Standards conversion
Dec 1, 2007 12:00 PM, By David Tasker
Get content in the right frame rate every time.
Still picture management is also important. While it seems like an easier task, some technologies turn stills into a picture with moving bits. Even seemingly benign images can go to pieces, such as a still shot of a building exterior, with the windows in motion.
Moving roller credits are a particular challenge. It takes sophisticated motion estimation to account for the movement of small objects within a picture. Phase-correlated motion compensation can enable high-quality deinterlacing as well as precise, clean frame-rate conversion even for complex graphics, fast-motion sports, film and variable speed camera outputs.
Exceptional motion compensation is not just higher accuracy in creating pictures, but also how it makes mistakes. A mathematical byproduct of phase correlation is a reliability indicator that can tell the system when it is working effectively and when it must tread more carefully. This information provides a graceful fallback mechanism for concealing any errors the system inevitably will make.
Added image and audio
When motion-compensation standards conversion became available, phase correlation was so effective that often the only residual artifact for the viewer or clue for the downstream broadcaster was that cuts were no longer clean. Since then, different technologies have been developed to ensure clean video transitions between scenes and programs. Some solutions allow operators to choose the field dominance of the converter output. Prior to this, converters scrambled field dominance whether or not it was correct on the source.
The problem with fluctuating field dominance is that it becomes difficult to edit programs. It is also an issue with international program exchange. Post-conversion master editing is tricky when the field dominance isn't consistent.
Because most content is compressed somewhere within the delivery chain — whether on DVD or over a broadcast media — fluctuating field dominance is also a problem here. The issue makes it difficult for a downstream compression system to insert a single clean I-frame. The efficiency of both workflows — and the quality of the end product — can be compromised when standards conversion doesn't provide clean transitions.
Consider pixel accuracy
If content is shot using an interlaced camera and the content contains interlaced content on a field-by-field basis, the only transparent way to convert between formats is to use motion estimation to measure the movement between fields and compensate for the effects of the movement between them.
The first step is to use motion estimation to nullify the effects of any movement and to make sure that pixels within the input frame are aligned in time. Within this deinterlacing process, it is the motion estimator's job to deliver motion information that can be used to near-perfectly compensate for movement.
Another issue accompanying modern standards conversion is aspect ratio. The transition to a 16:9 aspect ratio is still relatively new to most U.S. broadcasters. This means production companies and broadcasters must deal with at least two aspect ratios. Fortunately, every standards converter made today includes built-in aspect ratio conversion with various preset and user-definable modes.
The audio side
Dealing effectively with audio also has become the standards converter's responsibility. Standards converters should accommodate 16 channels of audio, or eight AES pairs, and be able to resample audio and perform sample rate conversion from the input rate to the output rate. The Dolby audio standard is used extensively throughout the broadcast industry for multichannel surround. This means handling up to 16 channels filled with Dolby E, discrete 5.1 audio, an additional stereo mix and perhaps a second language or soundtrack information.
Dolby E brings its own requirements to conversion because it is locked to the incoming frame rate of video and must be decoded, recoded and relocked to ensure that the audio can be re-edited downstream without corrupting the Dolby E signal.
Product development
Investing in standards conversion technology is an important decision in the life of a broadcast, post or duplication facility. Advanced solutions today include comprehensive film tools for 23p, 24p, 25p, 30p and sF formats, along with 3Gb/s capabilities to handle 1080p. Standards conversion platforms are capable of operating within both the hardware or software domain.
Standards conversion issues do not disappear in the file-based workflow. Content must remain at the correct frame rate through the entire workflow right to the end of the consumption chain.
David Tasker is head of technical sales for Snell & Wilcox.
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