Re: [Discuss-gnuradio] different decimation rates on one USRP

[Matt Ettus]

Brent M. Ledvina wrote:
>
> On Sep 4, 2007, at 2:11 PM, Matt Ettus wrote:
>
>> Brent M. Ledvina wrote:
>>> Hi,
>>>
>>> I am running into a USB bandwidth issue. I'm sampling data
>>> simultaneously from 2 daughter boards at a decimation rate of 10. This
>>> produces 2*6.4 MS/s * 4 bytes/I and Q sample = 48 MB/s.
>>
>> Actually, the correct calculation of data rate is:
>>
>> 6.4 MS/s * 2 * 2 = 25.6MB/s
>>
>> One factor of 2 is for I and Q.  The other is for 2 bytes per sample,
>> since 16 bit samples are sent over the USB.  They are converted to
>> floats on the host computer.
>>
>
> So, for two daughter boards, I'd expect twice your calculation: 51.2
> MB/s.

Yes, that is correct.

>>> My application requires one board to have a decimation rate of 10, but
>>> the other can have a decimation rate of ~30. My understanding of the
>>> USRP is that you cannot easily (i.e. in software) use different
>>> decimation rates for different daughter boards on the same USRP. My
>>> questions are: 1.) Is this easy to do at the FPGA level? 2.) Has
>>> anyone tried this before?
>>
>> It is not possible with the default FPGA build to use different rates
>> for 2 RX streams.  The TX and RX can have different rates though.
>>
>
> In your judgement, how challenging is it to modify the FPGA build? I
> could tolerate hardwired decimation rates.

You need to come up with some sort of packing order for the data, and
then unpack in the host.  The changes won't be hard, but debugging will
be...

It is possible that the inband signaling may make this easier.

Matt