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  #1  
Old 03-23-2010, 12:04 PM
K.B. K.B. is offline
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Default Fixing AGC...?

This must seem like the dumbest of dumb questions, but right now it would be a godsend to Canon 5D Mk II users:
Quote:
One shortcoming of these HDSLR cameras is their lack of high quality audio recording. Ironically, the audio recording hardware isn't terribly bad, and the final audio recorded is actually uncompressed.

The monkey wrench is that these HDSLR cameras record the audio with automatic gain control (AGC) always on. AGC is a method of compressing and/or limiting audio input levels so they do not overload the camera's audio preamps. Think of it as an automatic volume knob for incoming sound.

This is normally a good thing as it safeguards the camera's internal audio hardware from inexperienced users that may override the preamps with signal that is too hot. However, an AGC system that cannot be turned off is a curse in any professional environment. It creates a noticeably higher noise floor that can be very annoying.
Surely, someone somewhere must have dreamed up a way of anticipating and fixing this in post - some clever way of reverse-attenuating the sound...
(And yes, I know, "Don't call me Shirley!" )

Some pics of a full HDSLR rig: http://hddslr-hire.com/


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Old 03-23-2010, 12:07 PM
FajitaTone FajitaTone is offline
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Default Re: Fixing AGC...?

looks like they solved it by using that Zoom recorder and a RODE mic.
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  #3  
Old 03-23-2010, 12:12 PM
K.B. K.B. is offline
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Default Re: Fixing AGC...?

Would be a lot better to record the sound in the camera in sync... (though I bet it has RCA inputs!) Saves marrying them together later - the camera has no timecode facilty (yet!) so you'd have to use a clapper or something.


EDIT: I forgot there's software that can sync it for you these days, but that's another stage that costs time and money, and users of this camera have low budgets.

Seems to me what I'm suggesting is a bit like the old Dolby B I had on my cassette deck once upon a time, so could even be seen as a good thing
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Old 03-23-2010, 12:30 PM
FajitaTone FajitaTone is offline
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Default Re: Fixing AGC...?

Strip Silence is your friend... cut out the garbage, keep only the Speech, and add your own nats... it's called Audio Post Production you know
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Old 03-23-2010, 02:04 PM
tom_lowe tom_lowe is offline
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Default Re: Fixing AGC...?

With the new Canon software, 2.0.4 AGC can now be turned off. Using this with one of the many XLR adapters would be suitable.

I recently did a shoot on a 5DmkII and recorded sound separately with a slate.

No issues.
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Old 03-23-2010, 03:48 PM
mikevarela mikevarela is offline
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Default Re: Fixing AGC...?

On a professional shoot, they'd handle it just like a conventional camera. record the sound separate using a slew of mics and HDD recorders.

I'd still record the audio to the camera though, as the editor can use this as temp material for the edit and as a sync guide.

The only way to fix issues with this type of audio would be to use a mix of expansion, de-essing and noise reduction/automation. It's almost not worth it though because it's already being processed and probably over-modulated by the onborad compressor. Also, by sheer convention, the distance from sound source will most often prevent a clean recording. Byt he time the editor levels the audio, room and ambiance will follow, to a degree that will void a professional sound.

If you're shy on a recorder, use a laptop and mbox. I've done this and it works fine. just use the camera audio as guide for sync.
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Old 03-23-2010, 04:12 PM
K.B. K.B. is offline
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Default Re: Fixing AGC...?

The audio on this camera is uncompressed 16-bit stereo PCM at 44KHz, and I wasn't thinking of using the on-board mic!

I was wondering if maybe you recorded a continuous tone at a low-ish level on one channel, and your live sound on another, then the AGC would react to the live sound and affect both equally. Later you could use the tone level variations as a guide in automating a fix to the level in the live sound. It really would work like Dolby B!

But if you can now switch AGC off with software 2.0.4 my query is academic...

Cheers for answering!
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Old 03-23-2010, 04:53 PM
philper philper is offline
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Default Re: Fixing AGC...?

Quote:
Originally Posted by K.B. View Post
The audio on this camera is uncompressed 16-bit stereo PCM at 44KHz, and I wasn't thinking of using the on-board mic!

I was wondering if maybe you recorded a continuous tone at a low-ish level on one channel, and your live sound on another, then the AGC would react to the live sound and affect both equally. Later you could use the tone level variations as a guide in automating a fix to the level in the live sound. It really would work like Dolby B!

But if you can now switch AGC off with software 2.0.4 my query is academic...

Cheers for answering!
Your idea is essentially how the Juiced Link adapter dealt with the Canon AGC. It worked, sort of.

Now with manual levels you CAN set levels, but with the following caveats (we did a lot of testing):
--realize that you can ONLY look at the meters and set levels when you are NOT rolling
--realize that you still cannot monitor what the camera hears except in playback
--the input is still mic level only and is UNBALANCED.
--the sound of the camera audio compared to an outboard audio recorder was thin, brittle, noisier and had a lot of peak distortion.
--in setting levels, as to an external mixer, we had better luck re peak distortion by setting the mixer zero level at half-scale on the camera meters, rather than the minus 12 mark Canon provides.
--if you are going to do some verite audio with the camera, with unpredictable levels, or are recording what will just be a ref track, I would recommend leaving the AGC ON--to avoid peak distortion.

Philip Perkins
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Old 03-23-2010, 07:06 PM
Carl Kolchak Carl Kolchak is offline
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Default Re: Fixing AGC...?

I actually had a nightmare of a shoot last year (helping out on a low budget short - a vastly different experience to how these things used to be a decade ago. Cheap digital workflow does not = due diligence involved in expensive film / sound stock) using one of these.

I really don't want to go off on a massive rant about how much easier the whole thing would have been if I'd been consulted up front, but in a nutshell a dual system is the way to go.

Just think of it as recording on a Nagra & a BL4 using crystal sync.

The other thing is, the Canon only shoots 30fps.
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  #10  
Old 03-24-2010, 01:34 AM
K.B. K.B. is offline
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Default Re: Fixing AGC...?

I believe the latest upgrade does 25p and 24p.

And I would agree that quality standards on low budget shoots have taken a nosedive over the past decade. I don't do micro budget stuff any more. It's just not worth it.

Shame about the Canon sound. But that'll be fixed in a couple of years. We live in interesting times...
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