QAM 64 is the STANDARD-USED MODE when you use SINGLE CHANNEL VIDEO MODULATORS as the bandwidth is more than 25 megabit/s more than enough for broadcast video quality

The QAM-64 is also very robust and not affected by cables, connectors, signal attenuation, noise, coax amplifiers distorsion, and typically runs for 30% more FT distance than QAM 256

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QAM 256 is typically used when you use multiple video programs in the same frequency AND is a more-delicate transmission as it can be easy-affected by cables and amplifiers distorsions

No need to use QAM 256 on MINIMOD-2 / MICROMOD-3 / VECOAX ULTRA unless your Televisions does not support QAM 64 ( all TVs support QAM 64 as standard main modulation )