by Wayne V. Martin, CLVS
When monitoring audio, the use of both headphones and V.U. (volume unit) meters at each component of the audio chain will help ensure a quality audio program. During a deposition, listening through headphones is the best way to monitor audio quality and detect noises, buzzes and interference. V.U. meters give you an accurate visual indication of the level of your audio signal. They also help to detect excessive level and loss of signal.
Unity Gain is a method of calibrating the audio signal as it passes through multiple audio processing components. The audio level indicators on the mic mixer, the VCR and audio cassette recorder should all read the same decibel (dB) level when passing a signal through the audio chain. This helps minimize the problems associated with low audio level (tape hiss, inaudible) and overmodulation (distortion).
The ability to perform a unity gain calibration is a required skill for a CLVS candidate. As stated in the March 1999 CLVS Guide, standard #18 reads, "Prior to the test recording, the videographer shall calibrate the audio mixer and the audio input levels of the VCR by using a tone oscillator and unity gain procedures."
When setting up and checking a video deposition kit, a legal videographer should perform a unity gain calibration between the microphone mixer and the video cassette recorder. The audio cassette recorder and additional VCRs should also be included if they are being used. If there is an automatic gain control (AGC) or automatic level control (ALC) switch on the unit it should be switched off before beginning the Unity Gain calibration. At the audio mixer, activate a tone oscillator. Using the master gain control, adjust the VU level to zero dB on the mic mixer. Then adjust the audio levels to zero dB on the camcorder, VCR and audiocassette recorder by using the gain controls on each successive audio component in the audio chain. Unity Gain is achieved when all components in the audio chain display the same dB level as the mic mixer.
On some mic mixers, such as the Shure M267 microphone mixer, the tone oscillator may have its own attenuator. With these mixers, the tone oscillator attenuator and the master gain control should be set in such a way that there is room to both increase and decrease the gain at that attenuator. If the mic mixer is not equipped with a tone oscillator, an external tone oscillator can be used. External tone oscillators can be purchased from Shure Brothers, Inc.
So what happens if your camcorder does not have V.U. meters or audio gain controls? You deprive yourself of the ability to control and visually monitor the audio where it counts the most, at the unit which is recording your original master videocassette.
For example, you use a Panasonic AG-456 camcorder with a portable mic mixer to record your depositions. The AG-456 is not equipped with manual audio gain controls and V.U. meters. You know the correct level on your mic mixer to get the best result with your camcorder and you've never had a problem. One day your mic mixer goes down on the job and you borrow another model from the A/V department at the hotel where the deposition is being held. What level do you set the mic mixer to? The new mic mixer may be outputting at a different level than your old one. The audio sounds okay through your headphones; but on playback, you discover there is significant tape hiss because it was recorded too low. This is because the automatic gain control on your camcorder was able to adjust for the different audio input level enough for you to hear it well through the headphones, but was not getting enough signal to saturate the tape. You end up with a low signal to noise ratio and tape hiss or, worse, an unusable recording.
Another common problem can happen when the videographer, using the same AG-456, monitors the audio from the mic mixer. Since it is upstream from the VCR, the mic mixer can only tell you what's happening with the audio before it is recorded. Audio that sounds fine at the mic mixer can be under or overmodulated at the VCR. If you are sending line signal to a mic input on the VCR, the audio is likely to be indiscernible. This is because of the level difference between mic and line signals. The use of a line/mic attenuator (available from Shure Brothers and Radio Shack) between the mic mixer and VCR will solve this problem. Some mic mixers can put out a much stronger signal than can be handled by the miniphone jack input of many camcorders. In this situation, a master gain setting at level 6 may cause distortion when the audio sounded fine at level 5.
You may also have similar problems when adding different types of microphones to your system. Dynamic mics, mics operating on phantom power and wireless microphone receivers may require significantly different settings on the mic mixer in order to produce the same gain level. Also, an audio signal from a microphone or line feed could be overmodulated before it reaches your mic mixer. Even though your system is calibrated correctly, there could be distortion originating at the audio source. So it is important to listen to the quality of the audio in your headphones.
After performing the Unity Gain calibration, remember to make and playback a test recording that includes each microphone and audio channel employed in your system. Only after listening to the test recording can you be sure your system is ready to go.
So put on the headphones and watch those V.U. meters. Get it right the first time. There are few opportunities for second takes in the deposition business, especially if your client is rushing to catch a plane or the witness has left the country.
Special thanks to Martin P. Feldman of The Camera. Wayne Martin can be reached at wayne@legalvideo.net.