The whole point of the converter boxes is to tune and convert ATSC digital to the NTSC format.
The modulator usually has a 2 channel output, 3 or 4, That is what pipes to analog TVs.
Usually equipped with analog video as well as digital out.
The old NTSC format used vestigial sideband for transmission and was susceptible to noise, signal fade, weak signal noise, and ghosting(multipath distortions)
Digital improves on that in that weak signal or severe multipath distortion, causes excessive missed or uncorrectable packets and the video decoder just goes blank, it works good, or not at all.
The 'blockies' is a marginal signal where display segments are droppeed but still decodes packets.
>>So you know, every level of today's ATSC broadcast is digital from camera/audio to your display monitor. The studio, the link to the transmitter, the transmitter to your home.
Even before the switch, much was digital up to the transmitter.