A/V Timing Errors

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Jan 14, 2011 12:35 PM, By Ned Soseman

Are you the person who fields engineering office phone calls? If you are, then you've probably listened more than a couple of that proposed with these typical words: "There's something incorrect with your station." This is often followed by a few sort of accepting complaint. As the number of viewers examination DTV on considerable prosaic screens grows, you may be conference more complaints about lip-sync problems than you used to.

Lip-sync errors do not always beginning at the station. At the finish is the spectator and his TV system. Complaints about mouth sync may not always simulate a complaint with your hire or signal.

The next time you're in a big TV set tradesman or box house, step back and watch as many flat-screen HDTVs concurrently as possible. Typically, many of the screens are related to the same video source. As scenes change, watch keenly with your engineering eye and you may uncover that all the TVs do not always manifestation stage changes at precisely the same time. All prosaic screens routine video, and a few routine it with more latency than others. Some LCD and plasma displays can increase up to 3 frames of check by conducting digital processes, which, if the audio isn't behind to match, can emanate a important lip-sync problem.

Lip-sync problems turn more evident on incomparable screens, and they may be exacerbated by the viewer's audio wiring. For example, a flat-screen manifestation may be related with a DVI or HDMI line from a line or heavenly body set-top box (STB), whilst the analog audio outlay of the STB is related right away to a stereo system. If there is any video-processing check in possibly the STB or display, the audio will pierce forward of the video. Other home drama gizmos such as scalers or vigilance converters can moreover increase video delay.

Another non-static is the module smoothness way the spectator chooses to use. Although there is a direction toward more DTV off-air reception, there are still far more people examination local channels on line and heavenly body than those using antennas. Does your hire continually guard all the leading smoothness systems many expected used to take your station? Not all line and heavenly body providers pass every vigilance precisely as transmitted at its source.

The high quality your hire transmits probably means a great treat more to you and your station's advertisers than it does to a module provider traffic with hundreds of channels. It's not unusual, quite with basic-tier channels such as local stations, for a module provider to occupy information shrinking on the simple harvesting machine to set aside more bandwidth for reward channels. This routine can infrequently deliver more video delay. You should customarily approve that all module providers retransmitting your vigilance are carrying out so with the top high quality and accuracy, inclusive mouth sync.

What A/V timing errors are acceptable? Written in 2003, the ATSC IS-191 cabinet inform stated, "Under all functional situations, at the inputs to the DTV encoding devices, the sound module should be firmly synchronized to the video program. The sound module should never lead the video module by more than 15 milliseconds and should never loiter the video module by more than 45 milliseconds." More recently, the AES, SMPTE and IEEE have all shaped investigate groups and committees to investigate assorted standards for lip-sync blunder showing and analysis systems. Incidentally, the lip-sync standard in the movie industry, infrequently called "lip flap," is in addition to or minus 22ms, that is a lead or loiter of roughly one-half a support at 24fps.

What ordinarily is referred to as a 60i DTV support lasts 33.366ms (at a 59.94 margin rate). A margin is half that, or 16.68ms. Thus, if the video lags the audio by a singular field, it exceeds the endorsed limit. The limit adequate audio loiter is somewhat less than 3 fields. As an aside for your reference, the speed of sound is roughly 330y/s, or about 33ft/frame. Based on the ATSC cabinet close and movie attention standards, people may begin to understand a sound check when a sound-producing earthy event, such as a firecracker or the fissure of a ball bat, is celebrated at a stretch of as small as 18ft. Perhaps that's because people can endure more audio loiter than lead.

Several creditable institutions have researched this same check notice question, and all resulted in identical conclusions. As against to the earthy world, where sound check is commonplace, in DTV, the complaint is often the other way around.

There are many unintended ways a broadcaster, prolongation or post residence can pull video behind audio before the vigilance ever enters a recorder, DTV encoder or a essential element feed to a headend, that brings us to where the vigilance originates.

Video from a typical live CCD camera starts with at least one margin of built-in video check as the picture is captured, stored and digitally processed before it ever leaves the camera or is recorded. So similar to pushing 70mph in a 65 zone, you can often obtain away with it. Plug that camera in to a switcher and run it by a DVE, or punch it up on a switcher with frame-synched inputs, and you'll increase at least one support of check to the margin you already had. Now, you are pushing 85mph in a 65 and folks are going to notice.

