Do you know what an AGC is? How about depth multiplexing? If you don't know, you're in good company.
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.
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.
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.
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.
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.
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 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.
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
Audio Frequency Modulation. A noise-resistant method of encoding and recording audio information.
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.
A stereo FM recording standard used in VHS-family VCRs and camcorders.
Light emitting diode. Often used for metering audio signal strength.
A circuit which keeps signal levels below a certain point to avoid distortion.
The mixing of different signals into a single signal, to be separated later.
Any unwanted elements introduced into an audio signal. They can be electrical or ambient in nature.
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.
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.
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.