Defeating audio AGC?

[George]

This isn't really a design question as such, but I hope someone will
have an idea about it.

I have an Canon Elura 100 miniDV camcorder, which has automatic gain
control on audio. I'm sure Canon thought they were doing a good
thing with this, but it causes motor noise to be picked up quite
noticeably during times when the audio level is low and the gain is
ramped up to the maximum.

This model also has an external mic input, but the mic needs to be a
"self-powered" electret. So, I built a little amp for that, powered
by a 9V battery. Unfortunately, it appears the AGC is still there.
And while I don't get motor noise, it appears the amplifiers in the
camera aren't exactly low-noise, so when they crank up looking for
something to hear, Or it may be noise from my amp that gets
amplified, but I don't hear that when I use this setup on the
computer mic input.

Anyway, since I've already built this amplifier, I just wondered if
there's anything I could add to its output that wouldn't be audible
to humans, but would fake out the camera AGC. Or maybe center the
output level a bit above ground - well, ok, that's not likely to
work assuming the camera inputs go through capacitors.

Well, I don't have my hopes up on this, but just thought I would ask
in case there was a solution.

Of course I could record the audio separately on another device, but
that's a lot of trouble since the streams would have to be synced at
some point.

Thanks for any ideas.

 

[Jan Panteltje]

This isn't really a design question as such, but I hope someone will
have an idea about it.

I have an Canon Elura 100 miniDV camcorder, which has automatic gain
control on audio. I'm sure Canon thought they were doing a good
thing with this, but it causes motor noise to be picked up quite
noticeably during times when the audio level is low and the gain is
ramped up to the maximum.

This model also has an external mic input, but the mic needs to be a
"self-powered" electret. So, I built a little amp for that, powered
by a 9V battery. Unfortunately, it appears the AGC is still there.
And while I don't get motor noise, it appears the amplifiers in the
camera aren't exactly low-noise, so when they crank up looking for
something to hear, Or it may be noise from my amp that gets
amplified, but I don't hear that when I use this setup on the
computer mic input.

Anyway, since I've already built this amplifier, I just wondered if
there's anything I could add to its output that wouldn't be audible
to humans, but would fake out the camera AGC. Or maybe center the
output level a bit above ground - well, ok, that's not likely to
work assuming the camera inputs go through capacitors.

Well, I don't have my hopes up on this, but just thought I would ask
in case there was a solution.

Of course I could record the audio separately on another device, but
that's a lot of trouble since the streams would have to be synced at
some point.

Thanks for any ideas.

Sure, couple of periods of 30kHz once a second MAY perhaps help...
It all depends on the circuit.

 

[Jon Slaughter]

Of course I could record the audio separately on another device, but
that's a lot of trouble since the streams would have to be synced at
some point.

If you can easily record the audio on another device then you could record
them on both and then just replace the audio on the video. It should be very
easy to sink up because its just a matter of matching up which would be
extremely easy to do with the right software.

 

[Martin Griffith]

This isn't really a design question as such, but I hope someone will
have an idea about it.

I have an Canon Elura 100 miniDV camcorder, which has automatic gain
control on audio. I'm sure Canon thought they were doing a good
thing with this, but it causes motor noise to be picked up quite
noticeably during times when the audio level is low and the gain is
ramped up to the maximum.

This model also has an external mic input, but the mic needs to be a
"self-powered" electret. So, I built a little amp for that, powered
by a 9V battery. Unfortunately, it appears the AGC is still there.
And while I don't get motor noise, it appears the amplifiers in the
camera aren't exactly low-noise, so when they crank up looking for
something to hear, Or it may be noise from my amp that gets
amplified, but I don't hear that when I use this setup on the
computer mic input.

Anyway, since I've already built this amplifier, I just wondered if
there's anything I could add to its output that wouldn't be audible
to humans, but would fake out the camera AGC. Or maybe center the
output level a bit above ground - well, ok, that's not likely to
work assuming the camera inputs go through capacitors.

Well, I don't have my hopes up on this, but just thought I would ask
in case there was a solution.

Of course I could record the audio separately on another device, but
that's a lot of trouble since the streams would have to be synced at
some point.

Thanks for any ideas.

Generally, you will be out of luck, the camera audio was probably
designed by the RIAA marketing team, even the professional sound guys
complain about the really crap audio from cameras in the 20k$ ++
market, and the headphone returns can be somewhat mediocre as well.

