Sound Track: Audio Dubbing

Thanks to the modern miracle of audio dubbing, you can make changes or add new sound to your videotape.

Thanks to the modern miracle of audio dubbing, you can make changes or add new sound to your videotape.

You've probably heard the term "dubbing" spoken with great reverence, as if it was the very key to video and audio editing. While dubbing is not the key to editing, it is a powerful tool in the videomaker's toolbox.

In essence, dubbing replaces the taped audio or video signals (or both) while leaving some other part of the recording intact. In the case of audio dubbing, our goal is to erase and replace taped sound without altering the video. As you read on, you'll discover that some formats allow dubbing and some don't. For those who are working with a "don't" format, we'll offer some practical suggestions for working around the limits of your equipment.

Why would anyone want to lay new audio on old video? Ponder these scenarios:

  • You've got good dialogue and natural sound on tape from the shoot, and you want to add some mood-enhancing music.

  • You want to add narration or an occasional sound effect to an otherwise effective soundtrack.

  • The sound you got on tape while shooting is a complete wash--you need to replace it altogether.

    In each case, the simplest solution is to use a VCR or camcorder capable of audio dubbing to replace or augment the sound on tape.

    Will It or Won't It?
    If you don't know already, you're probably wondering whether your chosen format is capable of audio dubbing. As a general rule, how a given format stores its audio and video signal dictates whether or not it can dub. If the format blends the audio and video signals together, there's no replacing one with destroying the other. It's like wishing you had ordered baby blue paint instead of fluorescent orange after the paint store already mixed the pigment and base--there's no "unmixing" the two.

    In the beginning was VHS, with a monophonic "linear audio" track. This audio track is separate from the video track, allowing most VCRs and camcorders to dub audio without changing the video. Unfortunately, the fidelity of this track isn't the greatest. Enter VHS-family hi-fi audio, which offers excellent-quality stereo sound --with a tradeoff. Because the hi-fi signal sits under the video signal (blended onto the same area of the tape), you can't erase and replace the sound without destroying the video.

    But units with hi-fi audio still have the linear audio track, and you can dub to it. After you've replaced the audio on the linear track, you can mix it with the hi-fi tracks on playback. Not a bad deal, exept for the relatively low fidelity of the linear audio track. This applies to all VHS-family formats: VHS, VHS-C, S-VHS and S-VHS-C.

    The 8mm family went about things opposite that of the VHS family. The original 8mm machines only had the high-fidelity AFM stereo audio track, which can't be dubbed. The AFM signal is blended in with the video signal and is inseparable from it. Audio dubbing came later in the form of a PCM digital audio track--it sits on a separate area of the tape from the video signal. But PCM-capable machines are rare. Only a few decks (and just a couple of camcorders) allow recording, playback or dubbing of the PCM audio track. However, those units that do offer PCM audio give the videomaker some serious audio- tweaking power. Not only do the dub-friendly PCM tracks offer clean, near-CD-quality digital audio (8-bit, sampled at 31.5kHz), it's stereo to boot. You can, for example, record AFM stereo audio in the field with your 8mm or Hi8 camcorder, and later dub stereo music to the PCM track, behind your natural sound.

    We can't discuss audio dubbing without covering the latest format on the scene--DV. It should come as no surprise that DV's audio support is as impressive as its video quality. DV actually offers two PCM audio modes, one that supports dubbing and one that doesn't. The latter mode offers better audio quality, using all the available audio data "space" for just two tracks. The mode that allows dubbing divides the audio data into quarters for four tracks of audio. As a result, the two-track DV audio mode offers CD- quality sound at 16-bits and a 48kHz sample rate, while the four-track mode offers near-CD-quality, 12-bit, 32kHz-sample-rate performance.

    DV owners need to think about their audio needs before they shoot, because some DV camcorders will record in either mode. If you select the 16-bit, two-track mode for shooting, you won't be able to dub new audio onto those sections--it takes up all of the available audio data space. If you're not planning to dub to your original tape, this mode will give the best quality field recording. If your DV camcorder doesn't offer selectable modes, it's probably recording in the four-track mode.

    How's it Done?
    The specifics of how you perform an audio dub depends on the equipment you're using. In most cases, you'll run a line-level signal from your sound source or audio mixer into the RCA-style input(s) on your camcorder or VCR. Cue up your sounds, engage audio dub/pause mode on your camcorder or VCR, and enter the dub mode just before you start your sound source. Even folks with editing systems will likely need to get their fingers involved. Because even though many edit controllers generate a general-purpose- interface (GPI) trigger to start up your sound source when dubbing, CD players and tape decks that recognize this trigger signal are hard to find.

    If you're dubbing to the digital audio tracks of an 8mm-family or DV format, you'll be able to record the wider, more spacious sound of stereo sources. Simply run two cables (left and right) from your mixer or sound source to the camcorder or VCR. In the case of VHS-family units, you can dub only to the mono linear track--one cable will do.

    When squeezing stereo music down to a mono track, you'll get the best results by running the music through a mixer and panning each channel to the center. Take a single feed from the mixer and you'll get both the left and right channels of the music, instead of just one or the other. If you don't have an audio mixer, a simple Y-cable will often achieve the same effect, though some devices don't take kindly to having their stereo outputs joined in this fashion. If you hear distortion or any other problems, disconnect the Y-cable immediately.

