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RE: [Discuss-gnuradio] Output of usrp_rx_cfile and Input of ATSC demodul
RE: [Discuss-gnuradio] Output of usrp_rx_cfile and Input of ATSC demodulator
Thu, 29 Nov 2007 17:51:30 -0500
Do you know if the USRP is in fact little endian? I wrote a little C
program to test the endian-ness of my machine (which I am sure everyone
did in CS 101) and its big endian.
Endian-ness is based purely off the processor and not the OS right?
Sorry for the somewhat non-related SDR question, I generally work with
Java, so I sometimes forget about those silly low level details :).
From: Dan Halperin [mailto:address@hidden
Sent: Thursday, November 29, 2007 4:13 PM
To: Wuest Brandon-WTVR47
Subject: Re: [Discuss-gnuradio] Output of usrp_rx_cfile and Input of
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Wuest Brandon-WTVR47 wrote:
> I am having a problems with disk writing not being able to keep up
> with the high data rate, so I am trying to do a little manipulation of
> the IQ data to get around this. What I am trying to do it get 8-bit
> samples from the usrp (which my disk can keep up with) and then before
> I feed that in to interp.py, I convert every pair of bytes to a
A logical approach if your disk can't handle 32MBps linear write.
I'll answer question #0 first:
For converting pairs of bytes to complex, this might seem a bit (really,
a lot) silly, but a simple way to do it in GNU Radio would be
You could also write a gr.interleaved_char_to_complex() block (which
appears to not exist in gnuradio-core/src/lib/general) which would be a
MUCH better solution but would require effort :-D.
> 1. What is going on with byte order? From looking at the output of
> usrp_rx_cfile.py, it looks like everything is little endian, but if
> this is the case, I do not see where the ATSC demodulator converts
> little endian to big endian to ensure for cross platform compatibility
> (the GNU Radio website does indicate that this is possible to run on
> windows and I have not found any information regarding byte order
> concerns). I am running Ubuntu on an x86 processor which is big
> endian and it sounds like most users machines are big endian, so is
> there some conversion going on or am I just reading the output of
> usrp_rx_cfile.py incorrectly (when set to stream complex floats)? Or
> is it that the USRP does send down floats in little endian and the
> ATSC demodulator expects that the input is of the same byte order of
the machine it was compiled?
Source files are assumed to be in local-Endian, and sink files are
created local-Endian. Cross-platform is in the eye of the user :-D.
Also, I challenge your assertion that Ubuntu on an x86 processor is big
Endian. Endian-ness is a function of the architecture (except some crazy
and/or cool chips that have an Endian-switch like I think some older
> 2. What are the float values expected by the ATSC demodulator? In
> other words, what is the expected range of each float? Should they
> all be in the range (-1,1) or (0,1) or (min float value, max float
> value) or something else? It looks like the floats are all bounded to
> 2 bytes and the higher two bytes are just not used.
> 3. When converting an 8-bit sample to a float, do I need to scale it
> or just cast it to a float? So if I have a byte with the value of
> 100, could I just something like "scale = 100/255; i = FLOAT_MAX *
> Looking at some of the conversion methods included, it seems that a
> char gets simply casted to a float, but I believe this would have a
> negative effect since it causes all the amplitudes to appear weak.
Usually you'd just cast to a float. The demodulator should presumably
handle low-amplitude signals. But see my answer to (2).
> 4. When I specify 8-bit samples from usrp_rx_cfile.py, are these bytes
> signed or unsigned? I find this important because if I want to scale
> each byte to a float, instead of just casting, I would possibly need
> to do some bit masking to account for this.
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