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Old April 20th, 2010, 04:46 PM   #1
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'AGC Defeat' doesn't work with T2i? Tested...

I tested the AGC Defeat hack of sending a tone to one channel and a mic to the other with a T2i. I have an MP3 player playing to one channel and a shotgun to the other via a dual mono to stereo 1/8" cable. I confirmed that the tone is being recorded only to one side. I tried as many different scenarios as I could come up with - cranked the tone, different pitches, recording the shotgun and tone to different channels etc. The noise floor with the shotgun plugged directly into the camera is -30db. When the tone is blaring the noise floor goes down to -42db, but as expected bringing the levels up in post brings the noise floor right back up to -30db. The next step is to boost the shotgun's signal so no signal boosting is necessary in post and I still have the lower amount of noise. I have an H4n, but I was really hoping to keep the post-production workflow manageable by keeping the audio in sync from the start. A few other observations:

I noticed that there isn't the 'biting' down at the beginning of loud audio when the mp3 player is going. With just the shotgun there is a noticeable clipping of the signal.

There is a very slight bleed-through from one channel to the other when the tone is played at full volume.

~ T2i Audio Test ~
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Last edited by Joel Peregrine; April 21st, 2010 at 01:41 AM.
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Old April 21st, 2010, 09:31 AM   #2
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go old school

hey joel. my T2i is arriving later today. i plan to go old school and record the audio to my H4. "speed... action..."

the audio the cam records will act as sync track. we used to feel comfortable with double-system but we got spoiled with all the bells & whistles on the new video cams, and the ability to plug mics or feeds from mixers into the cameras.

i view T2i filmmaking as old bolex/nagra, and the old rules of engagement apply (better have a damn good reason to pan or zoom within a shot; establishing shot, medium shot, medium close-up, close-up...). for audio, always record the best audio you can. sure, it's more difficult, but the results will be worth the effort
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Old April 21st, 2010, 12:45 PM   #3
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There are two methods of AGC defeat: tone and noise. With a noise signal, you use it on one channel and record into the other. This works great on the 5D2 in auto mode, both the old and new firmware. With the tone method, you can mix the tone with the signal and record into both channels. That's guaranteed to work, even if the AGC is independent on the two channels.

One problem with an MP3 player and the tone method is that MP3 compression will kill everything over 16kHz or so and the tone gets distorted.

FWIW, with the juicedLink DT454, the signal to noise is slightly better with the noise method than the tone method, but it's very subtle.
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Old April 21st, 2010, 03:17 PM   #4
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Hi Jon,

Quote:
Originally Posted by Jon Fairhurst View Post
There are two methods of AGC defeat: tone and noise. With a noise signal, you use it on one channel and record into the other. This works great on the 5D2 in auto mode, both the old and new firmware. With the tone method, you can mix the tone with the signal and record into both channels. That's guaranteed to work, even if the AGC is independent on the two channels.
What frequency is best? I'm assuming either very high or very low?


Quote:
Originally Posted by Jon Fairhurst View Post
One problem with an MP3 player and the tone method is that MP3 compression will kill everything over 16kHz or so and the tone gets distorted.
I chose the Sansa Clip mp3 player because it has one of the best frequency response specs of any player: This review is from cnet and has response curve graphs for a bunch of different players:

SanDisk Sansa Clip - Audiophile MP3 players, by the numbers | MP3 Insider - CNET Reviews
...SanDisk MP3 players are not known for their audio quality, but the superaffordable Sansa Clip is a notable exception. With a frequency response deviation average of -1.36dB that plots steady from 20Hz-22kHz and an admirable signal to noise ratio, the Clip can hold its own.

The L.A.M.E. mp3 encoder I'm using now defaults at up to 20 kHz and actually has a low pass filter that you can disable to allow encoding above 20 kHz.

