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The soundtrack on a video movie is just as important as the pictures, and itís a whole lot easier to change. Even simple techniques, like adding background music, can transform your productionsÖ



Sound is a vital ingredient in movie-making, but how many of us make proper use of it? A lot of camcorder owners are content to stick with whatever the microphone picks up, however irrelevant, incoherent or distracting that may be. Unfortunately bog-standard natural soundtracks tend to be a bit boring, but it doesnít have to be like that. Thereís no excuse, itís easy to do something about it and youíve probably already got most of the equipment youíre going to need.


However, before you start messing around with soundtracks willy-nilly itís helpful to know a little bit about what youíre going to be dealing with. At the last count there were half a dozen different sound recording systems in use on domestic camcorders and VCRs. In order of importance and occurrence they are VHS linear, VHS hi-fi, 8mm mono, 8mm stereo and PCM sound, used on both analogue and digital equipment.



VHS mono linear is at the top of the list because itís the most common, itís the format standard, and can be found on all VHS and Super VHS recordings, whatever else is on the tape; itís also the easiest to change, but the weakest in terms of quality. The mono linear soundtrack is recorded along the bottom edge of the tape, by a stationary recording head. Because of the low tape speed the bandwidth, or the range of frequencies it can handle is quite narrow, from around 75Hz to 8kHz Ė and thatís on a good day. Being mono makes it sound a bit flat, it can also be rather noisy, though that usually only becomes apparent when thereís a lull in the soundtrack. Nevertheless, itís fine for speech and incidental sounds; it can be replaced or Ďdubbedí and mixed with other sounds easily and without affecting the picture, (on VCRs with an audio dub facility). Incidentally, some early VHS video recorders split the mono linear track into two, for stereo, however the quality was very poor, and needless to say it didnít catch on.


VHS hi-fi sound is a major improvement over VHS mono linear. Itís in stereo and the bandwidth is similar to Compact Disc, covering a frequency range of 20Hz to 20kHz, or roughly that of our own hearing. It can be a bit hissy; the amount of background noise varies from machine to machine and it is also affected by the quality of the tape. An extra pair of recording heads, mounted on the spinning head drum, records stereo sound along with the video signal. It uses a technique called depth frequency multiplexing or DFM. To cut a long story short, audio signals are modulated onto a FM carrier, operating at a frequency below that of the video signals and they end up buried deep in the tapeís magnetic layer. A fraction of a second later it is overwritten by the higher frequency vision signals, which stay close to the top surface of the tape. The by now weak audio signals have to be read through the video signals, which is where a lot of the noise comes from. Because the two signals occupy the same physical space they cannot be separated Ė at least not easily Ė so hi-fi soundtracks cannot be changed, without affecting the video.  


8mm and Hi8 camcorders use a slightly different system to DFM; moreover the same rotary heads records both video and audio signals. Nevertheless the end result is the same and like VHS hi-fi, 8mm sound cannot be dubbed or changed, after it has been recorded on the tape. Originally the 8mm ĎAFMí soundtrack was mono, with a bandwidth extending from 20Hz to15kHz; stereo AFM was developed in the late 1980ís and is now featured on all but the cheapest machines. The bandwidth is slightly narrower, and thereís a certain amount of background hiss though in comparison with VHS mono linear the quality is still very good.


A couple of semi professional 8mm and Hi8 camcorders, plus all DVC digital camcorders use a pulse code modulation or PCM sound recording system. In the case of 8mm/Hi8 equipment PCM is in addition to format-standard AFM soundtracks. PCM has actually been around for some time; it was even featured on one portable Betamax deck back in the early 1980ís. PCM digital audio has the advantage of a wide, flat response Ė similar to VHS hi-fi Ė but with very low levels of background noise. It is very robust and can be easily dubbed as it occupies a separate part of the tape, though not all DVC machines have an audio dub facility.


PCM audio data is normally written immediately after the video track, by the spinning recording heads. PCM quality varies according to the recording scheme, which in turn depends on the number of bits used to represent audio data, plus the type and amount of compression used, to squeeze the information into the space available. Generally speaking even the most basic PCM systems out-perform analogue recording systems, when it comes to frequency handling and background noise. 



Now youíve got the measure of the beast itís time to consider what you can do with it. In common with video editing, the process begins before you press the stop/start button on your camcorder. Think about the sounds that you are about to record. How can you improve the quality? Simple things, like monitoring sound as it is recorded with a pair of headphones, can help avoid a lot of mistakes. Donít shoot into wind, get closer to your subject or use an external microphone. Theyíre simple steps that cost nothing and can help to reduce the need to mess around with the soundtrack. 


