Re: [Discuss-gnuradio] Welcome and brief update

[John E. Perry]

Marshall White wrote:

> The answer is that you must have a "tuner" that takes the signal at 107.5 MHz and 
> "moves" it down
> to a much lower frequency (called an IF, or intermediate frequency).
>
> If you have a 40 MHz sampling rate, any IF less than 20 MHz should more or less 
> be okay.
>
> If you had a 400 kHz sampling rate, then your IF would have to be CENTERED at 
> 100 kHz.
>
> Marshall
Why does everyone insist on using an if?  I seem to recall from my radio 
classes (admittedly quite a few years ago) that direct-to-baseband IQ 
conversion keeps all the modulation information present in the signal.  

For all voice and program broadcast signals the bandwidth is well within 
even low-end PC sound cards.  Phase-locked loops are not trivial, but 
neither are they hard, to build. Essentially all common signals except 
hi-fi FM and video signals could be decoded in practically any low-end 
PC, couldn't they?

What am I not understanding?

John Perry