ProVideo Coalition.com: Creating Motion Graphics: Keyframes by Chris & Trish Meyer

Just as there are different graphic and video file formats inside a computer, there are different audio formats outside of it. Unfortunately, most hardware is not as versatile as software, and cannot “export” and “import” a wide variety of formats. And sometimes, you can accidentally jam one format into another - with less than optimal results.

There are four hardware audio “formats” parameters to keep straight: number of channels, signal levels, wiring, and connectors. There is no hard-and-fast rule about what piece of gear uses what, and there are always exceptions and special cases. However, there are some overall trends, which I will touch on here. Beyond that, armed with this background information, you can debug almost any situation, as long as you have a few “file format converters” - adapter cables and connectors, along with a few black boxes - to help you along.

Channels

The number of channels is the easiest to sort out, so we’ll start there. The majority of audio we hear is stereo: It has left and right channels, corresponding to what our left and right ears are supposed to hear. Virtually all of the connections mentioned above are stereo, and therefore needed both channels routed from device to device. However, the occasional monophonic (i.e. single channel) signal does lurk out there - such as a single microphone, or the “monitor” inputs and outputs that exist on some higher-end video decks and monitors. On the other extreme is Dolby 5.1 surround sound, which contains front left, right, and center channels, back left and right, and finally a subwoofer channel (the “.1”).

Quite often, you will want to convert mono to stereo. To do that, all you need is a simple “splitter” cable that will duplicate the lone mono signal into faux left and right ones. That is what was needed to go from the mono monitor output of the BetaSP deck to the VHS deck’s input in the example on the previous page. Instead, an incompatible stereo connector was forced into the mono jack, resulting in only one channel getting through (and poorly at that).

On the other hand, note that you should never use a simple adapter cable to combine together the two channels of a stereo signal on its way to a single-channel mono input (i.e. from a VHS deck’s stereo outputs to the mono monitor input on some monitors). Always use some form of mixer or signal combiner for this.

Signal Levels

We hear sound as vibrations in the air. The larger the vibrations, the louder the sound. Sound is represented in wires as fluctuations in electricity that correspond to these vibrations. However, how a certain size of electrical fluctuation relates to a certain loudness of sound is open to interpretation - and there are a few different interpretations out there you have to be aware of.

The most common interpretation is usually referred to as “-10dB” or “line level” signals. Most VHS decks, CD players, computers , computer speakers, and home stereo amplifiers use this interpretation of how electricity relates to vibrating air. Therefore, when you connect together the audio lines between these various devices, they usually agree on how loud is loud, and things tend to work.

However, most professional audio and video equipment - BetaSP decks included (as well as 3/4” U-matic and even some professional VHS decks, plus higher end desktop video interface boxes) - have a different interpretation, often referred to as “+4dB” or “pro level” signals. These devices use a higher electrical level to correspond to the same “loudness” as line level devices. Connect a pro level piece of equipment to a line level device, and audio the pro device thinks is at a good level will be brutally loud to the line device - resulting in distortion. Conversely, run an audio line from a line device to a pro one, and a “good” level will suddenly seem far too quiet, often resulting in a lot of hiss and noise getting mixed in with the desired signal. This is what happens when you try to connect a BetaSP or 3/4” deck to a VHS deck or computer.

The format converter you need in a situation like this is a line level converter. The old-school device used in video studios was the Henry Matchbox, which runs about $190 from places like Markertek (click here to see one). It takes two pro level signals and converts them to two line level signals, and can also take two additional line level signals and convert them to pro level.

Another useful variation available from some musical instrument stores is the Ebtech Line Level Shifter (available in two versions: one that will take a pair of signals in either direction, and one that will take eight signals in either direction). If you are introducing a pro video deck into your desktop world, consider one of these devices to be a mandatory purchase to go along with it. For example, we originally used a Matchbox to make our Sony UVW-1800 BetaSP deck look like an ordinary line level device to everything else in our studio when we first got it; since we’ve grown, we now use an eight-channel Ebtech to convert between a variety of devices.

Level issues don’t stop there. Microphones generally use a much lower electrical level than either line or pro equipment, and usually need amplification to bring them up to a matching level. This is the main job performed by microphone preamplifiers, which are built into many audio mixers. In a pinch, you can sometimes get away with a “microphone transformer” of the kind sold by Shure and others. On the other hand, headphones and speakers usually use a much higher electrical level (in addition to a different “impedance” - another electrical interface issue we won’t even get into here), and in general should not be used in place of normal audio connections except in cases of emergency.

Monday, February 17, 1997

I Can’t Hear You: Correct Audio Connections

After we get our computers stable and video cables connected, it seems to be the audio wiring that befuddles us the most.

Different Formats

Channels

Signal Levels