Sound Reasoning: Audio Tracks and Editing

All but a very few camcorders and VCRs can record audio signals along with video; that’s pretty common
knowledge. But do all camcorders record audio in the same place and in the same way on every tape? Well,
not quite.

Knowing where and how your audio tracks record will help you get the best sound quality and
flexibility. This knowledge can also help when it’s time to buy new gear. To learn more about audio tracks,
we’ll consider the problems with editing in both the 8mm and VHS formats; better yet, we’ll cover ways to
sidestep these problems, giving you more control over your audio.

VHS Tracks
VHS camcorders and VCRs began life with a single linear audio track. Linear means that the
track runs in a straight line along one edge of the tape (see figure 1a). The sound quality of this single mono
track has never been great–only slightly better than a standard cassette deck. For those who like to know, the
frequency response at the fastest speed (SP) doesn’t get much beyond 50Hz to 11KHz at best. Still, it gets the
job done.

In response to the wave of stereo movies released for tape rentals, VCR makers split the mono audio
track in half to create linear stereo (see figure 1b). Because there was no more space on the tape, each of the
two tracks were now about half as large as the mono track, and the sound suffered further. But linear stereo
worked, and it was fully compatible with mono VCRs (a single mono head will pick up both stereo

Knowing that linear stereo placed limits on sound quality, developers came up with the present
VHS standard–VHS hi-fi. But if the video, linear audio and control tracks took up all the space on the VHS
tape, how did they find room for the hi-fi audio? The answer lies in a process known as depth

Put simply, depth multiplexing records more than one signal along the same area of the tape. To do
this, hi-fi VCRs and camcorders use audio heads with a wider gap between the two poles of the head. This
records the audio signal deeper into the tape than the video. The hi-fi stereo track goes down a fraction of a
second before the video, which is then laid over the top of the hi-fi audio track (see figure 2). On playback,
the video and audio heads ignore each other’s signals. This advance gave true hi-fi stereo to VHS.

But there are problems. Because the hi-fi signal sits beneath the video, you cannot dub over the
video without replacing the hi-fi track. The reverse is also true; you cannot dub to the hi-fi track without
destroying the video.

To relieve this problem, the mono track remains on most hi-fi decks for dubbing in other

8mm Tracks
8mm camcorders and VCRs have never had a linear track. 8mm started life with an AFM (audio
frequency modulation) signal that includes both the mono audio and video mixed together. It records onto
the tape with the same set of heads. No audio (or video) dubbing is possible with this system for the same
basic reason that you cannot dub to VHS hi-fi–recording one signal will erase the other.

With the demand for stereo, engineers created a "stereo difference" signal which contains
the difference between the left and right signals. This signal is recorded with the mono audio and
video signals. At playback, circuitry separates out one video and two audio signals. In stereo VCRs and
camcorders, the mono and stereo difference signals are combined to derive a left and right signal.

The new stereo standard is fully compatible with the original AFM mono standard; if your VCR or
camcorder doesn’t have the required circuitry, the mono AFM signal will still play back as normal.

Unfortunately, AFM stereo audio offers no improvements in editing flexibility–you still can’t dub it
independent of the video. So engineers got together and came up with Pulse Code Modulation (PCM) audio.
In this system, stereo audio tracks are first encoded into digital information, then recorded onto a special area
of the tape at the end of each helical video scan line (see figure 3). On playback, the digital data is converted
back into analog audio signals. The sound is very good, and the design allows for full dubbing of PCM audio
independent of the video. Unfortunately, only higher-level Hi8 decks offer PCM sound.

To consider how to get around some of the problems that audio editing poses, we’ll look at each
format, VHS and 8mm, separately.

Editing VHS Sound
In VHS, we’ve seen that the mono linear audio track records separately from the video track. This results in a
track that’s easy to dub to without affecting the video, if an audio dubbing feature is present on your

VHS hi-fi is a different story. Like we said before, hi-fi audio tracks mix and record with the video
signal. You can’t record the hi-fi tracks without wiping out your video.

There are further problems. When hi-fi became available on consumer VHS decks, manufacturers
retained the mono linear track for compatibility with other VHS machines. Few of these early units,
however, allowed dubbing to the linear track.

Most VHS hi-fi decks now offer dubbing on the linear track. With these decks,
you can add narration, special effects and even music through the audio dub process. If you dub music this
way, keep in mind that you are dubbing to the lowest quality audio track.

If your deck is older and has only a single linear track, or if you have an older hi-fi deck without
access to the linear track, you face limits in your ability to edit your audio tracks at the deck. In this case, you
might want to think about running your audio through an audio mixer during editing.

