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 later.
The 8mm Family
The designers of the 8mm (and Hi8) format intended from the very beginning to blend the audio and video signals together on tape. But they didn’t do it with depth multiplexing (and two sets of heads). Instead, 8mm formats blend the two signals together electronically before recording. It’s easy to separate the audio and video signals on playback because they’re recorded at two different frequencies. Audio quality of 8mm and Hi8 AFM is very good. Its technical specifications are not quite on par with VHS hi-fi but, audibly, the difference is nearly indistinguishable.
Though simple and inexpensive to implement, this AFM (audio frequency modulation) system has one big drawback: you can’t dub AFM audio independently of the video. (Sound familiar?) Blend the audio and video signals together as AFM does, and there’s no recording one without destroying the other.
Thankfully, 8mm designers left room on the tape for a different type of audio signal–a digital one. Called pulse-code modulation (PCM), this method holds audio as a string of binary numbers on a separate section of the tape (called the PCM sector). Fidelity is quite good with the digital system; frequency response and noise performance are better than the 8mm AFM track (see figure 1b).
Certain consumer-level Hi8 VCRs offer PCM audio, though no Hi8 camcorders do. This is not really a great shortcoming–PCM audio is of greater benefit during post-production than when shooting. Most Hi8 VCRs with PCM audio allow you to control the relative level of the AFM and PCM audio tracks on playback.
Because the PCM audio has its own dedicated section of tape, it’s possible to dub audio onto it while leaving the video and AFM audio intact. The PCM sector also makes it possible to dub video onto a tape that has existing PCM audio. But manufacturers offer this feature only on industrial and professional Hi8 VCRs.
The DV Family
The DV (digital video) format is just what its name implies–a recording system that stores everything as a series of digits. DV uses a spinning head drum much like the analog formats, but it records billions of ones and zeros instead of a smoothly changing magnetic waveform. Audio and video "data packets" sit in their own discrete areas (sectors) on tape, making it possible to replace one without affecting the other. DV, to cut straight to the point, can audio dub (see figure 1c).
DV tape’s audio sector has space for a certain number of bits of audio data, and it doesn’t really care what gets recorded there. This allows DV hardware to configure the available space for two different audio modes: one non-dubbable stereo signal or two dubbable stereo signals. Most DV camcorders default to the latter, with no way to toggle between the two modes.
DV, like the 8mm family, uses the PCM method to record digital audio data. The single stereo mode records 16-bit audio with a sampling rate of 48kHz. This gives us audio quality to top that of compact disc. The dual-stereo mode uses 12 bits per sample, with a sampling rate of 32kHz. Its audio quality is somewhat higher than the 8mm-family PCM audio, which uses 8 bits per sample, and somewhat lower than DV’s single stereo mode, which uses 16 bits. Do the math on the two DV modes and you’ll see they add up to the same amount of data. (48kHz x 16 bits x 2 channels = 1.536 megabits per second = 32kHz x 12 bits x 4 channels.)
DV editing decks allow you to dub two more audio channels to the two you already recorded with your camcorder. Then, on playback, you can control the relative level of these two stereo pairs. It’s worth noting that there’s no way to take a two-channel DV recording and convert it to four-channel (to add more audio) without copying your tape.
Audio Track Tips
When it comes to handling audio with consumer video formats (especially the older ones), we’re not living in a perfect world. Here are some tips to make editing and dubbing audio a little less painful.
- If your DV camcorder gives you a choice between two- and four-channel modes, choose wisely before you start to record. There’s no easy way to go from two channels back to four, and only the four-channel mode gives you hassle-free dubbing.
- VHS hi-fi tracks have lots of headroom, and you can safely light up the red LEDs when setting record levels manually. DV and 8mm-family digital audio are a different story altogether. When digital audio exceeds 0 decibels (full strength), it distorts in the most ungraceful way. Digital audio users: you’re dealing with a very unforgiving system. Watch your record levels like hawks.
- If you’re planning to mix any two audio tracks together from the same tape, make sure they don’t contain the same audio. Using two different record methods (VHS hi-fi and linear, 8mm PCM and AFM) to play back the same audio causes severe interference and phase cancellation. If your music sounds hollow and your people sound like robots, you’re probably doubling your audio on playback.
- Avoid silent hi-fi tracks on your final VHS tape. Hi-fi VCRs will mute the linear track if they sense the presence of hi-fi tracks, silent or not. If you’re not sure what audio is going where, you may want to disable the hi-fi tracks altogether as you dub or edit your final tape. You can disable the auto-sensing feature on most VCRs, but it may require the remote.
- Planning to mix hi-fi and linear tracks on playback with a VHS-family hi-fi VCR? Remember that you can’t control linear audio record level with most decks. Nor can you control the balance of linear to hi-fi audio. The only way to change the relative balance of these tracks is by raising or lowering the hi-fi record level controls manually. And the only way to get the correct setting is by trial and error. Do a few test edits and dubs to learn the proper hi-fi record level. On the same topic,
- Keep in mind that every hi-fi VCR sets a slightly different mix of hi-fi and linear tracks when playing back in mix mode. Your perfectly blended masterpiece may sound very wrong through a different playback deck. It’s often better to go down one more generation so you can set the blend of the two tracks with your own VCR.
Good luck–and keep an eye on those record levels.