The attribute between video and audio may be compromised every time the vigilance is encoded or decoded, or when digital signal-processing occurs. The complaint builds as signals gush by a hire or prolongation trickery and as more processing, conversion and placement gadgets are switched in or out of the vigilance chain.

There are several ways to address this issue. As with scarcely all in the world of announce engineering, the way that functions most appropriate for you and your trickery will essentially rely on the coercion of the complaint and size of your budget. Some services take the shotgun draw close and simply insert an audio check section downstream from chief control, dial in a couple of frames of check and call it a day. More veteran methods to authorize and manage lip-sync problems are widely separated in to two groups: out-of-service and in-service.

In the out-of-service group, apparatus must be offline for testing. It may be as simple as using an A/V source showing the applaud and sound of a clapperboard, and using your eyes and ears to 0 in A/V sync manually with an audio check device. This simple way is as biased as it is primitive, but it has worked for about 90 years, and infrequently it's all a bill can afford. It may moreover be the only assessment instrument existing at the time.

Many other out-of-service collection are existing to the announce attention written to tie in audio delays to video delays. Typically, these products are found in the fortitude of a modern facility's infrastructure, such as A/D conversion, audio processing, distribution, embedded audio, SD-HD conversion, synchronizers and video processors. Typically, out-of service methods are more frequently used in mobile prolongation vehicles, where distances and delays can change with scarcely every new setup but sojourn firm once set.

The in-service organisation of gadgets is written to occasionally or every time detect end-to-end lip-sync errors without interruption. One early thought was to use watermarking. Watermarks are dark visible and audio queues updated to the initial material intended to secure calm and safeguard it from without official authorization copying. The complaint with watermarking is that it can obtain mislaid in digital-processing equipment, size or aspect-ratio modifications, or digital encoding and decoding.

A sufficient more intelligent and strong in-service lip-sync monitoring thought has not long ago emerged, ordinarily referred to as fingerprinting. The fingerprinting technique has several names, inclusive digital signatures, A/V signatures, descriptors, information correlations, underline vectors, partial digests and strong hash. Generally, but not always, fingerprints are level-based and taken over a period of time.

Fingerprinting moreover has many uses, that include calm management, calm verification, ad insertion verification, media monitoring, cataloging and safety and robbery management. YouTube, for instance, uses fingerprinting technology to prevent lawsuits by safeguarding calm owners from without official authorization uploads. In addition to all these absolute solutions, fingerprinting moreover has the skill give powerful, nonintrusive, online, in-service solutions for lip-sync corroboration and management.

A local more aged server may be located at any remote place to compare the local source to the one sent to it, or a centralized more aged server may be used to hoop incomparable amounts of information that must be compared or to compare assorted outputs simultaneously. Each finish will require a fingerprint-generation system. A more aged server can gather fingerprint information over IP networks to perform lip-sync blunder showing and fairly portion delays over long distances.

While several manufacturers moreover sell addressable estimate apparatus with the skill to remotely arrange delays to put together identified lip-sync errors, services can moreover resolve problems with stand-alone, locally tranquil audio check devices, updated to the audio outlay of any device that delays the video. In the box of incomparable prolongation switchers with multi-part DVEs, with a small engineering, the itemize network may be used to manage an addressable audio check section to tie in the varying video delay.

One of the many splendid benefits of using nonintrusive fingerprint technology is that it may be used finish to finish in the actual world. You can portion A/V delays in your trickery from your college of music to your digital encoder with a high grade of accuracy. It can even be used to guard your duct and portion check at the outlay of a heavenly body STB, line STB or demodulator with identical accuracy. Then, when you are asked "What's the matter with you people?" you can reply with a confident, "The vigilance is leaving here OK."

Clearly, there is no subject the attention needs to settle an interoperable fingerprint standard. It's not the use of fingerprints that needs standardization, but, rather, the way used to produce a fingerprint. The SMPTE 22TV Lip Sync Ad Hoc Group is immediately questioning the probability of producing a SMPTE standard is to fingerprint vigilance and/or the methods of metadata carriage.

That being said, the earlier the attention can consent on a universal, open-source algorithm for A/V fingerprints, the better.

Martin Jolicoeur, product executive at Miranda Technologies and Norman Rouse, selling growth executive at Snell , contributed to this article.