...My mistake, I still don't quiet fully get what the bit rating relates to colourwise - 8x8x8x8x8x8x8x8? or 8 to power of 8, of 8 divided by 2 times O/S bit rate multiplied by frequency or whatever colours.
Mb, Gb, G/M-htz etc I understand...
Do not index with any sort of dithering in gimp, select 'none', otherwise the colours will be grainier & have random white spots or other colours across convered image - BRIGHTER doesn't work well for me - lack o remembering user settings.