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[Discuss-gnuradio] Re: delay through dsp pipeline

From: Patrik Eliardsson
Subject: [Discuss-gnuradio] Re: delay through dsp pipeline
Date: Thu, 13 Aug 2009 08:38:58 +0200

The DDC in the dsp_rx pipeline is build with several filters. Depending on the decimation rate you choose different filter configurations. See the FAQ regarding DDC and decimation values.
I guess you want to sample the source from the antenna synchronous in both usrp2s. The case is the that the dsp_core_rx is reset at start up of the usrp2. So the receiver will start to fill their pipeline at different points of time. To get rid of this problem we have used the 1PPS-signal to reset the DDC in both usrp2 at the same point of time. Take a look in the dsp_core_rx.v file.
To start receiving the samples synchronous we then just polled the timestamp and when we received a sample with timestamp = 0 we started our process.
-Patrik Eliardsson

Från: address@hidden [mailto:address@hidden För Pham, Thanh
Skickat: den 12 augusti 2009 21:45
Till: address@hidden
Ämne: [Discuss-gnuradio] delay through dsp pipeline


In several discussions, I recall it was mentioned that the timestamp reference is at the end of the dsp pipeline. Is the dsp pipeline consisting just of the DDC or anything else? 

It was also mentioned that the delay through the dsp pipeline depending on some factors such as decimation rate. If we have multiple usrps2 using the same decimation rate, center freq, what other factors would contribute to the difference in arrival of the signal at the end of the dsp pipeline? (if two usrps2 are receiving from the same antenna and using a 10MHZ gps reference signal).

Is there a way to figure out the delay from the ADC to end of the dsp pipeline?

I also saw a mention about a patch for the start_rx_stream_at, which tells the usrps2 to start streaming at a specific time. Can someone elaborate on that some more? Whose time does the usrp2 based on to do that, and does that ensure two usrp2 start streaming at the same time?

Thanks for any inputs!



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