DDC SDRs do as you have described nickcarr, but sound card based SDRs (like the one in this thread) send the I/Q to the computer in the form of L/R audio inputs. And then the ADC in the sound card brings the data to the computer for processing.
Sound card based SDRs are a lot of value for the money. For about the cost of a mid market portable you can get a sound card based SDR that will outperform pretty much every portable on the market in several ways. In many ways it can perform at a level of most entry desk top SW receivers, such as the Icom R-75 or the Alinco DX-R8T, in some ways, such as filter settings and filter performance, it can exceed these radios. Typically the desktops will have a slight MDS or sensitivity advantage over the sound card based SDRs though. But the average user might not notice this sensitivity delta.
However, sound card SDRs do not perform at the same level as a DDC (direct digital conversion) SDR (such as the RFSpace SDR-IQ, SDR-IP, NetSDR, Microtelecom Perseus, WinRadio Excalibur, etc). Particularly in areas such as dynamic range, close signal rejection, and image rejection, the DDC SDR will be significantly better. This is primarily caused by channel to channel phase relationship issues on sound cards, even the best sound card is not designed to tightly control the phase relationship across frequency that would help the sound card SDR.
Also, the best sound card and sound card SDR combination cannot give you the instantaneous bandwidth of the lowest end DDC SDRs, and not even 1/20 the bandwidth of higher end DDC SDRs. Typically the sound card SDR yields something less than 190 kHz of range, with 90-100 kHz being a common max width, while the lowest cost DDC SDR yields 192 kHz, and bandwidths of 4 MHz are possible at reasonable prices.
Still, even with the shortcomings of the sound card based SDR most of them perform at a level that is quite good, and would not have been possible for ten times the money a few years ago.
T!
