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Re: [Discuss-gnuradio] Question of tx_ampl range: [0, 1) or [0, 2^15/2)

From: Josh Blum
Subject: Re: [Discuss-gnuradio] Question of tx_ampl range: [0, 1) or [0, 2^15/2) ?
Date: Mon, 08 Nov 2010 01:07:21 -0800
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20101027 Thunderbird/3.1.6

In transmit_path.py ( ..->  packet_transmitter ->  Multiply Const ->  USRP
Sink),  I see that the range of Multiply Const is [0, 1).

However, in one of the GRC example usrp_tx_dpsk.grc (Random Source ->  DPSK2
Mod ->  Multiply Const ->  USRP Sink), Multiply_Const = tx_ampl*p2p/2 , where
p2p = 2^15.

There is a multiply_const(2**15) in the transmit_path example. See generic_usrp.py.

The gr-usrp sink always takes samples between +/-2**15. See the docs in the USRP sink block in GRC.

Does anyone know why the Multiply_Const is set to tx_ampl*p2p/2 and what
does p2p refer to?

It means peak to peak and the purpose is to scale the signal to the expected range of the usrp sink block. I believe that p2p should actually be 2**16 in this case, but it works.


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