How Does Camera Know to Bypass its Pre-amp? at DVinfo.net

Peter Moretti · July 21st, 2007, 03:19 AM

One of the many good arguements for using a mixer is that mixers have better pre-amps than cameras do. Make sense.

But if you are recording to your camera (not to a separate recorder) your mixer's output is pugged into the camera's mic input. Doesn't this run the signal through the camera's inferior pre-amp circutry? Or does the camera detect that the signal from the mixer is amplified and bypass its own pre-amp stage?

Thanks as always!

Bill Davis · July 21st, 2007, 01:45 PM

Peter,

On most consumer level camcorders - you're right - when recording via a mic input you can't typically bypass the camera's pre-amps and/or AGC (auto gain control) which is a hassle.

As you move up to "prosumer" and beyond, expect to find the ability to "go direct" with audio and bypass these consumer style circuits.

At the pro level - any camcorder that accepts audio via balanced XLR connections won't bother with auto-gain (tho MUCH more useful peak limiting often becomes an option!) and their pre-amps should be up to the task of generating very clean audio.

Giroud Francois · July 21st, 2007, 03:08 PM

most of the camera accepting mic/line level on their minijack, have a switch (hard or soft) to do so.
the autogain feature is another part of the story and is set after the input, so it should be still active on either LINE or MIC input.
the big problem is the autogain usually try to maximize the input level and set the preamp to high level of gain . This induces a lot of background noise. You can easily get rid of it , simply switching to manual level.
feeding a weak line level with autogain ON could lead to the same problem (while you usually expect line level to have a strong signal).
So , the way to get good audio is either to choose a mic that give strong signal (some RODE video mic do this) or try to get a strong signal with an external preamp of good quality.
Most of the cheap audio mixers (like behringer) have the same problem. Their mic input is ok as long as you keep the gain button turned half the way or less. If you use the line input you usually do not need to turn this button very far , but if you do, the noise will be back again.
So whatever you use (line/mic level , internal/external preamp) make possible to match equipement in a way that preamp button do not need to be turned on at maximum.
Another trick is that most of time, you are turning input gain higher at recording because it sounds weak. Better to turn up the volume of your headset so you get a better feel on the sound without adding noise.

Daniel Epstein · July 22nd, 2007, 06:54 PM

You really have to look at the signal path in the camercorder to know whether you are bypassing the pre amp in the camcorder or not. Years ago it turned out that Sony was using mic level signals directly in the Betacam BVV series while changing Line level signals down to mic with IC's. Of course you had to look at the schematics to figure this out and a lot of sound people still insisted Line was a better way to get into the machines. I tend to use Line level for the robustness of the signal to avoid hum on the way to the recorder and not worry so much about which electronics I am going through inside the recorder unless I can hear a difference.

Marco Leavitt · July 23rd, 2007, 07:17 AM

You may still benefit from a mixer in that it allows you to boost the gain so that you can turn down the gain on the camera. You can't turn the gain off completely, but you should be able to keep it under 1/3 of the way up (on the camera). It improves the s/n.

July 21st, 2007, 03:19 AM	#1
Peter Moretti Trustee Join Date: Mar 2007 Location: Sherman Oaks, CA Posts: 1,259	How Does Camera Know to Bypass its Pre-amp? One of the many good arguements for using a mixer is that mixers have better pre-amps than cameras do. Make sense. But if you are recording to your camera (not to a separate recorder) your mixer's output is pugged into the camera's mic input. Doesn't this run the signal through the camera's inferior pre-amp circutry? Or does the camera detect that the signal from the mixer is amplified and bypass its own pre-amp stage? Thanks as always! Last edited by Peter Moretti; July 21st, 2007 at 03:55 AM.

July 21st, 2007, 03:08 PM	#3
Giroud Francois Inner Circle Join Date: Feb 2004 Location: switzerland Posts: 2,133	most of the camera accepting mic/line level on their minijack, have a switch (hard or soft) to do so. the autogain feature is another part of the story and is set after the input, so it should be still active on either LINE or MIC input. the big problem is the autogain usually try to maximize the input level and set the preamp to high level of gain . This induces a lot of background noise. You can easily get rid of it , simply switching to manual level. feeding a weak line level with autogain ON could lead to the same problem (while you usually expect line level to have a strong signal). So , the way to get good audio is either to choose a mic that give strong signal (some RODE video mic do this) or try to get a strong signal with an external preamp of good quality. Most of the cheap audio mixers (like behringer) have the same problem. Their mic input is ok as long as you keep the gain button turned half the way or less. If you use the line input you usually do not need to turn this button very far , but if you do, the noise will be back again. So whatever you use (line/mic level , internal/external preamp) make possible to match equipement in a way that preamp button do not need to be turned on at maximum. Another trick is that most of time, you are turning input gain higher at recording because it sounds weak. Better to turn up the volume of your headset so you get a better feel on the sound without adding noise. Last edited by Giroud Francois; July 21st, 2007 at 03:47 PM.

July 21st, 2007, 01:45 PM	#2
Bill Davis Trustee Join Date: Jan 2004 Location: Scottsdale, AZ 85260 Posts: 1,538	Peter, On most consumer level camcorders - you're right - when recording via a mic input you can't typically bypass the camera's pre-amps and/or AGC (auto gain control) which is a hassle. As you move up to "prosumer" and beyond, expect to find the ability to "go direct" with audio and bypass these consumer style circuits. At the pro level - any camcorder that accepts audio via balanced XLR connections won't bother with auto-gain (tho MUCH more useful peak limiting often becomes an option!) and their pre-amps should be up to the task of generating very clean audio.

July 22nd, 2007, 06:54 PM	#4
Daniel Epstein Trustee Join Date: Jan 2006 Location: New York, NY Posts: 1,267	You really have to look at the signal path in the camercorder to know whether you are bypassing the pre amp in the camcorder or not. Years ago it turned out that Sony was using mic level signals directly in the Betacam BVV series while changing Line level signals down to mic with IC's. Of course you had to look at the schematics to figure this out and a lot of sound people still insisted Line was a better way to get into the machines. I tend to use Line level for the robustness of the signal to avoid hum on the way to the recorder and not worry so much about which electronics I am going through inside the recorder unless I can hear a difference.

July 23rd, 2007, 07:17 AM	#5
Marco Leavitt Inner Circle Join Date: Jul 2002 Location: Albany, NY 12210 Posts: 2,652	You may still benefit from a mixer in that it allows you to boost the gain so that you can turn down the gain on the camera. You can't turn the gain off completely, but you should be able to keep it under 1/3 of the way up (on the camera). It improves the s/n.