May 28 2007
The other half of the picture
Any camera you buy today will produce images that rival those from 10 year old cameras costing 10 times the price.
Unfortunately the same is not true of audio. A $50 five year old minidisc audio recorder will produce better audio than any video camera under $10K.
The problem is cheap pre-amps, and it can only be properly solved by using a mixer or external pre-amp. Neither of these solutions is really practicable in the run and gun world of videojournalism.
Fortunately there are a couple of settings you can configure in camera to boost the signal to noise ratio and mitigate the problem.
First switch off the AGC (automatic gain control). THE AGC is the most common cause of noisy audio. Sure it will prevent most clipping but it will also significantly boost noise levels, particularly in quietersegments.
Set the levels manually so that the signal is peaking around -12db. In the digital world 0db is the ceiling - go over that and and the audio is unusable. The -12 db setting should give you enough headroom to cope with variations as the subject moves closer to the mic etc.
Check the volume control. The sweet spot for the pre-amp on most cams is in the 30-50% range.
If the volume control is pushing towards the half-way mark or is over it then switch on the attenuation, “MIC ATT”. This will cut the strength of the signal but providing you can keep the gain pot (volume control) below 75% it will improve the signal to noise ratio, giving you noticeably cleaner audio.
Using a hot mic like the sennheiser me66 I just about always have attenuation switched on.
An extra level of protection can be added by sending two different levels to the 2 audio channels. So channel 1 gets a feed with peaks set at -12db and channel 2 at -18db. That way if channel 1 does clip you just switch over to the audio from channel 2.
So the recommended pratice.
- switch off AGC
- use mic attenuation if possible
- send a slightly quieter audio feed to channel 2 as a safety net
YMMV - test thoroughly before going into the field - and ALWAYS use headphones.
Note for canon users
Using a Canon xlH1 I can control the audio levels of the stereo channels from the stock mic separately. I can also send the signal from a mono mic to both audio channels - but I lose discrete control of the channels.
The solution is to use an XLR Y adapater. A Y adapter splits the signal from a mono mic to feed two XLR inputs. This lowers the signal voltage slightly, is not a problem with a hot mic like the me66, but check first.
Do not use a Y adapter to feed unmatched inputs - that can create all sorts of problems.
Thanks for this tidbit of info - I was struggling with the quality of my audio due to leaving it on auto.
I shoot with a couple of SONY TRV950’s and a Rode VideoMic. Am I ok with this setup or should I look at something like a BeachTek Adapter to improve my audio signal???
Thanks again
short answer - no big advantage using a beachtek with your set-up, unless you want to add a second mic - lav or whatever. Longer answer - see today’s post.