Audio On Videotape

Back in 1987, TV Technology published an article that briefly traced the historical evolution of audio recording on videotape formats, and held that audio was clearly subordinate to video in early VTR designs.
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Back in 1987, TV Technology published an article that briefly traced the historical evolution of audio recording on videotape formats, and held that audio was clearly subordinate to video in early VTR designs. The first professional videotape format, two-inch quad, had but one broadcast-quality audio track, and its distortion, noise and frequency-response properties could be characterized as less than salutary.

When BTSC stereo television broadcasting became a reality in the mid-80's, audio in television broadcasting plants, as well as in television sets, received much-needed attention and improvement.

There were, at that time, a certain number of two-inch quad machines still in service. Most of these were of the cartridge variety, such as the venerable RCA TCR-100 and the Ampex ACR-25, which were principally used for playback of commercials, and, on particular cable networks, music videos.

These applications generated the need to modify these machines so that they could record and play back stereo sound. In the case of the TCR-100, at least, this was mainly accomplished by splitting the single audio track into two narrower tracks. The oldest professional video recording format was made to accommodate stereo sound this way.

The three-quarter inch U-Matic tape format had two audio tracks, but the tracks were not designed to be stereo, and their fidelity is worthy of no more than a passing mention.

The workhorse one-inch "Type C" format, which replaced two-inch as the broadcast standard, featured three audio tracks. Two were well-matched in performance and suitable for stereo. The third, typically used to record longitudinal timecode, was of lesser quality and was not designed to be used as one of a stereo pair.

The audio tracks of one-inch machines demonstrated improvements in signal-to-noise ratio and distortion over the two-inch predecessors; audio performance potential was subsequently enhanced with aftermarket audio noise reduction, either outboard or incorporated into audio-circuit assemblies that could be retrofitted to certain one-inch machines.

One-inch Type C machines with digital audio tracks appeared in the marketplace late in the format's lifespan, but not a great many of those were ever sold. By the time digital audio enhancement arrived on the scene, one-inch machines were being replaced with the next generation of video recording equipment.

HALF-INCH COMPONENT

The half-inch, component video formats that augmented or replaced one-inch machines in many installations, such as Beta and M-II, featured additional forward steps in the audio realm, albeit not without some compromises. Both Beta and M-II incorporated four audio tracks, but they were in the form of two matched pairs, not four identical tracks.

Both formats had a pair of linear audio tracks with integral noise reduction as well as an additional pair of matched audio tracks that employed technology borrowed from consumer videocassette recorders, FM audio recording.

These tracks carried audio signals that were frequency-modulated onto carriers, which were written onto the tape by the rotary video heads. Additionally, these audio signals were compressed upon recording and expanded upon playback, maximizing their signal-to-noise ratio. Provided that they were not overdriven, FM tracks produced very high-quality audio channels, with high signal-to-noise ratio and low distortion, but the tracks had two drawbacks--they were not phase-coherent with the linear audio tracks, and they could not be edited separately from the video.

Although both linear and FM tracks bore the proper timing relationship with the machine's video tracks to assure proper lip-sync, the tracks were not timed sufficiently to be phase-coincident with each other. If audio signals from the same source were recorded on both linear and FM tracks and subsequently mixed, comb-filtering and "swishing" sounds resulted. The second drawback to FM tracks was that they could only be recorded on tape simultaneously with video, making split audio/video edits impossible. In situations where one-time recording and playback was done, however, they provided excellent audio quality.

The next generation of broadcast videotape machines featured digital video and audio. These formats provided the best of all possible audio worlds: four identical digital audio tracks, offering the ability to mix, edit and "track bounce" among all four audio channels. They employed the professional 48 kHz sample rate, and at least 16-bit linear quantization. Among the benefits offered by digital audio recording are the absence of interchannel phase problems, very wide dynamic range (more than 90 dB for 16-bit quantization), very low distortion, and if the digital audio interface is used for dubbing and editing, virtually no generational noise or distortion buildup.

An added advantage is the ability to embed the digital audio data within the digital video signal, facilitating a single two-wire serial interface that carries both video and audio data.

Today, we are several quantum leaps ahead of yesterday's video and audio storage devices and technologies. It is well-recognized that audio quality is every bit as important as video quality, and the recording devices available today reflect this. Today's HD and SD videotape formats offer the capability of either four audio tracks at 24-bit quantization, or eight tracks at 16-bit quantization. The capability to record eight tracks is important when 5.1-channel sound is employed.

Although past video recording formats relegated the videotape recorder to the status of one of the weaker links in the television production and broadcast audio chain, today's digital video recording formats feature state-of-the-art audio sections. With these developments, the VTR is no longer the limiting factor in television audio quality.