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Saturday Audio Class - Unbalanced, balanced, mic, -10, line level??

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Ty Ford
Saturday Audio Class - Unbalanced, balanced, mic, -10, line level??
on Sep 15, 2007 at 12:31:26 pm

Hi,

I just answered a friend's audio question and thought others might benefit from the info. Most of us on this board use pro gear, but sometimes we may run into semi pro or consumer gear with unbalanced audio inputs. If you don't know how to deal with it, problems can occur. I'm not trying to talk down to anyone. Please don't take it that way. Anyway........

A balanced circuit provides a slightly hotter signal than an unbalanced connection. A balanced connection offers better rejection to outside noise by a phenomenon known as CMR, common mode rejection. For example, audio is AC at any moment at any single place on a twisted pair balanced cable, the audio is both +5 and - 5 V. If external noise is inducted on to the cable at, say +1 V, that +1 V is added to the + 5 and the - 5. As an arithmetic trick of elementary electronics, the noise is cancelled out inside the circuitry of the next stage. Pretty weird, but it works.

Shielding is another issue. Unbalanced audio uses one central conductor for audio and uses the more exposed "shield" for the other side of the circuit.

Bottom line; unbalanced is lower in level and less protected. Mic level via XLR is even lower, but is balanced and, therefore, more protected.

I don't like the passive Beachtek adapters. Too much stuff between the mic and the camera. I feed cameras with unbalanced 1/8" TRS stereo inputs from my Sound Devices mixer's balanced XLR outputs.

I use my regular balanced hose to get to the camera. At the end, I have my "special problem solver." It's two female XLR to male RCA cables. I've clipped them together with plastic wire ties. At the end, I use a simple, single Y-connector; two female RCA to stereo male mini TRS.

Now, there is also a possible level situation. Some semi pro gear I run into with RCA inputs is designed for -10 dB consumer levels. I have worked with several CD recorders that are designed this way. If my mixer output is set to +4 line level, it's too hot for that CD recorder. Fortunately, my Sound Devices 442 mixer has switchable outputs; mic, -10 and line. (in essence, it has it's own built in adapter.)

You'll know if your camcorder audio inputs are mic level, -10 or +4 by reading the manual or by feeding line level and LISTENING. The weird thing is that you can feed line level tone to a mic level input and actually set the level at what LOOKS proper on the meters. This is novice audio mistake #1. "I set tone at -20 and everything looked fine."

If you were listening during tone and bars, you would have heard tone in the headphones, but if you listened CLOSELY with good phones, you would hear a little "hair" around that tone. Some low level distortion. If you can't hear the diistortion on the tone, try recording something at actual level and then playing it back. If you have "hair", you have a problem.

Some consumer camcorders MAY have -10 consumer inputs. I don't know.

Finally, what to do about XLR mics fed directly to cameras with 1/8" stereo inputs? I have a special cable; XLR female to 1/8" male TRS plug. The cable is specially wired so that pin #2 is connected to BOTH the itp and ring of the mini TRS. This puts the audio from the mic on BOTH inputs. NOTE: Using the "regular" female XLR to male 1/8" TRS cable WILL NOT WORK ON A STANDARD STEREO INPUT. You end up with the positive peaks from the mic on one channel and the negative peaks on the other. If you get any sound, it sounds like crap.

Regards,

Ty Ford

Ty Ford's "Audio Bootcamp Field Guide" was written for video people who want better audio. More at: http://home.comcast.net/~tyreeford/AudioBootcamp.html
or http://www.tyford.com


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Ray Palmer
Re: Saturday Audio Class - Unbalanced, balanced, mic, -10, line level??
on Sep 17, 2007 at 5:08:26 pm

It is always good to re-enforce the basics.

Ty,
If you are going to cut and paste this post, you transposed two letters in the last paragraph. "itp" instead of tip.



Respectfully,
Ray Palmer




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Ty Ford
Re: Saturday Audio Class - Unbalanced, balanced, mic, -10, line level??
on Sep 17, 2007 at 7:21:23 pm

Thanks!

Ty

Ty Ford's "Audio Bootcamp Field Guide" was written for video people who want better audio. More at: http://home.comcast.net/~tyreeford/AudioBootcamp.html
or http://www.tyford.com


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