29th July 2012
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#1 | | Lives for gear
Joined: Dec 2005 Location: San Francisco area
Posts: 2,895
Thread Starter  | DSLR+Beachtek audio issues in post
A friend of mine who is an excellent filmmaker, who has produced a large number of high-quality productions (incl features), has a side business as a one-man-band producer of small videos for non-profits (...in these times...). He recently asked me to clean up and mix the audio for such a film he had done with his Canon 7D. I was expecting AGC artifacts, which can be dealt with in audio post somewhat, but instead got interviews that no matter how much I reduced the level of the file still seemed to be pinning all the meters. They also included huge numbers of tiny clicks. To see what was going on I exported a single clip to Renovator, and saw what you see below (this image is of about 4 or 5 seconds of audio) . In this image of the file, time is L>R, freq is up/down and volume level is color, with white being the loudest and black being silence. As you can see there is a tremendous very high volume level pitch running through the whole clip, at about 18.9kHz (and you can see the parade of clicks running through the audio as well--narrow vertical lines). The volume of that 18.9k pitch was so much louder than the voices that reducing the level of the file down to unacceptable levels for the voices still had the meters FAR too hot to pass tech eval. I realized then that without telling me, the filmmaker had used one of those DSLR audio adaptors that uses a mega-loud high-freq tone to paralyze the camera's AGC. But a track edited and mixed with this audio untreated (as done by most picture editors) would cause significant problems down the line for broadcast, streaming, etc. Even with the offending high freq pitch removed the audio still has a very odd quality to it--hard to describe, but definitely distorted and lowfi. The blizzard of click events had to be removed one at a time by hand, to avoid causing further damage to the voices by using an auto declicker plug in. My point in this is that I think this is a very bad idea for how to get sound into a DSLR, as prevalent as it may be. On this forum I know that sentiment very much runs towards using double system audio for sound with these cameras (with good reason), but people will nonetheless still try to cut corners and use the camera audio anyhow. Does anyone know if ALL the adapters of this ilk do this goofy 18.9k thing, or just the BeachTek?
Also, as you can see by looking at the image closely, the system isn't actually completely neutralizing the AGC--you can see the level of the 18.9k pitch "grow" in level with each pause in the dialog.
phil p
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29th July 2012
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#2 | | Gear maniac
Joined: Feb 2008 Location: NYC |
Oh man, that's horrible. I thought you had the option to disable AGC on the 7D.
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29th July 2012
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#3 | | Gear addict
Joined: Sep 2007 Location: Orlando, FL
Posts: 322
| Quote:
Originally Posted by RecRoom  Oh man, that's horrible. I thought you had the option to disable AGC on the 7D. | Only on 5d's. They also need to have specific firmware revision or later.
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29th July 2012
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#4 | | Gear addict
Joined: Aug 2007 | Quote:
Originally Posted by philper Does anyone know if ALL the adapters of this ilk do this goofy 18.9k thing, or just the BeachTek? | AFAIK the JuicedLink also has an AGC disable feature.
The 18.9kHz tone should only be sent down one channel as well - if you received stereo audio, try throwing away the right channel and working with the left channel as mono audio only. AFAIK the 7D records stereo sound.
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29th July 2012
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#5 | | Gear nut
Joined: Oct 2003 Location: Chatsworth, CA
Posts: 85
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To me, the soundtrack on DSLR's should be a scratch track only. The real sound should only be recorded double-system... period. | |
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29th July 2012
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#6 | | Lives for gear
Joined: Jan 2005 Location: Oberlin, Ohio
Posts: 4,084
| Quote:
Originally Posted by MusicTrax To me, the soundtrack on DSLR's should be a scratch track only. The real sound should only be recorded double-system... period. | 
I know a lot of people use them but the Beachtek units are not quite what they would like you to believe. We purchased one for our Sony FX-1 and after using it for a couple of days I sent it back. It was not really a good device for us and the sound we were getting was noisy and frequency restricted. I went back to doing sync'd sound with our Fostex FR-2 LE chip recorder and got much better results. I am sure they are good units for certain situations but not for what we were using them for which was recording music. FWIW
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Thomas W. Bethel
Managing Director
Acoustik Musik, Ltd.
