Is there a general rule about where in the signal chain you should go from analog audio (such as from a microphone) to digital audio (such as a digital video deck)?
Analog audio is susceptible to noise being injected into the signal from outside sources, such as radio interference or from power transformers inside gear. Digital audio does not have this problem. Therefore, the sooner you can move the signal from the analog to digital realm, the better.
Failing that, the sooner you can move your analog audio to +4 dB balanced audio connections, the better. The higher reference voltage plus the balanced wiring both help either drown out or reject outside noise. Microphone signals in particular have the lowest reference voltage, and therefore are the most fragile.
What’s the difference between audio meter readings on analog and digital gear, and how do you line up the two?
With analog equipment, “0 dB” (dB = decibels) is supposed to be the optimal average signal level. You’re allowed to go over it occasionally, as long as you don’t go too far. With digital equipment, “0 dB” is the absolute loudest peak level you can record; anything over it will be “clipped” and therefore distorted - akin to overexposing a shot, thereby posterizing your highlights.
The most common standard for lining up the two is that –20dB in digital should equal –0dB in analog. That said, I found that BetaSP video decks tended to sound too distorted on audio peaks with this setting, so we adopted an in-house rule of –12dB digital would equal 0dB on the Beta deck. Along those lines, it’s a good idea to notate on your tape labels what assumptions you’ve made.
We’re recording city council meetings in a room with fluorescent lights, and there’s a lot of buzz in the audio feed. Is this a ground loop problem?
Your problem is coming from the fluorescent lights injecting noise into the power and ground systems in the room. You might try running a heavy duty extension cord to an outlet down the hall or in another room where the lights are off, hopefully getting off the same circuit the problematic lights are on. Then run all the gear you can off of that alternate circuit. If you can’t find a “clean” outlet nearby, you can buy a power strip with noise filtering built in; I use a Tripp Lite Isobar in my studio to plug my audio racks into (available on Amazon.com
A popular but unfortunately discontinued pair of audio processing devices were the Roland SN550 and SN700, which had notch filters that removed ground hum as well as the “harmonics” of hum (noise at frequencies which are multiples of the AC current frequency of 60 Hz in the US, or 50 Hz in Europe). These notch + harmonic filters also can help remove fluorescent light buzz. Sweetwater Music closed out the last of their stock of SN700s for $300 each years ago...so sad. I did a little Google searching when I returned from NAB and found the Hum Bug Noise Eliminator, but it’s probably not an option for many at $1750 for a single channel. Note that a simple “hum eliminator” - which is really just a ground loop isolation transformer (such as the Ebtech and Radio Shack units I’ve mentioned elsewhere) - may not do the job.
If you can’t find a hardware device to deal with the noise before you record it, better sound processing software such as Apple Soundtrack or Adobe Audition can selectively remove known noises. Record a bit of the noise solo with no one else talking in order to give these programs a clean scent as to what noise to go after and eliminate.
By the way, if you hear a fluorescent light buzzing, there’s a good chance the ballast inside the light fixture is bad. Replacing the ballast often helps, if that’s an option.
We sometimes record in old buildings where there is no third prong on the AC outlets, and we have ground buzz. How can we properly ground our equipment in these situations?
Buzz is a sign of an insufficient ground connection, compared to hum which is a sign of too many ground connections (resulting in ground loops). So, you need to get a good ground from somewhere.
Even though there is no ground pin on the AC outlet itself, the conduit and wiring boxes connecting and holding the outlets should be grounded. Pick up a 3 prong to 2 prong AC adapter at Radio Shack or a hardware store, and connect the wire or metal lug on the 2 prong side to the screw holding the cover plate for the AC outlet.
If the building is in bad shape, there is a possibility that the conduit has a break in it somewhere, and ground is not reaching the specific outlet you want to use. If ground buzz persists, try a different outlet.
The absolute best solution is to do what audiophiles do to ensure the best possible ground, therefore reducing their noise and interference problems: Drive a copper stake into the ground outside the building, run a very thick copper wire from that stake to where you’re connecting power, and use that for your ground.
Our audio sounds fine until the talent touches the microphone, at which point we hear hum. What’s going on?
Your talent is acting as a new ground path, therefore causing a ground loop – akin to a human lightning rod (although thankfully nowhere near as dramatic). One solution would be to have the talent stand on a non-conductive (insulating) rubber mat. Another would be to try using a ground loop isolation transformer to break the loop elsewhere in the chain – in other words, this is a case where the Radio Shack or Ebtech hum eliminators may help.
Audio • Production •
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