[USRP-users] Questions on USRP N200 data streaming

Marcus Müller marcus.mueller at ettus.com
Wed Sep 17 10:29:23 EDT 2014


Hi Nicholas,

over your cable you send 2x16bit, but you chose 2xfloat32 complex as GNU
Radio stream format (the link between the usrp source and the file
sink), and that's exactly what is stored in the file. I'm not quite sure
what I can tell you about the difference between both data formats --
one is two floats between (-1;1) constituting real and imaginary part,
the other is 16 bit integers doing the same within their value range.

Now, for less data than you are expecting there's a lot of possible
explanation. Maybe you underestimate the time your flowgraph needs to
get started; maybe you're seeing a lot of overflows and just miss half
of the samples.

I suggest using a head block, and watching the console output for "O"
messages.

Greetings,
Marcus

On 17.09.2014 16:17, Nicholas Hu wrote:
> Hi Marcus,
>
> Thank you for your reply. Could you please clarify this for me? Are you
> saying that in my streaming settings, I am actually sending 12.5e6 S/s
>  with 32 bit I and 32 bit Q in every sample through my 1 gigabit Ethernet
> cable ? If this is the case, why am I only getting ~500 MBytes instead of
> ~1 GBytes in 10 seconds? And could you also explain the difference between
> the wire data format (complex int16) and the host  data format (complex 32
> float)?
>
> Thanks again,
>
> Nicholas
>
> On Wed, Sep 17, 2014 at 9:08 AM, Marcus Müller <usrp-users at lists.ettus.com>
> wrote:
>
>>  The In-GNU Radio "blue" data format is complex float32, so it's 64b == 8B
>> per file sample; the complex int16 format is converted between the cable
>> and the GNU Radio USRP source output port to complex float. So:
>> 8B*12.5e6 S/s * 10s = 100e6B* 10S = 1e9B ~= 1GB.
>> You probably want to use "output type"="complex int16" in the USRP source,
>> but you would also need to write your own Matlab loader (which is not hard
>> to do using the fread method).
>>
>> You don't really need GNU Radio to just dump samples to a file; UHD comes
>> with tools like uhd_rx_cfile that do exactly that.
>>
>> Greetings,
>> Marcus
>>
>>
>> On 16.09.2014 17:43, Nicholas Hu via USRP-users wrote:
>>
>> Hi,
>> I am working on a USRP project. I am using a USRP N200 with a BasicRX
>> daughter board to perform data streaming. I am using GRC 3.6.4.1 on a
>> Ubuntu 12.04 machine. The USRP N200 is set to have a decimation factor of 8
>> to achieve a streaming rate of 12.5 MSPS, the daughter board is set to A:A
>> to perform single channel real sampling. The I Q data wire format is set to
>> be 16 integer each, and the host data format is set to be complex float 32.
>> The settings above are also in the attachments. For analyzing data, I am
>> using Matlab R2013b on Windows 7. My question is: when I am streaming data
>> for about 10 seconds, I am getting a file around 500,000,000 bytes which is
>> reasonable ((16 bits/sample I+16 bits/sample Q)*(10 seconds)*12.5 MSPS*(1/8
>> bytes/bits)=500 Mbytes), but when I use read_complex_binary.m provided by
>> GNUradio to read the file in Matlab, I am only getting half of the expected
>> samples (~62,500,000 samples) instead of 125,000,000 samples. Could anyone
>> please explain why this is happening? Doesn't 16 bits I and 16 bits Q wire
>> format give me 32 bits per sample? Any help would be appreciated.
>>
>>
>>
>>
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