Re: [Discuss-gnuradio] Question on DDC/USB interface

[Eric Blossom]

On Wed, May 04, 2005 at 10:21:07AM -0400, Anastasopoulos Achilleas wrote:

> How are the 4 complex streams out of the 4 DDCs multiplexed into the USB
> port?

Depending on the nchannels setting you either get 1, 2 or (someday 4)
DDC outputs muxed onto the USB.

Assume that the DDCs are numbered 0, 1, 2, 3.

* If you select one channel, you get only the output of DDC 0

* If you select two channels you get one complex sample from DDC 0,
  followed by one complex sample from DDC 1, etc

* If you select four channels you the one complex sample from DDC 0,
  one from DDC 1, one from DDC 2, one from DDC 3, etc.

> If the 4 ADCs sample at 64MSps, and each of the DDC's is downsampling by
> N, then there are 4 complex streams of 64/N MSps waiting to be transfered
> through the USB port.

> How do we control (if we can control) which of the 4 is transfered ?

See above.

Note that you control which A/D's are routed to the DDC inputs using
the mux control word.  See usrp_standard.h for docs.

Eric