Digital Modulation onto Electromagnetic waves

[WarrickF]

Hi Guys,

I was hoping someone could explain why commercial satellite uplinks (like those used in TV broadcast) are able to achieve such high bandwidth rates vs. radio uplinks like HAM OSCAR uplinks.

I don’t completely understand how digital modulation is done onto radio wave, but at 1st glance on would think that since all electromagnetic waves travel at the same speed, that you’d be able to modulate an equal amount of data into the wave.

I understand that the frequency must play a role, but one would imagine that the form of modulation in use could be modified to accommodate the frequency in use and achieve the same rate.

I’m clearly missing something though as I’m sure someone would have done this by now.

Thanks
Warrick

 

[berkeman]

Here's a good intro to modulation schemes at wikipedia.org:

http://en.wikipedia.org/wiki/Modulation

What are the carrier frequency and data rate of the HAM OSCAR link? Most HAM stuff that I deal with (voice and packet) is at audio rates, without any compression to speak of.