With an audio mixer, you can mix your original audio along with audio CDs, audio cassettes, reel-
to-reel audio tape, outputs from sound effects generators and/or additional dialogue from a mike. Further,
you can direct all your audio from the mixer’s output to the high quality audio tracks of a hi-fi edit recorder.
You perform this kind of audio post production "on the fly" (manually, as the tape rolls), but with practice
many videomakers have made great audio tracks this way. In fact, many VHS videomakers prefer this
method, even when they have access to the linear track.

Several high-level S-VHS decks offer hi-fi audio along with two fully accessible linear tracks.
These tracks often feature noise reduction to improve their quality. Such decks clearly offer the best
flexibility in VHS audio editing.

Editing 8mm Sound
As we said before, 8mm has always had a single audio track, just like VHS. Also like VHS linear, when
8mm turned to stereo, it remained compatible with its original mono track. But unlike VHS, the 8mm audio
tracks mix with the video during recording. Once again, if you try to dub to the audio track(s), you will lose
the video.

Most videomakers working with the 8mm format have adopted the audio mixer method of editing
we talked about for VHS use. This offers the same control and flexibility, and the resulting sound quality can
be very good.

A few high-level decks offer both standard stereo AFM tracks and PCM stereo tracks. These decks
offer full access dubbing to both PCM tracks at the same time. As with industrial S-VHS decks with dual
linear tracks, Hi8 decks with PCM tracks offer the most options in 8mm audio editing. Unlike VHS linear,
however, PCM tracks offer very good audio quality.

Time Code
Time code gives each video frame an address in hours:minutes:seconds:frames to assure better
accuracy in your edits. Longitudinal time code (LTC) is so named because it’s recorded in a straight line
down the tape, usually on the linear audio track. It is not an audio signal–at least not one we wish to hear.
But it’s at the mercy of our audio tracks, especially in the VHS format.

Videomakers using VHS gear who wish to use time code face a few problems. If you have a mono
VHS camcorder, you can dub time code onto the tape after shooting. Note, however, that this process will
wipe out all audio. The same is true of recordings done on a mono VCR.

When using hi-fi VCRs, you can dub LTC to the linear track. This leaves the stereo audio signal
untouched, but here’s the problem–you can’t access the linear audio separately from the hi-fi tracks during
playback. Hi-fi consumer decks have no dedicated linear output, unless modified by a company such as
Carlson-Strand (San Clemente, CA).

Some industrial-level S-VHS decks have two linear tracks with independent outputs. You can
record LTC on one of these and leave the other for audio use. Some professional S-VHS decks offer a "cue"
track you can use for LTC. This track records in a line down the tape without affecting the
video signal. Because this track exists apart from the audio tracks, you can use it for LTC time code without
losing any audio.

At least one S-VHS camcorder and a few high-end decks will record and play back vertical interval
time code (VITC). This records in the vertical interval of the video (the horizontal black bar you see when
your TV picture rolls) and does not affect your audio tracks. External time code generators are also available
that will allow you to inject VITC into your video signal. To use these devices, you must place them in-line
between your camcorder and a deck while dubbing or shooting.

The method of choice depends on your edit controller and what, if any, time code it will work with.
You can easily see that the decision to use time code in VHS requires a lot of consideration.

It’s a much different story in 8mm. As there are no linear tracks, you can’t use the VHS method
described above to record linear time code. But 8mm manufacturers thought ahead on this. Several Hi8
camcorders and VCRs have adopted Sony’s Rewriteable Consumer Time Code (RCTC). RCTC records at the
end of each video scan line (just before the PCM audio info) and does not effect AFM or PCM audio tracks.
This means that time code editing in the 8mm family offers fewer problems than VHS.

In Summary
By now, you should have a good idea of where and how your VHS or 8mm audio tracks record. You should
know about VHS linear and hi-fi tracks, 8mm AFM and PCM tracks, and how you can (or can’t) dub or
insert to them. You should also know how using time code can impact your audio if you work in VHS.

Even if you’re a beginner, this information will give you good audio food for thought when you’re
ready to start editing your tapes or purchasing new equipment. Also, if you’re planning to buy new
equipment, be sure to read our Editing VCR Buyers Guide in the April 1995 issue of Videomaker.
It’s an excellent resource for finding audio features on editing decks.

Mike Wilhelm
Mike Wilhelm
Mike is the Editor-in-Chief of Videomaker and Creator Handbook

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