Okay, let’s be honest. How many of you videomakers out there know how sound
gets from your camcorder’s microphone to the finished video? Do you know what
an AGC is? How about depth multiplexing? If you don’t know, you’re in good company. All too many
videomakers focus their energies on understanding the visual aspect of their
trade, while turning a deaf ear to the way in which a camcorder handles
sound.
It is important to understand the way camcorders record audio. On the most
basic level, this knowledge can help you buy the right gear. More importantly,
it will help you to make better sounding videos by working with the hardware
itself to get the best results.
In order to learn more about camcorder audio, let’s follow the audio signal
from the microphone to the audio output. We’ll look at both the VHS or 8mm
families, pointing out all the major differences as we go along.
Getting In
Virtually all camcorders have a built-in mono or stereo mike. Most units also
have a plug for connecting an external mike; this cuts off the built-in mike.
One of the advantages of the external mike is that it will not pick up motor
noises from the camcorder, even if it’s attached to it.
Several methods help to get rid of motor noise from the built-in mike. Some
manufacturers mount the motors in rubber casings and use rubber bushings on the
motor mounts to help stop vibrations from reaching the mike. Brushless, very
quiet running motors eliminate static and mechanical noises. Of course, higher
priced camcorders tend to have more success in getting rid of motor noise. The
manufacturer’s ultimate goal is to have the mike pick up only desired sounds.
Once the mike turns sound into a changing electrical voltage, this signal runs
down a wire to a microphone preamplifier circuit. If your camcorder is stereo,
there is a second amp for the other mike’s signal. The mike amp is usually a
single IC (integrated circuit) chip. In fact, many camcorders use a single IC
chip to handle most every audio chore in the camcorder.
The mike preamp boosts the audio signal level up to that required by the rest
of the camcorder’s audio circuitry. Mike signals are very weak, sometimes even
in the order of micro volts (1/1,000,000 volt). That’s too weak a signal for
the rest of the circuitry to work with, so the amp boosts this signal up to
around 0.7 or 1.0 volt.
This is a crucial stage for your sound, because the mike preamp has to boost
the signal so dramatically. If the camcorder’s mike preamp isn’t of good
quality, your audio will suffer from excess noise.
Some camcorders accept line level audio in VCR mode. These signals, usually
from another VCR, come into the camcorder already amplified. The amplified
signals from the mike path and the line level path meet at a switch that
toggles between camera and VCR modes. In the VCR position, the switch
bypasses the mike to record the sound from the line level inputs.
Audio Muscle
Okay, we’re past the switch and now we come to the record amplifier circuitry.
This area processes your sound signal for actual recording. The first section
of this circuitry is where we adjust the signal level (you can think of
it as audio signal strength) for proper recording. Setting the recording level
is important. A level that is too low will become plagued with tape noise, and
may be difficult to hear when played back. Too high a recording level will
saturate the tape (record at a level higher then the tape can handle),
resulting in distortion of the sound.
A number of circuits designed to assist in the record level process may or may
not be included in the recording amplifier. The simplest are manual level
controls. These consist of potentiometers (devices that create a variable
electrical resistance) placed directly in the signal path. You turn the
attached knobs to set the recorded signal level.
It’s pretty much like controlling the flow of water at your sink by turning
the faucet handle. If you turn it down, the flow gets weaker and vice versa.
The signal, once adjusted by the level control, also drives a set of VU (volume
unit) meters or LEDs (light emitting diodes) that give you a visual reading to
set your levels by.
All consumer camcorders incorporate an automatic gain control (AGC). The AGC
looks at the highest level of the incoming audio signal and compares it to a
predetermined reference (generally a level just below tape saturation). The AGC
then raises or lowers the gain of the incoming signal to match this level. In
other words, it sets the levels for you.
Manual or AGC?
There are advantages and disadvantages to AGCs and manual controls. One
advantage of AGCs is that they will set fairly decent levels if you are in a
hurry. Their main drawback is that they’re try to bring every sound up to the
perfect signal level. If your talent stops speaking for a second, the AGC will
crank up any sound it can find–be it a droning refrigerator motor or a
sniffling cameraman.
