Re: [Discuss-gnuradio] I&Q on same or separate channels?

[George Nychis]

> This leads again to my question some time ago about how the USB data stream
> is organized. With the partial answer from that thread my understanding
> boils down to the following:

Our questions are on completely different waveforms :P  I'm talking 
about way before the FX2.


> However the last postulate gives me some headache. How do we make sure that
> the user process always fetches 512 byte blocks. 

The user application does not worry about this, the lower level 
interface that Eric has written to the USRP worries about this.  If you 
take a look at some of the code in usrp/host/lib/legacy, it is ensuring 
that all reads are 512 bytes else it will print an error and return -1, 
not performing the read.

> Can a crashing user process manage to read less then 512 bytes? 

Of course, but when the process is restarted the buffers reset in the 
USRP and FX2.

> What if a rogue process reads less then 512 bytes. 

The low level USRP USB drivers will fail the read.

> If the user process gets out of sync with the 512 bytes for some
> reason, is there any way to resynchronize?

The user process can "miss" 512-bytes if it's not reading the data fast 
enough and the buffers become full... this is an overrun.  The error 
will be reported to the host and the host has to live with it and move 
on with life.

- George