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Tektronix's AVDC100: Lip-sync error correction

Broadcasters today have a variety of issues to deal with, all of which require a great deal of their time and consideration. However, in an era where the cost of coming in behind a competitor can be huge, being first to market with a quality product is critical.

That's where maintaining the audio-to-video timing relationship of a television production can be so important and one of the biggest challenges to delivering a quality viewing experience to the customer.

Even in the "all-digital" facility, analog and digital often coexist in a hybrid arrangement requiring some level of separate video and audio signal processing. Routing considerations are also involved that, if not properly accomplished, can result in a mismatch of the video and audio timing. Timing variations in the network and video processing delays can be another source of lip-sync errors in programming generated outside of the broadcast facility and then later backhauled to the studio for final editing and distribution.

A new solution to the problem of monitoring and correcting lip-sync errors caused by factors outside of the broadcast plant is now available from Tektronix with the AVDC100 audio-to-video delay monitor and correction product. The AVDC100 is a totally new concept in controlling lip-sync errors. Imagine if, at the point of program creation, a time reference representing the program audio could be embedded in the live video signal. This embedded audio reference would then provide a permanent reference for later retiming of the audio and video signals in the event lip-sync errors developed during distribution of the program. This is exactly what the Tektronix AVDC100 accomplishes with the use of digital video watermarking technology.

Digital watermarking Digital watermarking is a well-known buzzword for a technology that is most often associated with the identification and copyright protection of intellectual property on the Internet. Originally developed in the mid-90s to protect still images, it is now being applied to other digital media including web music, DVD and digital television broadcasts. Basic digital watermarking technology uses an arrangement of low-level patterns, representing digital bits, to encode extremely low-level ID information. This pattern embedded within a video signal creates an identification mark that's invisible to the viewer but easily decoded by the copyright owner for positive proof of ownership. The AVDC100 uses digital watermarking technology to record the desired audio-to-video timing relationship by encoding the program video signal with an audio reference code derived from the audio program's natural envelope signature. At any point downstream another AVDC100 can decode the digital watermark from the program video and recover the audio timing reference for comparison to the program audio signal. Time shifts between the watermarked audio timing reference and the original audio signal indicate lip-sync error. The system measures the lip-sync error and uses the measurement to control an internal audio delay circuit to ensure constant and proper alignment of the program audio and video signals as referenced at the point of original watermarking. The system is designed with ample processing power for lip-sync monitoring and correction in real time and in service because, unlike current methods for measuring A/V delay, it uses no interfering audio test chirps or video test signals that disrupt the live program material.

Ease of use A common scenario for the occurrence of lip-sync error is in the use of remote production trucks, which gather program content and then compress the video signal for RF transport back to the studio facility for final editing and distribution. More often than not the compression is limited to the video signal and thus produces latency in the video signal with respect to the audio signal. Additionally, the studio frame synchronizer, which is used to lock the incoming ENG signal to the studio master reference, adds another video field or two of delay during its processing step. If the video signal is not constantly retimed to the audio signal, lip-sync error can occur that can degrade the program quality considerably if severe enough. Attempts at manually monitoring and correcting this problem usually meet with limited success because of the subjective nature of the process and the constant attention it requires from a qualified engineer. Engineers have much more productive tasks on which to be focused.

A more plausible solution for this scenario involves two systems automatically monitoring and correcting for unexpected lip-sync errors as they occur in real time. (See Figure 1.) The first system is configured for encoder mode and installed in the remote production truck at the output of the router. Note that the system is configurable for both encoder and decoder in a single package, so there's no confusion over encoder and decoder functions. The second system is configured for decode and lip-sync correct mode and installed at the studio at the output of the IRD just after the studio frame synchronizer. The system installed in the studio detects watermarked video and embedded audio time reference for retiming of the audio and video programs to their original state. Usually, the video is delayed relative to the audio due to the processing latency in the MPEG codecs. This is referred to as "audio advanced to video." The AVDC100 is capable of correcting up to 30 fields or 1/2 second of variable lip-sync error when the video is delayed relative to the audio.

A slim 1/2RU-wide, 1RU-high packaging provides a compact, functional solution for maintaining lip-sync during remote production activities. (See Figure 2.) Once installed and configured for either encode or decode mode, all processing is entirely automatic with instrument status instantly available from the two-line LCD panel and front panel LEDs. Another useful benefit is simultaneous video and audio status monitoring (i.e., loss of signal, format checking). This is possible because of the constant, real-time video and audio processing occurring in the unit during normal operation.

When used properly, the Tektronix AVDC100 provides an automatic solution to the problem of controlling lip-sync errors and monitoring video and audio status, assuring a quality product in an increasingly competitive marketplace.

For more information on Tektronix's AVDC100, circle (455) on Free Info Card.