Sound Track: Makin' Tracks

When it comes to audio and video dubbing, where--and how--a format stores its audio tracks makes a world of difference.

The ability to insert audio and video separately is key to completing many types of video productions. Unfortunately, few consumer-level video formats make it easy for us to perform this kind of edit (often referred to as an audio or video dub). Here's a quick look at the various formats and how they handle their audio tracks.

The VHS Family

When the VHS format first appeared on the scene many years ago, it had just a single audio track. Video folks call this the "linear" track, because it runs in a line along the edge of the tape. Its fidelity is relatively poor, and the linear track does not support the spacious sound of stereo recordings.

Later, manufacturers split the linear track in two to record stereo. This made the linear track's noise performance even worse, so engineers added noise reduction to keep tape hiss at a bearable level. There was no room on the tape to add more audio tracks so, dismal as it was, VHS audio quality had maxed out.

Then, manufacturers figured out how to record two different signals onto the same section of tape. We call this process depth multiplexing. In it, one signal penetrates deeper into the tape's magnetic layer than the other. With VHS hi-fi, the audio signal lies beneath the video signal. Because the audio heads sit on the spinning head drum, the effective tape speed is very high--this explains VHS hi-fi's CD-like audio quality (see figure 1a).

But there's a large drawback to VHS hi-fi--you can't dub audio or video. Because the two signals share the same magnetic material on tape, you can't replace one without erasing the other. JVC experimented with a feature called Video on Sound (VOS), in which you could record hi-fi audio first and dub video after the fact. The VOS system never worked all that well (you only got a few tries to get it right), and JVC eventually discontinued it.

With stereo hi-fi growing more popular, there was no reason to continue splitting the linear track in two. Manufacturers dropped back to the original mono linear audio track (without noise reduction). This track is still in use today and is included on all VHS-family machines.

Why, you may ask, do manufacturers bother with the linear audio track at all? Two reasons: first, it's cheaper to make a VCR or camcorder if you leave off the hi-fi heads and circuitry. For this reason, there are still lots of VHS-family units out there that don't support hi-fi stereo audio. Second (and this is where videographers take notice), you can dub audio and video with VHS linear audio. Because the linear audio and video signals lie on separate areas of the tape, you can re-record one without affecting the other.

VHS hi-fi VCRs sense when hi-fi tracks are present, automatically switching to them on playback. While it seems like a nice "feature" at first glance, this tendency to play back only the hi-fi tracks can get unwary videographers in trouble. More on this …