AGC circuits are also somewhat slow to respond to level changes, so a sudden
blast of noise may cause severe distortion before the AGC reacts. Or it may
respond to a loud background noise and cut, for a few agonizing seconds, the
overall level of the sound you are trying to capture.
In most camcorders, you’re stuck with AGC. This makes it all the more
important to use earphones to check the overall sound during the shoot. Be sure
that what you want to record balances well with the background sound.
With manual control, you can eliminate most of the problems associated with
AGC. You can also fine tune the recording levels and adjust the balance between
stereo channels. Many expensive high-level camcorders use manual controls
because in professional settings, control is paramount.
A serious disadvantage of manual input level controls is that each new scene
or sound source requires that the settings be readjusted. It is very difficult
to reset your levels while shooting–can you imagine trying to twist knobs on
the camcorder and keep it steady at the same time? Manual controls demand a
sound check, a luxury you may not be given.
Another level control is the limiter. A limiter circuit simply reduces any
part of a signal stronger than a pre-determined level to prevent
distortion. This is usually a point just below tape saturation. The limiter’s
main disadvantage: when it kicks in, you can really hear it squashing your
audio.
Camcorders with manual level controls often include a limiter. While the AGC
is the primary audio control of consumer camcorders, many industrial camcorders
use all three of these level control methods, allowing the user to switch
between them in different combinations.
Signal Processing
Let’s move on down the wires to the processing section of the record amplifier.
This circuitry changes the signal based on the method used to record it on
tape. It is here that record equalization takes place. To understand record
equalization, we have to consider the effect that different tape speeds have on
audio recording. At the higher tape speed (SP for VHS), the changing audio
signal is spread out over a longer section of tape than when recorded at slower
speeds. This makes it easier for the heads to read during playback.
At the slower recording speeds (EP), high audio frequencies tend to get lost
because they’re packed onto a small section of tape. The tape is just not
moving fast enough to properly record or playback these frequencies. The
equalization circuit boosts the high frequencies at the slower speed to
compensate. This tends to improve the recording quality of the higher
frequencies because they are now more dominant on the tape.
Record equalization tends to boost any high frequency noise along with the
original signal. But the advantage of better high frequency response outweighs
this.
Now we’ll look at the major differences in how each format–VHS or
8mm–actually records audio.
Recording VHS
VHS camcorders, by design, have always had a linear audio track. It’s called a
linear track because if it were visible to the eye, it would run in a straight
line down the edge of the tape. It’s recorded by a stationary record/playback
head in much the same way a reel-to-reel or audiocassette deck records. At
first this track was mono, but when Hollywood began to release movies on VHS
tape the linear track split in half to become linear stereo.
The linear track has never been very high in fidelity. It offers, at the
most, only 50 to 11KHz signal response at the fastest SP speed. Most consumer
decks top out at closer to 5KHz. This is about the same sound quality as a
cheap mono audiocassette recorder. With the audio track cut in half for stereo,
the quality only got worse as the space for each track got smaller.
Along came the VHS hi-fi standard, a successful effort to improve the audio
quality of the VHS family. But where did engineers put the hi-fi signals? In
the original VHS design, the linear audio track, video track and control track
covered the full width of the tape. Having no place left to go on the surface
of the tape, the developers of VHS hi-fi decided to record under the
video tracks using a process called depth multiplexing.
Depth Multiplexing
Multiplexing means mixing two or more different signals into one. To do this,
the camcorder assigns different carrier frequencies to each signal. The carrier
signals can be thought of as radio broadcast signals–though they occupy the
same space, you have to tune in to their frequency to pick them up.
The carrier current energizes the audio heads to record the audio on the tape.
This is frequency modulation or FM for short. The system uses 2 separate audio
heads on the video head drum–at least one is in contact with the tape at all
times.