The simplest form of editing is the audio dub. It doesnít require any special equipment, other than a suitably equipped camcorder or VCR, some connecting leads and a sound source. This can be a hi-fi, personal stereo or a microphone. Though unless the VCR has a microphone input, youíll need a simple audio mixer as well Ė more on that in a moment.


Audio dubbing is fairly limited and a bit brutal in that it can only replace the VHS mono linear Ė weíll assume youíre not using a high-end equipment with dubbable PCM sound Ė but it is still a very useful technique. Itís the easiest way to add a musical backing, ambient sounds like a Ďwildtrackí (crowd noises etc.), or voice-over to a previously assembled recording, but only when the finished copy has been made on a stereo VCR. The tape in question has three soundtracks; there are two hi-fi tracks Ė they are untouchable -- and the mono soundtrack that can be replaced Ė with music or a commentary. When the dubbed tape is played on a stereo VCR, the mono track can be mixed with the original sound on the stereo hi-fi tracks.  Itís worth bearing in mind that this technique is not advisable on tapes with only a mono soundtrack, unless you are prepared to sacrifice the original sound, or the VCR has a microphone mix or sound-on sound facility; very few do.


In addition to a VCR with an audio dub function, you will need one or more of the following: a set of audio connecting leads, a microphone Ė if you want to add speech Ė and a simple mixer. Audio mixers are cheap; prices start at less than £20, but make sure you get a stereo model Ė even if you are editing mono-on sound at this stage. It should have at least three inputs, two for line-level signals from the audio source (hi-fi, VCR, camcorder etc.), and one microphone input. Most mixers have a bank of faders, to control input and output levels, one or two exotic models have tone or equalisation controls, though they rarely needed and itís not really worth paying any extra for.


The connections are very straightforward. If you just want to replace the mono soundtrack with music, connect the audio output of a hi-fi system or personal stereo to the VCRís audio input. Itís worth making a short test recording first and experiment with the output level or volume, and input level on the VCR (if it has one), to get the desired effect. Connecting an audio mixer between the VCR and source component will make life a lot easier, it also means you can mix sounds from two or more sources, including a microphone.


Audio dub on a camcorder sometimes comes in handy if the original sound is too poor to be of use. On S/VHS/C machines itís usual for the new audio to have to come from the microphone, or via the external microphone socket, and that can cause difficulties. Microphone inputs require a high-impedance feed, connecting it to the output of a personal stereo or hi-fi will at best produce badly distorted sound, at worst it could blow the camcorderís microphone input stage. If you want to dub using line-level signals you will have to get hold of a microphone mixer, or an external microphone with on-board mixing facilities



Editing sound and video together involves a little extra work, but itís worth the effort, and itís a lot more flexible than simple dubbing. It also means you can mix in sound effects, dub over voices or cut out unwanted sounds, all without loosing the original soundtrack. You wonít need a lot of extra equipment to get started, a simple audio mixer, or mixing facilities on a video processor/editor is usually sufficient.


Once again the basic set-up is simple. The mixer connects between the sound sources Ė normally a camcorder or VCR with the master tape thatís being edited plus a CD player, tape deck or microphone  -- and the recording VCR; this time the new sound is recorded at the same time as the video. Plan whatís going to happen on each scene. If youíre using a storyboard, mark down what you want to appear on the soundtrack and make a note of any critical times or events. Always do a run-through first, to make sure the levels are correct, and practice timings, if youíre going to add in a special effect, do a voice-over or cue in music. Carefully monitor whatís happening on headphones; donít wait until itís on tape, if you get it wrong Ė and you will -- youíll only have to go and do it all again.


Try to avoid abrupt changes in the soundtrack, theyíre the audio equivalent of the visual jump-cut, where rapid scenes change can be very distracting. Itís liable to occur when thereís continuous background music, or a sudden change in volume. Audio jumps can be avoided with judicious use of the faders on the mixer. 


There is a bit of a knack to successful audio editing and your first few efforts will probably sound pretty awful. However, itís not a difficult skill to acquire. It doesnít require any fancy or expensive equipment, just some practice, and a little imagination.




It is becoming obvious that in a few years time, powerful PCs with fast digital video processing capabilities will replace traditional editing methods, but present-day machines already have an important role to play in audio editing. Even modestly equipped PCs can be used as sophisticated multi-track audio tape recorders, precisely mixing sounds, timed to coincide with events on tape, or the computer screen. Theyíre also good at creating new, original sounds, either from scratch, or samples of other sounds. Editing audio clips on-screen is quick and easy, and way beyond what is possible using conventional tape recorders and mixing equipment.  Unlike video, which is devilishly difficult to process on a computer, most recent PCs have stereo audio inputs and outputs, and since audio processed in the digital domain, the quality can be excellent.



” R. Maybury 1998 2601



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