I think we need to standardize on some terms. We're talking about data transmission down a pipe rather than broadcasting EM radiation, so "bandwidth" is a better term than "spectrum" or "frequencies". Each pipe provides a fixed amount of bandwidth, and within each pipe, regardless of the physical technology (coax versus fiber versus beamed microwave [sat]), the bandwidth is divided into channels of a fixed bandwidth each. And there's the rub: Uncompressed video needs at least 6 megabits/second (or its equivalent 6MHz analog channel) to transmit low-rez (NTSC/PAL) video. The pipe has a fixed amount of total bandwidth, and the more you can compress each channel the less bandwidth it consumes and thus the more channels you can pack into the fixed pipe...which is why it is an economic issue. Each type of provider uses a technology that gives it a certain amount of bandwidth, and adding more is expensive. They can essentially create more "product" at zero expense by cranking up the compression, quality be damned. Maintaining quality while adding channels means investing big bucks in new infrastructure.

BTW, resolution is another factor that the carriers can play with to wring more revenue from their systems. The broadcaster may transmit 1920x1080p, but nothing prevents the cable company from downsampling to a poorer resolution which requires less bandwidth.

Back to bits/second...The fact that each channel has a fixed bandwidth is why breakups and tiling are compression artifacts. The break up case, e.g. an explosion in which one frame is entirely different from the previous frame, instantaneously exceeds the available bandwidth--more bits have to be transmitted in 1/30th of a second than the channel will accommodate--and so you get breakup and pixelization (tiling). The more you choke the channel to wring more channels out of a fixed pipe, the more vulnerable each channel is to compression artifacts.

The physical media makes all the difference in whether cable or FIOS or satellite does a better job. Only fiber has the raw bandwidth to let each channel have a decent bandwidth and low compression, but here in North America and I'm pretty sure most of Europe, the companies don't take fiber all the way to the subscriber. In Japan and S. Korea they do, lucky bastards, but then they have the advantage of nationalized PTTs that aren't driven by beancounters to amortize the last bitter penny out of obsolete copper (Zap! Pow! Take that, Tom!).