Re: Generating Scalar Data from Complex Data

John E. Hadstate wrote:

"Jerry Avins" <jya@xxxxxxxx> wrote in message news:HLidnVdruvDfvgHanZ2dnUVZ_h6hnZ2d@xxxxxxxxxx
John E. Hadstate wrote:

Even though this method works well, I have some concerns. Upsampling involves the generation of high-order images which have to be filtered out before frequency translation. Filtering is a somewhat inexact process that is subject to several "engineering approximations", not the least of which is selection of the filter. Therefore, we must be somewhat altering the generated scalar data so that it isn't exactly the same as the complex data from which it originated.

Should we conclude from this that conversion between scalar and quadrature data representations will always involve some loss of information?

The sampled information is not exact; quantization error limits it. You don't say how many bits are used, Let's in any case assume that the converter is perfectly linear, so that the maximum error is +/- 1/2 LSB. With 16-bit data, that amounts to about 8 ppm. It won't be easy, but you can filter the images that well (-100 dB).


I don't think quantization or sampling enters this picture. I'm talking about a stream of digital information being represented in scalar or complex form, irrespective of where it came from or by what process, and about converting from one representation to another.

You had concern that image vestiges would corrupt the data. I tried to point out that also corrupts the data, and if the effect of quantization exceeds the effect that images have, nothing of substance is lost. It's understandable to mourn a loss of perfection. Recognizing that the original data were imperfect ought to ease the pain.

Jerry
--
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