Television standards conversion
Phase Correlation is perhaps the most computationally complex of the general algorithms.
Phase Correlation's success lies in the fact that it is effective with coping with rapid motion and random motion. Phase Correlation doesn't easily get confused by rotating or twirling objects that confuse most other kinds of systems converters.
Phase Correlation is elegant as well as technically and conceptually complex. Its successful operation is derived by performing a Fourier Transform to each field of video.
A Fast Fourier Transform (FFT) is an algorithm which deals with the transformation of discrete values (in this case image pixels).
When applied to a sample of finite values, a Fast Fourier Transform expresses any changes (motion) in terms of frequency components.
What is the advantage of using FFTs over simply trying to predict the motion vector on a pixel by pixel basis?
- Mathematically, it's far easier and faster to recognize and process frequency signatures from which very accurate motion vectors can then be calculated.
- Rather than having to measure where every pixel goes from frame to frame the FFT rather results in representing just the changes from one frame to the next.
Since the result of the FFT represents only the inter-frame changes in terms of frequency distribution, there's far less data that has to be processed in order to calculate the motion vectors.
- Unlike other motion vector calculating methods, the FFT technique is not easily fooled by objects that have rotational or spiraling motions.
- What results from the FFT is a three dimensional frequency distribution represented mathematically by peaks in a three dimensional wave pattern.
- The 3rd dimension in this coordinate system represents subsequent fields of video.
In summation: Objects in motion can be mathematically correlated to their peaks in the frequency distribution. Once the FFT is performed it becomes a computationally simple matter for the computer to track just the peaks and assign them the appropriate motion vectors. This conversion technique is both elegant and computationally involved. Sophisticated software and large amounts of processor "horsepower" are required for these complex computations.
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PAL & NTSC conversion for HDTV broadcasting
A digital television adapter, (DTA), or digital-to-analog converter (box), is a device that receives, by means of an antenna, a digital television (DTV) transmission, and converts that signal into an analog television signal that can be received and displayed on an analog television.
These boxes cheaply implement HDTV (16:9 at 720 or 1080) to (NTSC or PAL at 4:3). Very little is known about the specific conversion technologies used by these converter boxes in the PAL and NTSC zones.
As most of the time downconversion is required, very little image quality loss is perceived by viewers at the recommended viewing distance with most TV sets.
See also (Americas)
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Offline conversion
A lot of cross format television conversion is done offline. There are several DVD packages that offer offline PAL <--> NTSC conversion -- including cross conversion (technically MPEG <--> DTV) from the myriad of MPEG based web video formats.
Cross conversion can use any and methods commonly in use for TV system format conversion, but typically (in order to reduce complexity and memory use) -- it is left up to the CODECs to do the conversion.
Most modern DVDs are converted from 525 <--> 625 lines in this way, as it is very economical for most programming that originates at EDTV resolution.
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See also
- Reverse Standards Conversion
- IEEE papers on systems conversion
- AES/EBU papers on systems conversions
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