Inside Audio: Dave Moulton

The Truth About Stereo for TV

This column will be devoted to the basics of stereo for video, as part of my series on "the basics." Interestingly, stereo still isn't all that well understood in the video world. For the record, it is a powerful audio attribute that should neither be ignored nor carelessly used.

WHAT STEREO IS

Stereo refers to the practice of presenting audio information from two channels, usually both derived from separate microphones operating simultaneously in the same space. These two audio channels, when played back by matching loudspeakers and perceived by humans, yield a wonderful sensory quality of an alternative space and palpable "presence" in recordings, as well as quite remarkable and enjoyable "phantom images."

At the same time, a stereophonic presentation tends to mask defects in loudspeakers and the audio playback chain. These features add up to an audio modality that humans really enjoy. We do this intuitively, without really being aware of the stereophony as such. All we know is, "the music sounds really good."

This attribute of stereo makes it a very powerful tool for audio engineers to improve audio quality. The use of stereo in television is somewhat constrained. Most televisions have mediocre speakers that are poorly placed and underpowered for the purpose of generating compelling illusions. In my book, "Total Recording," I characterize the domestic stereo television as a boombox with video monitor added.

Meanwhile, the nature of television programming mitigates the elaborate production stereo detail work that we make so much of in music recording and production.

As I noted last month in my columnon reverb, all voice work tends to be bone-dry mono, and we have made it an aesthetic principle to not recover and transmit the acoustic ambience of the set in which a production is shot.

Further, from film practice, we have learned to not have voice-tracks bounce from left to right and back as the talent moves or as conversation takes place. That audio-source motion is distracting and even more troublesome when we cut from one camera to another.

The net result of this is that stereo is usually limited to, A) music beds, B) ambience for sports events, and C) the occasional live music-based production show. In short, stereo is used as an enhancer, a sonic perfume if you will, to sweeten production values but not to add realism or definition.

DOES STEREO TV WORK?

I personally think this sort of ersatz stereo works quite well, so long as we don't expect too much from it or invest it with mystical values. The music beds and occasionally poignant or dramatic stereo ambiences are quite entertaining, and they seldom mess anything up. Film and DVD sounds pretty good and thereare few annoyances. Meanwhile, it's simple and cheap to produce.

The secret here is not to expect too much and buy into the idea of spatial reality being represented by stereo, with all the attendant confusion and complication that entails. If we keep it simple and just use it as a perfume that underlies a mono presentation, it's hard to go wrong.

MONO COMPATIBILITY

The one wrinkle in the ointment is mono compatibility. For a variety of technical reasons, it is often necessary for a stereo signal to be "summed" to mono, which is to say, the left and right channels are mixed together to yield a single monaural signal.

This happens automatically in any legacy TV or small mono TV set; it is not under the control of anybody in the production chain. Therefore, it is essential that the audio sound good in both stereo and mono, simultaneously and all the time.

This can present a problem. One of the more powerful ingredients in stereophony has to do with small time differences (from 0.1 to 35 milliseconds) between channels. When used well, these delays can add a powerful sense of spaciousness to the recording.

However, when summed to mono, such delays result in a very annoying timbral coloration called "comb filtering," which strongly detracts from the audio quality. Mostly, we've learned how to head it off, but I still occasionally hear it in some NFL and other sports broadcasts, where crowd noise is picked up by two or more mics some feet apart and inadvertently summed to mono, or else some time offset is happening to one of the stereo channels down the processing path.

The solution to this problem is to stick to coincident stereo microphone configurations and to carefully maintain the integrity of the stereo audio signal pair during transmission. Good housekeeping, as my mother used to say. That's all it is.

WHAT DOES IT ALL MEAN?

Stereo can be your friend. Without causing much trouble, it can easily enhance your productions. Use it in music beds for occasional ambience (when you add reverb, for instance), and in real musical and dramatic productions, where it is a key element.

Pay attention to the stereo quality of your music beds, learn to hear the difference between stereo and "summed-to-mono," and reap the benefits that stereo will bestow on your productions. Don't get caught up in worrying about realism. Don't try to make stereo happen in ENG or documentary production, except where it is really called for.

Next month, we'll end this Basics series with a similar look at multichannel production. Uh-oh!

Thanks for listening.

Dave Moulton is either on the left or on the right. Sometimes he's even a phantom image! You can complain to him about anything at his Web site, www.moultonlabs.com.