And these are the specs for the T2i:
Linear PCM
(1) Internal monaural microphone
(2) External stereo microphone jack
* Recording level automatically adjusted.
* Sampling frequency: 48 KHz, Bits: 16 bit x 2 ch

So should I change the signal from the mp3 player to a noise that covers all frequencies rather than the single-frequncy tone?
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Old April 21st, 2010, 06:57 PM   #5
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A high frequency is best for two reasons: 1) you can record two channels of signal and remove the tone without too much damage to the signal. With a low tone, you'd be prone to clipping, and with a mid tone, you'd step on the content, and 2) even in a dual mono setup, the crosstalk would be near 20kHz and wouldn't get in the way of your signal.

The problem with a HF tone is that a bad MP3 encoder, decoder, D/A, anti-aliasing filter, or output amp can get in the way of delivering a pure, loud tone to the camera. From that standpoint, having a pink noise signal in one input and signal in the other is a better way to go - assuming that the two channels' AGCs are linked.
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Old May 4th, 2010, 03:23 PM   #6
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Hey guys, I also tried numerous configurations this weekend with a Rode NTG-2, an ipod with tones/ noise, and a Zoom H4 feeding into the T2i input.

The problem I kept having was crosstalk in the camera, the H4 experienced no crosstalk with any of the tests, but the camera did every time.

I used Pink noise, added a lot of static to the voice channel, I tried all sorts of tones, 50 Hz added strange noise, so did the super high tones. I used a wav file on the ipod to see if was aliasing with the mp3, but that made no difference.

Does anyone know of the right configuration with this? I saw a link a long time ago, but can't find it anymore!

Thanks for any help, for now I'm using the H4, but to not have to sync it all up would be amazing, and they don't make Pluraleyes for Premiere Pro...
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Old May 4th, 2010, 03:25 PM   #7
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I should also add that the AGC WAS being defeated beautifully, it was just the noise that was the problem! I'm sure the line out of the H4 is definitely part of the problem, but using the headphone out and lowering the volume didn't do much to help....
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Old May 4th, 2010, 08:02 PM   #8
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The T2i is set for mic level. I've put a 50db pad between the Tascam DR-100 and the T2i. I feed pink noise into one of the channels on the Tascam.
Gold Line Goldline GLPN Compact Pink Noise Generator with Phantom Power Indicator Phantom Power Supplies at Markertek.com

Everything works fine.
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Old May 5th, 2010, 05:35 PM   #9
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tell me more

I don't quite understand how it is you'd be using your dr 100 with t2i. If you're using it as a separate recorder you wouldn't need to have a pink noise channel into the t2i right? ????
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Old May 5th, 2010, 06:15 PM   #10
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Use a Zoom or buy a JuiceLink.

Last night we shot our Iron Man 2 show after a screening. I have had some decent success plugging my Sennheiser G2's into the T2i and taking the mic down to -40. Well, I did a commercial shoot using my lav (the TV show uses the handheld butt plug with a vocal mic) and forgot that I took it down -30 and the output on the receiver -12. This caused the floor to be too low and the hiss was really audible.

So, today I did some new tests and discovered that no matter what you do with your mics, the ACG will produce some hiss.

So, I sure wish I could afford a JuiceLink, but right now my solution is to use my Zoom and Plural Eyes (which is a $150 bargain if you ask me, especially considering the multicamera uses).

I'm concerned about these DIY options. The Zoom has been in my bag for three years and is a go to. I have the old H4. Today's tests convinced me that my option is sync sound until I can afford the JL.

And Iron Man 2, it is pretty darned good guys! Gotta love Scarlet in that leather catsuit!
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Old May 6th, 2010, 03:47 AM   #11
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Quote:
Originally Posted by Steve Lustgarten View Post
I don't quite understand how it is you'd be using your dr 100 with t2i. If you're using it as a separate recorder you wouldn't need to have a pink noise channel into the t2i right? ????
I wouldn't if I was just going to use the Tascam DR 100 but it doesn't hurt to have the audio going into the t2i.
If the t2i audio is OK then I don't have to do any syncing.
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