@Fred Rodrigues
Thanks and no worries, I try to read posts giving the benefit of the doubt to people. Written words can be easily misunderstood and taken in the wrong way, and unfortunately examples can easily be found everywhere. On that note, I have made a few observations of my own to share that hopefully won't come across offensive to you or others as well. Here we go:
Lets beak down what the OP said,
jefflee wrote:Hi, I just purchased the ATEM Television Studio and plan to connect two cameras via HDMI cables for our elementary school's assemblies.
To me this says that they have a simple setup that and they want a simple & economical solution for connecting the cameras to the switcher.
jefflee wrote:I'm having a problem where I can only get a good signal from my cameras when I use a really short (6ft) HDMI cord, but all my longer cords flash in and out.
This says to me that they have tried a few cords already that haven't worked reliably further than 6ft. Note that they have already gotten them to work via HDMI into the ATEM as well.
jefflee wrote:Can anyone help me with a (hopefully cheap) solution where I can have my cameras 20-30 ft from the switcher and get a reliable signal using HDMI cords?
The answer is yes there are cables that should work that I have had positive personal experience with that I believe would suit this application well. I answered because everyone was saying that SDI is the only way to go and I believe HDMI is another possible solution to consider also. Some thoughts that apply to both:
1) You are going to be dealing with said issues of HDMI ports & cables any way you go because of the camera, so complaints in this area are moot. If you don't want HDMI issues, get an SDI camera!
2) Properly secured and relieved cables should give decent service regardless of connection type.
3) BNC ports are just as easily trashed by abuse just like anything else, Ive seen my share of BNC's ripped from cameras & gear by someone not paying attention.
4) Many pieces of BMD gear are picky about signal regardless of connection type, I have dealt with this issue several times.
Some thoughts on the HDMI->SDI converter box transmission method mentioned by others:
If you plan on taking the cameras further than 50-60 ft. cable wise away from the switcher, then this is going to be the cheapest way to do it and will allow for good flexibility in the distance area and signal reliability hands down. There are some extras to know about that are necessary for doing it this way.
1) Flexibility can bring complexity, sometimes unnecessarily. In this case you have double the connections to be concerned with, double the gear, not to mention the weakest link of most electronics, another power supply. Not necessarily bad things, just need to be aware of them.
2) It needs to be powered from somewhere near the camera if you want to minimize HDMI cordage. Power options range from having an A/C cord with an extra outlet for the converter to powering it via camera battery or its own battery.
3) If you need to repair a BNC cable properly, you will need the proper end for the cable you are using(not all coax is the same!!), and the proper tools (stripper & crimper) & the know-how to install that end successfully in minutes (all cost extra $). There could be a "tool less" end out there, but I have not seen or looked for one yet. If you know of one, I might buy a few for using in a pinch until proper ends can be used later. A "tool less" end may make this point moot, but being "tool less" may compromise the ends ability to make a proper connection or be more limited in the scope of cables it would fit.
4) Coax cables are decently resilient to being run over by heavy objects
5) Spools of coax are readily available locally, ends to fit it and/or tools for those ends may be another story.
Some thoughts on the HDMI cable method:
If you are not taking the cameras beyond 50-60 ft. cable length from the switcher, then this could be a very simple and inexpensive option. It requires no extra things to deal with, plug one end in the camera and the other in the switcher and you're done.
1) More limited in distance from the switcher than SDI.
2) Unknown resiliency to being run over by heavy objects.
3) If the cable gets damaged, you either get it warrantied or buy a new one. No repairs.
4) In the case of redmere cables, they are unidirectional (they are well labeled). Not necessarily a bad thing, just good to keep in mind.
5) Only one cable is needed to be run to the camera and requires no external power source. The camera is free to be powered by any available means necessary.
6) Outside spec? the cables are made to go these lengths.
7) The 50ft redmere cables we have are roughly the size of rg6 (a tad bigger). However, flexibility is more akin to a standard HDMI cable of the same diameter, so its pretty decent to work with.
IMHO, you could not do much better than either of these options. However in this case, going via redmere's are a perfectly viable option that will likely meet the OP's needs with simplicity/cost savings and should not be so casually dismissed.