[Discuss-gnuradio] confuse with the relation between ADC and DDC

[John Wu]

the ADC and DDC scheme is shown in:
http://swigerco.com/gnuradio/usrp_diagrams/mux_example_2.jpg

From what I know, DDC take a signal as an input and output the I and Q signals.
But in usrp scheme DDC take I and Q signals from ADC as inputs. So what does ddc do in usrp?