Recording heads, be they audio or video, work a bit like a horseshoe magnet.
There’s a gap between the two poles of the heads, and the tape bridges this
gap. The width of this gap is the key to depth multiplexing.
In the VHS hi-fi system, the audio heads use a wider gap than the video heads.
This generates a stronger magnetic bias (the force that actually affects
magnetic tape). Since it’s stronger, it records deeper into the tape. Following
right behind the audio head is the video head, which records over just the top
magnetic layer of the tape. This results in a video signal with the audio
signal just underneath it. During playback, the video and audio heads ignore
each other’s recorded signals.
VHS hi-fi offers 20 to 20KHz frequency response, which approaches CD quality.
It is available on many VHS and all S-VHS camcorders.
The audio tracks excite the magnetic field in the audio head gap and the
resulting signal travels on to a playback amplifier. From there, it separates
into right and left channel hi-fi signals again. Then it travels out to the
audio jacks on the back of your camcorder. Separation between the left and
right hi-fi track is excellent and time code recorded on one of the tracks will
cause no audio or video problems.
Because you cannot video dub over VHS hi-fi, or insert hi-fi audio under the
video, hi-fi audio isn’t very flexible for editing. All camcorders that feature
VHS hi-fi also offer at least one linear track, and editors rely on this track
for editing.
Sadly, consumer hi-fi decks offer no separate input or output for the linear
track. You can dub to it, and switch your output to choose it alone or mixed
with the hi-fi tracks. You can’t have linear and hi-fi audio coming out of the
deck on separate jacks. One other drawback is that when using lesser grade
video tapes, the hi-fi signal has a tendency to cause minor noise in the video
picture.
The JVC HR-S10000U consumer deck and the BR-S378U industrial deck offered a
feature called VOS (video over sound) which did allow video inserts over the
hi-fi audio. The decks turned off the flying erase heads and laid down a strong
video signal to replace the original signal. Each attempt at inserting video
weakened the original hi-fi audio, so you only got one or two shots at it at
best. Still, it was true video insert editing under hi-fi audio. JVC should
consider bringing this feature back for their present consumer editing decks.
8mm Recording
8mm recording is another matter entirely. 8mm was born with AFM (audio
frequency modulation) audio and has never had a separate linear track. 8mm AFM
multiplexes audio and video together, and then records them as one signal with
the video heads. During playback, the video heads pick up the multiplexed
signal and an amplifier strips the audio signal from the video, sending it on
to an audio playback amplifier.
From there, it travels to the audio output connectors on your camcorder. This
is the scheme with 8mm mono audio, which is common on less expensive units. As
with VHS, however, the demand for stereo produced a new audio recording
technique.
The solution involved adding a stereo difference signal that uses a different
carrier frequency and records right along with the mono track. During playback,
the camcorder extracts the two tracks and the result is AFM stereo. Stereo
recordings are compatible with mono camcorders and VCRs, because the actual
mono signal remains unchanged.
Like VHS hi-fi, this process creates problems when editing. You can’t dub to
AFM stereo without erasing your video. But it generates high quality sound
which approaches CD quality. Unlike VHS hi-fi, AFM stereo separation is poor
and use of either track for time code as previously described causes crosstalk
problems (the crossing of sound from one track to the other).
One other method of recording sound shows up on expensive industrial and high
level consumer 8mm camcorders. Called PCM (pulse code modulation), this stereo
system encodes the incoming audio into digital information. The digital signal
records diagonally on the tape through separate heads in the video head drum.
Instead of sharing the same area of tape as the video signal PCM records on its
own small track at the end of each video track.
During playback, the PCM heads pick up the digital signal and send it to a
digital-to-analog (DA) converter. The DA converter restores the analog audio
signal, routing the audio on to camcorder’s output connectors.
Unlike VHS hi-fi and 8mm AFM, you can easily dub to PCM. Hence PCM decks are
capable of audio insert editing. PCM audio has excellent separation, frequency
response and noise performance.