    Some camcorders will only allow dubbing through the built-in mike (or mike input, if it has one). If such a camcorder has no mike input, your only option is to speak or play music directly into the camcorder's mike when dubbing. This is neither convenient nor high-fidelity, especially in the case of dubbing music. If your camcorder has a mike input jack, you'll get much better sound quality by dubbing with it. In this case, you'll need to run your line-level signal through an attenuator to step it down to mike level.

    If your camcorder or VCR will not allow audio dubbing, you really have just one option at your disposal--copying your tape. As you make your copy, intercept the audio from your source deck and run it into an audio mixer. Add in other sources as you copy, control their relative levels at the mixer, and you'll have a second-generation master tape with "dubbed" audio. The main drawback? Adding another video generation to your final tape.

    Two Tasty Tricks
    We videomakers working with consumer-level equipment sometimes have to trade off low price tags for skimpy post-production features. To be frank, until 8mm PCM and DV PCM audio hit the scene, the state of "audio editing" in consumer-level video was pretty dismal. If you're not currently using one of these formats, there are a few tricks you can do to make your dubbing tasks easier. One requires specific hardware, one does not.

    Those videomakers with certain S-VHS VCRs can perform a "loop-back" dub. This gives you access to the audio from the deck's hi-fi tracks while you're dubbing. The advantage of this mode is that it allows you to take the hi-fi audio, add in other elements with an audio mixer, and return the combined audio back to the linear track. Once all of your audio is on the linear track, you can video dub at will without breaking the continuity of your soundtrack.

    To perform this function with most decks, you'll need to toggle the audio monitor output (often using the remote) to hi-fi after engaging audio dub/pause. If you select anything but hi-fi only, you'll get feedback. Your manual probably won't mention this feature, so you'll need to experiment.

    Our second dubbing trick should work for everyone. Basically, the "trick" is to make a separate mix of all your non-synched audio elements onto another videotape. VHS-family hi-fi, 8mm-family AFM and PCM and DV digital audio all offer excellent stereo fidelity--why not use them as advanced audio recorders? If you mix all your elements onto one of these formats, even two-finger editors can perform a "video" edit with the two tapes. Engage the audio dub mode instead of record, and perform the edit like any other. If you have an edit controller, let it do the synchronizing work for you. Only folks with frame- accurate edit controllers should attempt audio dubbing of dialogue and other tightly synched sounds.

    The Fine Print
    Now that we've talked about how dubbing is supposed to work, let's cover some of the snags that can make it a real pain for a few of the older formats. Those of you working with 8mm- and VHS- family decks and camcorders--listen up!

    First, let's discuss VHS-family dubbing. The toughest challenge here is balancing the levels between the hi-fi tracks and your dubbed linear audio. Most decks will mix the hi-fi and linear tracks, but only in roughly equal proportions. What if your hi-fi tracks drown out your freshly dubbed voiceover, or the music you just laid down on the linear track obliterates your on-camera narrator? There's no way to adjust their relative balance on playback, so you're stuck with whatever you've got on tape. Because an auto gain circuit (AGC) controls the level of the linear audio track, you can't cheat the volume as you record. Some decks will allow you to adjust record levels of the hi-fi tracks, but most camcorders won't-- you can't cheat there, either.

    Solution #1: purchase a deck with a dedicated linear audio input/output. When you make copies of your master, you can adjust the relative hi-fi/linear balance through an audio mixer. Solution #2: retrofit an existing deck with a dedicated linear audio input/output (try Carlson-Strand, San Clemente, CA). Solution #3: try the loop-back dub technique described earlier to set the balance of your audio elements permanently.

    Which brings us to our second hi-fi snag. When you have a mix of hi-fi and linear audio elements, you have to be extremely careful to avoid playing back the same audio from the hi-fi and linear tracks simultaneously. Because these tracks are recorded in dramatically different fashion, they cause severe phase cancellation when played back together. Remember--a hi-fi camcorder or VCR is recording to the linear audio track as well as the hi-fi tracks unless instructed not to. With a potentially confusing mix of hi- fi, original linear audio and dubbed audio on one tape, you have to be diligent to keep everything sorted out correctly.

    Solution #1: perform a loop-back dub, then disable the hi-fi tracks when making subsequent copies. Solution #2: If you're going to do any audio dubbing on a given tape, record over the linear audio track for the length of the production. If you leave bits of original audio on the linear track, it will clash violently with the hi-fi tracks when playing back in mix mode.

    Our third snag lies in wait for both 8mm- and VHS-family videomakers, and it involves playing your master (or submaster) tape back in someone else's deck. If it's a VHS hi-fi deck, it will automatically detect your hi-fi tracks and switch off the linear track. If you've done any audio dubbing, you'll need to turn on the "mix" mode, and hope that the deck balances the hi-fi and linear audio tracks properly. Some decks will flip back and forth between linear and hi-fi audio monitoring almost at will, creating havoc with your audio.

    Those using a mix of 8mm-family AFM and PCM tracks face a similar problem. If the deck you're playing back on has an AFM/PCM balance control, it might not respond like your recording deck. And if it doesn't have this control, you're at the machine's mercy for how it mixes your precious audio tracks.

    Solution #1: control the mix yourself, by copying your tape to a "no brainer" format--VHS hi-fi or simple 8mm AFM. Don't try to avoid going down one more generation--the hassle isn't worth it. Solution #2: if you absolutely will not copy your tape, bring your own deck to the viewing (and don't forget the remote).

    Regardless of the format you use, you'll find the added control that audio dubbing gives you to be well worth the effort. Happy dubbing!

  • Issue: 

    Fri, 11/01/1996 - 12:00am