does the bandwidth of the scope have to match the speed of what i am trying to measure? (sampling speed isn't enough?)
how does anyone create a hdmi-out device without an insanely expensive scope?
Can it be done without observing the signal to make sure you have it right/debug?
most of my project ideas have HDMI-out in them.
You need to ensure adequate signal integrity[1], and then it is probably sufficient to flip into the digital domain with logic analysers or printf statements.
Scope bandwidth has to exceed that of your signal. Don't forget that the probe and probing technique are part of the measurement system.
Sampling rate is irrelevant, and for signal integrity purposes can be arbitrarily low.
Make sure you understand how to capture and interpret eye diagrams.
[1] by design, or by test, or by using a pre-built module