Defeating audio AGC?

[George]

[This may be a duplicate post. After posting it the first time on
Cox, I saw that Cox may have been UDPed because of spam, so I'm
posting it again on Octanews to make sure it gets out.]

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.

 

[Jamie]

[This may be a duplicate post. After posting it the first time on
Cox, I saw that Cox may have been UDPed because of spam, so I'm
posting it again on Octanews to make sure it gets out.]

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.

try a lower impedance to the input of the mic of the camera.

 

[George]

Jamie says...

try a lower impedance to the input of the mic of the
camera.

Not sure I uderstand what difference it would make. Anyway,
the amp I built has a 100-ohm output impedance as it is. I
thought that was already pretty low.

 

[mpm]

[This may be a duplicate post. After posting it the first time on
Cox, I saw that Cox may have been UDPed because of spam, so I'm
posting it again on Octanews to make sure it gets out.]

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.

Can you roll-off some of the low end? (I assume this is where the
motor noise resides)
Maybe by using a couple series .022uF capacitors?

-mpm

 

[Jamie]

Jamie says...


Not sure I uderstand what difference it would make. Anyway,
the amp I built has a 100-ohm output impedance as it is. I
thought that was already pretty low.

The idea was to lower in the input to the camera in attempt
to not allow the input to be so sensitive to white noise when
gain was maxed out.
or.
Experiment with injecting high frequency audio to see
if the gain loop will drop back.
It's very possible the high freq will get removed in the
cam further down the line.

 

[JosephKK]

mpm [email protected] posted to sci.electronics.design:

[This may be a duplicate post. After posting it the first time on
Cox, I saw that Cox may have been UDPed because of spam, so I'm
posting it again on Octanews to make sure it gets out.]

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.

Can you roll-off some of the low end? (I assume this is where the
motor noise resides)
Maybe by using a couple series .022uF capacitors?

-mpm

I suspect that the issue is mechanical noise rather than electrical
noise.