Recording Production Audio for miniDV, Dig 8, Hi8mm, etc.
by Fred Ginsburg C.A.S. Ph.D.
   
Small format video (miniDV, Digital 8, Hi8mm, SVHS) is being
used ever increasingly for professional application. The various merits of
shooting images with these relatively inexpensive, inobtrusive, and extremely
portable video acquisition systems are familiar to most readers. However,
interfacing these consumer and pro-sumer camcorders with professional audio can
be a nightmare.
Use the consumer shotgun mic that comes with the camera (or is sold as as
aftermarket accessory) and everything sounds fine. But try to plug in your
thousand dollar professional microphone and the result is a lot of buzz, humm,
and lower audio levels.
Let's examine why this happens and how to fix it.
Most camcorders will readily accept the industry standard 250 ohm low-impedance
microphone input signal, so the problem is not that of matching impedance level
so much as it is a question of proper input cables. Purchasing an impedance
matching device is not the solution to your problem.
The mic input is generally a 3.5mm mini stereo jack. The output of a
professional grade microphone is XLR 3-pin. Therefore, the first task at hand is
to adapt XLR 3-pin output to mini stereo input. To do that, we need an adapter
cable that consists of a female XLR 3-pin connector at one end feeding a mini
stereo plug at the other.
Since the camcorder input is stereo, our adapter cable needs to split the
incoming monaural audio over both the left and right camcorder channels. If we
were to record onto only one channel, we risk serious damage to our soundtrack.
The second, empty, channel would fill with hiss and noise. When our audio is
transferred from analog video (Hi8mm) for post-production, a percentage of this
noise will most likely bleed into the good channel. Such is the nature of Hi8mm
audio, due to the proximity of the recording tracks and head placement. Some
digital also suffers from track bleed, though not as much. It depends on the
camera.
In addition to audio bleed, the presence of an empty track may cause confusion
or even havoc with the automatic gain control in the camcorder. Even those
cameras that offer a manual audio level control will revert to auto gain if the
power is turned off, tape changed, or battery changed. Unless you are
meticulous, the resumption of videotaping may be with auto gain ON.
Finally, recording to just one camcorder channel makes it difficult to monitor
some meter levels and to hear with both sides of the headphones (unless you use
cumbersome adapters with your headphones).
Bottom line: Always feed the Left and Right camcorder channels, either with
monaural audio going to both sides, or with discreet stereo audio going onto
respective Left and Right. Never assume that what is on the second channel will
not show up in some form on your first channel, so be very careful if recording
wild questions or cue tracks that you plan on editing out.
What about the buzz?
Earlier in this article we mentioned those aftermarket micropones that the
consumer manufacturers would so dearly like us to invest in. Most of those mics
are of the electret condenser design, meaning that they need to see a few volts
of DC power in order to function.
To power these mics, the camcorders produce three to six volts DC at the mic
input jack. Even those camcorders that have a separate DC OUT micro jack
alongside of the MIC INPUT jack, still root the DC circuit in the ground of the
MIC jack. Unless your plug-in mic is of the precise electronic formula of the
camera maker, you may experience symptoms of DC interference.
Depending on the particular characteristics of your pro mic, these DC related
symptoms may be: non-existant; a loss of gain/volume; buzz/humm; or even
intermittantly increasing buzz/humm. Different types of mics react differently.
To eliminate this buzz problem, the upstream DC voltage must be blocked by means
of capacitors. Choose your capacitors wisely, for too little will not cure the
buzz, and too much will affect the mic signal.
If you are wondering if anyone manufactures an adapter cable that remedies all
of these problems, then stop your wondering. Equipment Emporium in Misson Hills,
California designed their XLRH8 Audio Adapter Cable a few years ago to solve the
audio problems that many law enforcement agencies were encountering with the use
of their Canon LX100's and Sony's. Today's XLR-H8/DV cable is an improved
version of those cables intended for DV as well as Hi8mm/SVHS.
The basic XLR-DV cable is a short adapter that features one female XLR 3-pin
connector at one end and a right angled mini stereo plug at the other. Audio is
split over the left and right camcorder channels. Blocking capacitors housed
inside of the shell of the XLR knock out the DC interference.
Equipment Emporium recommends slipping a rubber band around the mini connector
and pulling it around the camera body like an oxygen mask, thus applying inward
tension on the connector. The XLR connector of the cable should be strain
relieved by tying or taping it to something sturdy on the camera body, such as
the strap lug.
Never allow any long cables or any weight to tug directly on the mini jack of
the camcorder; the jack is fragile and can loosen or damage easily.
We sell the Sign Video model XLR-PRO, which is also an adaper box that mounts
beneath the camera body. It features two XLR inputs for mic/line, mono/stereo,
and volume controls. A belt clip and extension cable are included, so that you
have the option of either wearing the adapter or mounting it to the tripod leg.
Price is only $159.
Equipment Emporium also sells the basic BeachTek XLR box that fastens underneath
most camcorders and provides two XLR mic/line inputs. Output is a stereo mini
plug that connects to the mic input jack of the camcorder. Price is approx
$179.
BeachTek also offers the DXA6 ($269.00)
 which features 48volt Phantom powering
for condenser mics. And their latest box, the DXA8 ($369.00) has actual mic
pre-amps that allow you to RAISE mic volume.
