Re: [Discuss-gnuradio] How to capture fixed length burst of samples (to avoid overflow)

[Zhonghua]

On 03/01/2012 12:22 PM, Rickard Radio wrote:
> Hi list,
>
> Two questions:
> a)  I would like to know the easiest way (of low complexity) to capture a 
> certain length/number of samples (or time period), preferably in gnuradio, and 
> then automagically turn off the uhd-reception (and save received samples to 
> disk) ?!
>
> b)  Are there any other (kernel, uhd, gnuradio) buffers and settings than "net.core.wmem_max" and 
> "net.core.rmem_max"  (set with "sysctl" command in Linux) which affect transmit and 
> receive buffering for uhd/gnuradio ?
>
> Background: I am capturing bursts of samples at very high sampling rate (like 25 MSPS @ 
> 16 bit I&Q, or 50 MSPS @ 8 bit I&Q) and save them to disk for further offline 
> processing. But for now I have to manually cancel the application (CTRL-C) after a few 
> seconds to avoid overflow since writing to a normal HDD does not keep up for saving in 
> continuos real time (which anyway quickly creates too large files). Also, I have tried 
> to maximize receive (and transmit) buffering (through mentioned buffers) in order to 
> capture (and buffer) as long sequences as possible in RAM before saving them to disk 
> and get the inevitable overflows (resulting in loss of received samples). However, to 
> avoid overflow without manual interception I would like to capture just as much as the 
> RAM-buffers can hold (or less) and then automatically turn off the reception so that 
> the burst of samples can be saved to disk without any overflows and loss of samples.  
> Feels like it should be an easy way to accomplish
>    this but how ?!   FYI: Hardware is USRP N210.
>
> Any help appreciated!
>
> / Rickard
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Hi Rickard,

Maybe you can use the new module of 'stream_selector' provided by great 
Josh to control your stream. Some simpler ways can also achieve that 
purpose, such as using 'lock' to control your stream or just set your 
receiver's gain dynamically. You can bypass the received data to a 
vector sink simultaneously when you receive them to a buffer, and 
calculate the length of this vector sink. Once the length meets your 
requirement, you stop the receiving stream and save these data. After 
that open the stream again.
I don't know could that help you or not.

Good lucky,

Zhonghua