Camcorder Sounds: Understanding What Your Audio Goes Through (page 3)
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.
