 600 ohm (user modifiable for 150 ohm) input to 2500 ohm output. The label their other transformers as low-to-high matching, like the a95uf rated 75-300 ohm imput to 41 kilohm output.

http://www.shure.com/pdf/userguides/...ories/a96f.pdf
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Old January 8th, 2006, 09:12 PM   #8
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Ok, Well I think I've decided I will buy the shure cable 3.5 mm to XLR. The #A96F if you guys say that is right one to get. But I still need to worry about the Automatic gain control kicking in. there is no option of cancelling it in my camcorder. Is says the options are

The only other options are:
• Use the camcorder’s line-level input, if it has one. On most camcorders, the line
input is not affected by the AGC. But since a microphone signal is not strong
enough to drive the line input directly, the mic must be connected to a mixer or
microphone preamplifier that has a line level output. Using a mixer also allows
for more precise adjustment and metering of signal levels, in addition to its basic
function of combining the signals from multiple microphones into one feed.
• Keep the external mic signal level very low. This causes the AGC to relax
and let the audio level rise to maximum. This tends to amplify the hiss of the
camera’s audio circuitry, but reduces the pumping action. The
microphone’s output signal can be reduced by moving it farther away from
the source, or by using a device called an attenuator or pad, which
decreases the signal level by a fixed amount. Some attenuators offer a
choice of settings, such as -15 dB, -20 dB, and -25 dB.

So first, I don't even know if I have a line level input, is that the same as the audio/video jack? and Second what would be a good small mixer to get, something for my purposes now and possibly for something i can use in the future when i use the Canon XL2? I'm just really worried about the AGC, I'd hate to invest in lots of audio equipment to get a really bad swooshing sound, how they describe it, is it really that bad or noticeable? what should I do?

Thanks very much for the replies thus far.

Luke
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Old January 9th, 2006, 06:21 AM   #9
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Quote:
Originally Posted by Luke McMillian
...
So first, I don't even know if I have a line level input, is that the same as the audio/video jack? and Second what would be a good small mixer to get, ...
Luke
You A/V jack is line level but you need to check the manual to see if A) it is an input/output or just an output, and B) if it will record audio as you shoot. Some A/V jacks are for dubbing to add new audio to prexisting audio or to replace it and only work when the camera is in VCR mode but not in camera mode. If you would tell us what make and model your camera is someone around here is bound to know for sure.

Mackie makes some good small mixers if you'll have AC mains available and Sound Devices MixPre, 302, or 442 are highly regarded battery powered models.
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Old January 9th, 2006, 11:46 AM   #10
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Hey thanks,

My camcorder model is Sony DCR-TRV27, and it says my a/v jack is an input and output, but i don't know if it can record audio as i shoot. IS there a lot of complication to mixers as well? I'll look into those mackie models, which one do you think would work best for using this cam now, and an xl2 type in the future? any other cables or anything I'll need?
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