Maybe carry something like a H4 (samson?)in your pocket, just for when
you need a some better sound, maybe with a cheap lav.

A handclap is a good sub for a clapper board ( in vision of course)
The advantage of a person recorder, is that you can do longshots....
and still hear a person talking


Martin

 

[Jim Thompson]

Generally, you will be out of luck, the camera audio was probably
designed by the RIAA marketing team, even the professional sound guys
complain about the really crap audio from cameras in the 20k$ ++
market, and the headphone returns can be somewhat mediocre as well.

Maybe carry something like a H4 (samson?)in your pocket, just for when
you need a some better sound, maybe with a cheap lav.

A handclap is a good sub for a clapper board ( in vision of course)
The advantage of a person recorder, is that you can do longshots....
and still hear a person talking


Martin

At least 20 years ago an employee of mine at OmniComp/GenRad had a
friend who was a sound man for "60 Minutes"

We made a wireless synchronizer to put a sync tone on the audio when
the video started.

I'll see if I still have a copy of the schematic.

IIRC we called it "bleep-bloop" ;-)

...Jim Thompson

 

[Martin Griffith]

At least 20 years ago an employee of mine at OmniComp/GenRad had a
friend who was a sound man for "60 Minutes"

We made a wireless synchronizer to put a sync tone on the audio when
the video started.

I'll see if I still have a copy of the schematic.

IIRC we called it "bleep-bloop" ;-)

...Jim Thompson

There used to be a scheme with a tungsten bulb in the gate of a 16mm
chemical cam that did the same, fortunately I have forgotten all
about it, and don't want to remember.

Loads of people use 433MHz and ancient smpte linear timecode these
days


Martin

 

[George]

Jon Slaughter says...

If you can easily record the audio on another device
then you could record them on both and then just replace
the audio on the video. It should be very easy to sink
up because its just a matter of matching up which would
be extremely easy to do with the right software.

My expeience trying to do that in the past was not good.
Two devices, even if both are operating perfectly, will be
operating off time bases which are inevitably just a bit
different - because crystals aren't perfect.

So what I found was that, for a long clip, if the A/V was
in perfect sync at the beginning, it no longer was at the
end.

So then you are looking at, say, stretching the entire audio
stream so that it's in sync the whole time. I know there is
software to do that, but I think that whole process would
get old pretty fast if I had to do it on everything.

 

[George]

Martin Griffith says...

Maybe carry something like a H4 (samson?)in your pocket,
just for when you need a some better sound, maybe with a
cheap lav.

Thanks, Martin, but an H4 is like $500, which is about $480
over my budget for this project.

It would have to be something like using my laptop to record
the audio, but that wouldn't be very convenient. Plus, my
laptop has lousy audio to begin with.

 

[Martin Griffith]

This isn't really a design question as such, but I hope someone will
have an idea about it.

I have an Canon Elura 100 miniDV camcorder, which has automatic gain
control on audio. I'm sure Canon thought they were doing a good
thing with this, but it causes motor noise to be picked up quite
noticeably during times when the audio level is low and the gain is
ramped up to the maximum.

This model also has an external mic input, but the mic needs to be a
"self-powered" electret. So, I built a little amp for that, powered
by a 9V battery. Unfortunately, it appears the AGC is still there.
And while I don't get motor noise, it appears the amplifiers in the
camera aren't exactly low-noise, so when they crank up looking for
something to hear, Or it may be noise from my amp that gets
amplified, but I don't hear that when I use this setup on the
computer mic input.

Anyway, since I've already built this amplifier, I just wondered if
there's anything I could add to its output that wouldn't be audible
to humans, but would fake out the camera AGC. Or maybe center the
output level a bit above ground - well, ok, that's not likely to
work assuming the camera inputs go through capacitors.

Well, I don't have my hopes up on this, but just thought I would ask
in case there was a solution.

Of course I could record the audio separately on another device, but
that's a lot of trouble since the streams would have to be synced at
some point.

Thanks for any ideas.

Another option, for motor noise, is to export the audio to an audio
editor, say cooledit, do an FFT analysis, and notch out the offending
frequencies.
My Canon XL1 has a very nasty, but sharp 150Hz buzz when used with the
internal mic, and a 30dB notch improves it no end. Tedious but it
works


Martin