Taping it Together
Manufacturers coat the videotape surface with billions of tiny magnetic
particles which change polarity in reaction to the magnetic charge created in
the audio record head. During playback, these particles alter the magnetic
charges in the playback head by virtue of their polarity, which recreates the
signal originally recorded.
Tape quality and formulation have a greater effect on sound quality for the
VHS linear track than the AFM hi-fi track. Cheaper tapes tend to infect your
linear audio track with more noise than a higher grade tape. Most tapes,
labeled “hi-fi” or not, work great for hi-fi audio.
Regular 8mm uses a metal particle (MP) tape with particles small enough and
tightly packed enough to give good response to the AFM recording method.
Hi8 MP uses an even higher grade of metal particle tape designed for increased
high frequency response. Hi8 metal evaporated (ME) tape is made by evaporating
metal pellets onto the backing of the tape. Any Hi8 formulation will deliver
excellent sound quality for AFM or PCM digital recording.
Tape speed has a much greater impact on audio quality than does tape
formulation. Here’s why. Imagine putting a dot on a piece of paper with a
pencil. Then start to put a second dot on the paper, but yank the paper away as
the pencil hits the surface. This leaves a long mark next to your fine little
dot. Your pencil was on the paper for the same amount of time, but the latter
mark was spread out over a much greater area.
Now have someone take the paper down the hall while you look at it. Which mark
is easier to see? The longer mark, of course! That’s also why faster tape
speeds record better signals that are easier for the heads to properly read.
Slower speeds also have more problems with irregularity or dropouts (a flaking
away of the oxide on the tape causing white pinholes or streaks in the image).
This is especially the case with cheaper tapes. The result is poor image and
sound quality.
Finally, the condition of the heads in your VCR have a direct effect on sound
quality. If your audio heads (or video heads for that matter) are dirty, or the
head gaps are packed with shed oxide or grit, there is no way that they can
record or reproduce a decent signal. For information on cleaning your
equipment, consult the Videomaker April 1994 issue.
And So…
Understanding the different audio recording methods will assist you in deciding
which is best for you. You might need more than just the single linear track of
VHS. If you want to edit the sound of your videos, the three tracks of the VHS
hi-fi/linear combination may be just the thing. Maybe 8mm’s AFM tracks will
meet your needs if you don’t plan to use time code. Or perhaps you can afford
to go all the way and use Hi8 with PCM audio for full audio editing
capability.
Whatever route you choose, you’ll have a better appreciation of your audio
when you understand what it goes through.
Glossary of Audio Terms
- AFM
Audio Frequency Modulation. A noise-resistant method of encoding and
recording audio information. - AGC
Automatic Gain Control. A circuit that adjusts the signal strength of
incoming audio signals to optimize recording. - Ambient Sound
Any natural sound (wind, cars, voices, etc.) at the location
of your shoot. - Hertz or Hz
A unit for measuring frequency. 1 Hertz=1 cycle per second. - Hi-fi
A stereo FM recording standard used in VHS-family VCRs and
camcorders. - LED
Light emitting diode. Often used for metering audio signal strength. - Limiter
A circuit which keeps signal levels below a certain point to avoid
distortion. - Multiplexing
The mixing of different signals into a single signal, to be
separated later. - Noise
Any unwanted elements introduced into an audio signal. They can be
electrical or ambient in nature.
PCM
Pulse Code Modulation. A method of converting analog audio to digital
data, and back again. Used in some industrial Hi8 camcorders and consumer VCRs.- Record Equalization
Optimizing the amplitude of high frequencies for better
record/playback response. - Saturation
The point at which a tape reaches maximum signal acceptance,
beyond which distortion occurs. - Signal to Noise Ratio
A ratio between the actual audio signal strength and
any noise in the signal. The higher the number the better. - SP/EP
Tape speeds. SP: Standard play, EP: Extended play. - VU Meter
Volume Unit meter. Measures average sound intensity with a moving
needle or bank of LEDs.
