Most of us know what we mean by “conversion.” In the transmission world, one may speak of “turning around” a signal from a C-band transponder to a Ku transponder. It might well be useful to think of digital conversion in the analog to digital conversion instance in the same context.
The science of analog to digital to analog conversion is one of taking a representation of reality (i.e. an image projected onto a sensor, which outputs a synchronous stream of electrons we think of as a continuous image). It is no less sampled than a digital video signal but, given infinite bandwidth, it is a good representation.
Digital representations can offer the same, for the difference is only a matter of how many bits it takes to represent reality. Theory shows that perhaps it takes 208 million bits uncompressed to represent a 525/625 image adequately. It would no doubt also show that there is no real advantage to doubling the number of bits, for the frequency being represented is less than half of the sampling frequency. In Figure 1, the same data is sub-sampled at one-fourth of the sampling frequency, with the predictable result that the reconstructed waveform is much harder to recognize as a sine wave.
Figure 1. When the same data is sub-sampled at one-fourth of the sampling frequency, the reconstructed waveform is much harder to recognize as a sine wave. Click here to see an enlarged diagram.
The number of bits is dependent upon just how much reality you want to represent. In Figure 1, doubling the sampling frequency might improve the result, but the difference would be slight. Clearly, perception becomes reality in digital.
Other conversions include encoding to compressed formats, color space conversions, scan format conversion, upconversion to HDTV formats or the reverse. It is representing data taken in one “universe” in a parallel universe. If we focus on the narrow definition, we miss the impact of the fact that we do conversions everyday and now cannot live without them.
In the context of more traditional usage, conversion is often used to denote simply the garden variety conversions between analog 525 or 625 video and their digital video equivalents. These conversions were once relatively expensive and now can be done for about the cost of an analog video DA — at monitoring-quality that is. High-quality conversion remains much more expensive, often topping $2000 for composite analog to component digital conversion. Some of the most critical tricks are in the filtering done in either the digital or analog domain.
Another important part of the conversion equation is removing the imprint of the composite color coding from the luminance signal without trashing the signal too badly. Component digital video looks fantastic, and it is the best way to input video to compression systems, which are inherently component as well. One special case of conversion is high-quality decoding and digital coding for exactly this purpose. Such a conversion might cost well above $2000, to the best part of $10,000 in the case of a conversion that includes noise reduction and other features.
On the other end of the scale is another conversion done quite often. Picture monitors tend to have expensive digital input converters. A great money-saving strategy is to use a converter intended for this purpose, which might cost well under $500 at retail. The output is typically Y,Pr,Pb components and is usually not intended for grade-one monitoring applications.
The form factor for converters has undergone considerable evolution, in part spurred by the silicon solutions available. Early converters occupied cards that took as much as 5RU per slot. They used discrete components and had extensive analog filtering. Now the components can easily fit in the volume of a small flashlight for monitoring converters, or even inside a patch panel! A converter with good quality 10-bit conversion now can fit on a card the size of a Palm Pilot.
HD conversions, while still not cheap, do not have to deal with composite signals. Thus, they are generally less complex in some strange twist of technical fate. But at the same time, analog HD is not of much use in the real world today, except as an input to a monitor.
John Luff is senior vice president of business development for AZCAR. To reach him, visit www.azcar.com.
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