There is merit to the concept that every mathematical operation on an image is potentially damaging to the data in the image. As one moves a photo from film to disc to computer to imag,setter, mathematical damage is done to that image at every conversion. The fewer conversions the image must go through, the better the image perhaps.
Each mathematical operation has the potential for rounding errors, and thus the introduction of noise into the image. Noise comes in the form of stray pixels that stand out from their neighbors and become exaggerated when the image is sharpened. Noise can be worsened by image compression and decompression, or enlargement through mathematical interpolation. In every case, the process of too many changes to a digital image cam be damaging to that image, and one should avoid it if possible.
Some of the process conversions for printing are less damaging than imagined and result in surprisingly good results. It is not always true that reducing the number of steps in a process makes the result better. For example, it is conceivable that an image could be converted from its scanned data into the YCC color space, and then converted to RGB and finally to CMYK for printing. In the conversion, some damage may be done to the image. But, at the same time, image enhancements - some possible only in RGB - can be made that make the image more pleasing and useful.
Some of the creative tools in Photoshop software and others may work only in RGB; the choice is more dependent on the software than on the hardware or the file form at.
If a group of images needs to be converted from Photo CD to CMYK, with little or no image modification along the way (color correction or creative work), then the best avenue is a direct conversion from YCC to CMYK through one of the software products that allows this.
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