Room with a View Productions
Oberlin, OH 44074 www.acoustikmusik.com
Doing what you love is freedom.
Loving what you do is happiness.
Celebrating 18 years in the mastering business in 2013
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29th July 2012
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#7 | | Moderator
Joined: Dec 2006 Location: NY NY |
You can disable AGC on a 7D. You have to do a little wiring trick externally. We shoot on Multiple 7Ds with AGC off.. Just to be clear that High Frequency tone is AGC. You can find pre made cables or make them yourself as we did.
...also what Tom said... Use a dual system when shooting DSLR.
We use a 8 track digital recorder for our main sound recording, and send a scratch audio feed to all the cameras for media management and editorial support ( sync, editorial, shot reviews, etc) But we use the 8 track digital recorder audio for final audio delivery. Just finished a national infomercial for a client using this method and it worked fine. (Three 7Ds being fed audio mix using cables we created to bypass AGC, and recorded with Lav's and stick to 8 track digital recorder.
cheers
geo
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29th July 2012
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#8 | | Lives for gear
Joined: Dec 2005 Location: San Francisco area
Posts: 2,895
Thread Starter | Quote:
Originally Posted by ThisIsSka AFAIK the JuicedLink also has an AGC disable feature.
The 18.9kHz tone should only be sent down one channel as well - if you received stereo audio, try throwing away the right channel and working with the left channel as mono audio only. AFAIK the 7D records stereo sound. |
All audio channels had this issue.
phil p
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29th July 2012
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#9 | | Lives for gear
Joined: Dec 2005 Location: San Francisco area
Posts: 2,895
Thread Starter | Quote:
Originally Posted by georgia You can disable AGC on a 7D. You have to do a little wiring trick externally. We shoot on Multiple 7Ds with AGC off.. Just to be clear that High Frequency tone is AGC. You can find pre made cables or make them yourself as we did.
...also what Tom said... Use a dual system when shooting DSLR.
We use a 8 track digital recorder for our main sound recording, and send a scratch audio feed to all the cameras for media management and editorial support ( sync, editorial, shot reviews, etc) But we use the 8 track digital recorder audio for final audio delivery. Just finished a national infomercial for a client using this method and it worked fine. (Three 7Ds being fed audio mix using cables we created to bypass AGC, and recorded with Lav's and stick to 8 track digital recorder.
cheers
geo | Yer preachin to the choir. I do DSLR shoots all the time (50% of my location work), have a very good method for sending sync ref audio to them on wireless and generally will turn down production sound jobs where the producers are insisting on single system sound recording with a DSLR. But as a post biz you are going to get audio that someone has shot w/o consulting you in this manner sooner or later, and there are a lot of people out there who will not consider doing double system sound for their DSLR shoots no matter how much I rant at them (they are usually one-man-band types). I've been asked to find my friend a better solution since he is not going to start hiring more help on location and is definitely going to keep using his camera. Can you turn off that stupid high freq tone in any of those interfaces? There's a Fostex box that bolts onto the 7D like the Beachtek and feeds the camera audio, but also records to its own card (and autostarts/stops with the cam), maybe I'll suggest that. (DCR302)
phil p
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29th July 2012
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#11 | | Lives for gear
Joined: Dec 2005 Location: San Francisco area
Posts: 2,895
Thread Starter | Quote:
Originally Posted by georgia | Thanks for the help. I don't want to do any of those high Hz tricks--I think they are a terrible idea. I think the presence of that freq in the audio at that volume makes the rest of the audio sound odd/bad even when it is taken out, and a lot of what people using that technique will record will go through a simple Final Cut edit+mix and that's it. They won't know that high freq is there, will pass the file along and it will make all sorts of trouble for broadcast or whatever down the line, and those people won't understand why they are having a problem. That high pitch also seemed to be causing the only partially suppressed AGC to add a lot of clicking etc to the audio. The best way is the Magic Lantern hack (but only on 5D, not 7D) or that Fostex thing.
phil p
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