Studio One makes the XLR-BP-PRO ($189.95), which is a beltpak adapter box
featuring two XLR inputs, switchable for mic/line, volume controls
(attenuation), mono/stereo, ground lift, as well as 1/4-inch and 1/8-inch inputs
(which you should be cautious about using). The box features DC blocking and
almost no internal loss of mic signal (unlike certain other brands).
Their XLR-BP-PRO3 is similar, but
features three XLR inputs (two on one channel and the third on the second
channel). Price is $218.95
Using a belt-pack has advantages and disadvantages (you be the judge).
Disadvantage is that it does not mount underneath the camcorder, so there is one
more item to deal with when you pick up or put down the camera. Be VERY careful
not to tug on the stereo mini connection to your camera (strain relief it!).
Advantages include the fact that the box does not mount underneath the
camcorder. Easier access to changing tape, and more secure mounting to tripods
and shoulder mounts. Any major tugging from the mic cables is absorbed by your
hips, rather than causing the camera to suddenly lurch while videotaping.
A special note. Some of the new digital camcorders from Canon and Panasonic are
fairly forgiving of external audio and do not require the special XLR-DV cables.
All that they need is just an adapter cable that converts from XLR female to
stereo mini. However, be warned, that some of the off-the-shelf adapter cables
are wired incorrectly. They are set up for a stereo mic (XLR) going to stereo
mini. Pin 2 of the mic goes to tip (left) and pin 3 of the mic goes to ring
(right). If you plug a monaural mic into that cable, the sound will be recorded
out of phase on left and right, which means that when you go to transfer the
audio will be low volume or not audible. A correctly wired adapter cable will
have pin 2 from the mic connected to both the tip and ring; pins 3 and 1 from
the mic both go to the sleeve of the mini connector. Equipment Emporium does
sell the properly wired adapter cables, as well. ($18)
Which brings us to the next topic in our Audio for DV discussion: line level
feeds.
Audio coming from a mixing board is usually at 600 ohm line level, which is a
much hotter and stronger signal than mic level. However, most camcorders only
accept mic level external input when used as a live camcorder; when the
camcorder is used as a VTR it will accept external line level video and external
line level audio via the RCA jacks in the back. When the camcorder is
functioning as a camcorder, the RCA inputs are disabled.
Therefore, to feed line level audio into a camcorder (which is mic level), we
need to reduce the signal by 30 to 50 dB.
Some mixing boards, such as the Shure FP33, the PSC DVpromix3, and some Mackies,
have a switch to reduce their outputs from line to mic level. You can't ask for
more convenience than that!
Otherwise, one needs to insert a pad or attenuator between the output of the
mixing board and the input of the camcorder. Shure makes a 50 dB line to mic
Attenuator. Audio Technica offers a switchable -10, -20, -30dB pad. Either of
these devices will work, since the camcorder input does have some range with its
gain or volume.
Sometimes when you feed from a mixing board, the DC interference at the mic
input jack may be a problem. Sometimes it may not. It all depends on the design
of the mix board.
To play it safe, use an input cable with DC blocking, such as the XLR-DVcables.
So far, we have only discussed the logistics of getting an audio signal into the
camcorder. Now, a few comments on what that signal should be.
There is an old computer programmers expression that goes "garbage in, garbage
out." That applies to production sound, as well. It does no good to have a clean
signal entering the camcorder if that signal is worthless to begin with!
Just because you are recording on an inexpensive medium is no excuse to be lazy
with technique. Small camera or not, a tripod and lights will still make the
difference between a home movie and a professional product. And proper
microphone deployment and mixing are still esential if you want a professional
sounding track.
Use the same mics and techniques that you would on a big budget show.
Mic your subjects from close overhead with a boompole, and use high quality ENG
or full condenser shotgun mics.
If using lavaliers or wireless mics, pay careful attention to proper placement
and rigging. Check for clothing and wind noise.
Adjust any cables that function as antennas. If possible, mount your receiver as
close to the action as possible, any pay careful attention to antenna line of
sight.
Mounting a radio receiver on a consumer camcorder can be tricky, since these
cameras are not as well shielded as their professional large format
counterparts. Camcorders tend to produce a lot of RF interference near their
viewfinders and near the recording heads. Try moving the receiver around to find
a "sweet spot" that is free of buzz or humm.
If your camcorder is equipped with an auto gain control, then your best strategy
for eliminating or reducing the "hunt for some sort of noise to amplify" is to
make sure that you feed a well mixed or well chosen signal to the camera.
Think of it in terms of working with a still camera that has auto light
metering. Frame up on a high contrast or unusual lighting condition and you get
horrible exposures. But compose a scene with some highlights, shadows, and a lot
of middle tones and the exposure comes out beautiful.
It's the same way with sound. Feed your camcorder a strong signal with dialogue
dominating over ambience; or sound effects dominating over background noise --
and the resultant soundtrack will be fine.
By the way, a neat trick when you are recording in mono and using a camera with
manual audio settings is to use a Y cable and bracket your exposure, so to
speak. Split the monaural output to both XLR inputs, and offset the volume so
that one track is better for soft passages while the other track is adjusted
down for the louder stuff.
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