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Re: [Discuss-gnuradio] GRC squelch block with separate control signal?

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] GRC squelch block with separate control signal?
Date: Thu, 8 Sep 2011 10:22:33 -0400

On Wed, Sep 7, 2011 at 7:58 AM, John Ackermann N8UR <address@hidden> wrote:

Tom Rondeau said the following on 09/06/2011 11:28 AM:
On Tue, Sep 6, 2011 at 10:22 AM, John Ackermann N8UR <address@hidden
<mailto:address@hidden>> wrote:

   I want to implement the equivalent of a carrier-operated relay -- a
   squelch block that provides an "on" signal at a threshold input
   signal level, which then passes or blocks data at some point further
   downstream rather than right at the point where the sensing is

   Are there GRC blocks that will allow this?



Yes, look in "Level Controls," there is a "Simple Squelch," "Standard
Squelch," and "Power Squelch" as well as a simple "Threshold." You'll
have to look closer into this blocks to find out how they work.


Hi Tom --

The challenge is that the three squelch blocks all sense and switch the data stream at the same point -- the input signal is passed or not based on its level.  What I need instead is to sense the stream at one point, and use that information to switch a different data stream.

I could picture two blocks -- one that sets (and updates) a boolean variable based on the current signal level, and another that switches a separate sample stream based on whether that variable is true or false.  Does GRC support message passing between blocks in that fashion?

The practical example is an FM repeater where the transmitted signal is turned on only when the received signal level exceeds a threshold.



You can use a message for this, or you can use the stream tagging interface where a message is tagged specifically to a sample number. It's a way of passing synchronous messages where the message passing interface doesn't.

The problem is, you really have to work inside of the C++ code to get to the stream tags, so you won't be able to work in GRC for it.


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