Ask HN: Looking to purchase a software-defined radio, any advice?

As an introduction to signals, The Scientist and Engineer's Guide to Digital Signal Processing is pretty good(http://dspguide.com/) you should also read this woman's blog http://www.windytan.com/

In general Ettus is the best around unless you have a really large budget and want to get something with a backplane (which I highly recommend you avoid unless you have really specific requirements). Ettus is about to come out with a low cost B200 mini which could be a good option. I think it's so they can compete with the BladeRFs and HackRFs of the world. Ettus boards aren't perfect but they've really come a long way since the USRP2 days when I started using them. The UHD is also a really nice library to work with.

The RTLSDRs are definitely the best value (get one with a metal box around it). Also look for the HackRF Blue.

Finally, think hard about what you want to do. You mention decoding TV signals but the RTLSDRs can't do that (in America at least) since they can only do about 3 MHz of instantaneous bandwidth and TV signals are 6 MHz wide. You could try using 2 or 3 RTLSDRs for that but then you'd have put them all on the same clock, which can be difficult.

If you can spend around $1500 I'd go for a B200 with a GPSDO. GPSDOs let you accurately timestamp signals and time evens (i.e. start transmitting at some specific time). In my eyes the 210 isn't worth the bigger FPGA and dual transceivers. The isolation between the transceivers probably isn't great (but I could be wrong) and a bigger FPGA isn't worth anything to me.