AC90: External Audio Input via XLR- AC90

J

ja_swaer

Member
This past weekend I filmed a track at a local concert. I asked the sound engineer for a feed from the mixing board. We were under the gun to get it going and I realize I should have used a headset but hindsight is 20/20 but this was 30 seconds before I needed to start shooting.

He handed me an XLR cable and I plugged it into the Input 1 XLR port, set the input to line with phantom power off. The sound meters on the LCD screen immediately jumped to maximum and stayed there. I turned down the gain on the camera but was not able to get a lower setting.

I then flipped the switch to Mic Level (as opposed to Line Level knowing that mic was hotter than line) and the signal came down a little. With the on camera gain set at around 25% the meter appeared to be fine (while the artist was talking) so I started recording and filmed the song watching the audio meter and while it stayed up near the max it didn't seem to go over.

Once back in my office I copied the track to my workstation and played it back. Video feed was great but the audio was terrible, clipping and clicking during any loud vocal.

Pulled it into audition but to no avail, the audio is unusable.

What I can't figure out is why the audio feed was so hot. Are there different kinds of feeds that come out of the boards ? I just assumed it would be a line feed but I was wrong.

Any thoughts will be appreciated.
 
God knows what sort of feed the guy gave you and how bad he cranked it. if you do this stuff often, buy an in-line pad - it's a male/female XLR barrel with circuits inside; you can buy them for several levels of gain reduction.

Also, plug into whatever camera input will feed both channels, and play with levels and mic/line. Or, worst case, use one channel for your feed and one set to the internal mic. At least you'll get something useable. And at small venues, the board mix is usually very "off" sounding - dry, not as much drums, no crowd, etc - having a room mic (on camera if need be) and a feed lets you mix the two.

Best way to do it? Someone dedicated to audio with a laptop, 4-track min. input, stereo board feed and two stationary matched room mics. There's probably dozens of kids in every town with such a setup in their bedroom. Use on-cam mic for reference and synch their tracks in post.
 
I never trust any mixing board!
What i usually do is: get the line from the mixing board, it may be usable. But also: get a directional mic on top of the cam as a backup, on the other channel (this will sound hollow and too roomy though). But mainly: put a little Zoom recorder ($100 ish) on a stand about one metre from one of the room/venue speakers and record everything. Make sure you record on the soft side (never automatic level), crank up in post is easy, repair distorted signal is virtually impossible. Usually I end up using this signal, as this is the sound (room sound) the mixing guy cares about! As a bonus you also get the right amount of room noise (applause, laughter).

D
 
Thanks for the advice, I guess a dear and expensive lesson learned.

I have a zoom recorder, under the pressure I never considered recording with it.
 
It's probably due to an impedance mismatch.

Line level outputs expect to see a fairly high impedance. Mic level inputs tend to be much lower. If you feed a line level output into a mic level input, there is a risk that the output impedance will be lower than the input impedance. No matter what your meters show, the output will be saturating and clipping when it hits a certain level, much like what you describe.

There is a device called an inline pad. These are typically XLR barrels that come in different amounts of attenuation. Get yourself a pair of -20 dB ones and keep them in your kit (Whirlwind makes them fairly cheap). Having them on hand would have solved your original problem whilst staying on line level.

Of course, there is the possibility that the sound guy was actually overdriving his output, but it's impossible to know until you knock the level down and hear what you are recording (shooting music without headphones? make sure you add a decent pair of closed circumaural cans to your kit too.)
 
I think there's more to it than just the feed you got from the sound guy. We just livestreamed a wedding with our new ac90 and had a terrible time getting good audio. In consulting the manual and doing some testing in the studio the next day we discovered that the ALC (auto limiter) on the camera can cause major problems with clipping using external audio sources. We were using an Audio-Technica at875r shotgun mic, the level according to the meters on the camera would jump from red-line to nothing with the slightest little turn of the audio dials. there was no in-between.

According to the manual, the red dot at the top of the meter on the LCD is 0db. Usually the little white line 80% of the way up the meter would be 0db. In our tests we determined that the audio actually sounded just fine when the level of loud audio (people cheering and singing, the band playing through a PA) was hitting the red, so long as it was just at the edge of the red, as soon as it was turned up a bit more, then the audio became distorted. We still need to do more testing to figure out how to prevent audio issues with the camera in the future but for now one thing is for sure, WE WILL NOT BE USING THE CAMERA WITHOUT HEADPHONES, which is annoying since our trusty hmc40 can be used just with the meters and always sounds great. Has anyone else had audio issues with the ac90. We are not a bit iffy as to weather or not to keep the camera or to return it and get a different one.
 
I have done 30 events over the past 12 months, with varying problems with sound board feeds (usually weak signals). When the signal is too strong, if I apply clipping on my AG-AC160A, it just lobs off the top so this gives totally unacceptable results rendering the feature useless, in my opinion. I have three settings on the camera -60, -50, -40db, with the 1st of these giving the strongest signal. I too find that moving the volume dials just a touch takes the meter reading from very low to red. Recommendations: Always use a headset, always have a shotgun mic on one channel as a backup, always set one channel lower that the other, always have a field audio recorder as your 2nd backup (I like the Tascam DR100 MK11). And, bring along in-line pads. I also carry around an ART dual passive direct box but have never had the occasion to use it.
 
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We rent AC160As all the time (we've had 20 events in the past 2 months, two of them with 7 cameras) and have never had the issue we're having with the ac90. Seems to me that the whole camera is very much "fly-by-wire." On the ac160a the mic level knobs are actually real potentiometers, you're really adjusting the level of the audio with an electrical device. On the ac90 all the controls are just sending instructions to the camera's computer. The problems that we had were with an on-camera shotgun, not with a feed from a sound board.
 
ShugPro Digital Studio said:
I think there's more to it than just the feed you got from the sound guy. We just livestreamed a wedding with our new ac90 and had a terrible time getting good audio. In consulting the manual and doing some testing in the studio the next day we discovered that the ALC (auto limiter) on the camera can cause major problems with clipping using external audio sources. We were using an Audio-Technica at875r shotgun mic, the level according to the meters on the camera would jump from red-line to nothing with the slightest little turn of the audio dials. there was no in-between.

According to the manual, the red dot at the top of the meter on the LCD is 0db. Usually the little white line 80% of the way up the meter would be 0db. In our tests we determined that the audio actually sounded just fine when the level of loud audio (people cheering and singing, the band playing through a PA) was hitting the red, so long as it was just at the edge of the red, as soon as it was turned up a bit more, then the audio became distorted. We still need to do more testing to figure out how to prevent audio issues with the camera in the future but for now one thing is for sure, WE WILL NOT BE USING THE CAMERA WITHOUT HEADPHONES, which is annoying since our trusty hmc40 can be used just with the meters and always sounds great. Has anyone else had audio issues with the ac90. We are not a bit iffy as to weather or not to keep the camera or to return it and get a different one.
Click to expand...

0dB can mean a lot of things (or nothing depending on how you look at it). If the top of the meter is 0 dB, it is probably telling you dBFS. At that point, you have "used up all the bits" and are causing digital clipping.

This is different from 0 dBv or 0 dBVU, which are analog scales. A true line level test tone should read 0 dBVU if all levels are set correctly.

Here's where it gets funky: 0dBVU should generally be calibrated somewhere between -18 and -12 dBFS (so about 60 - 80% up the meter). dBFS is a scale that is intended to show how much headroom you have before you reach digital clipping (which is 0 dBFS).
 
So AT 875r a good choice for recording events like concerts or performances like dances, debates and stage dramas ?
 
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