-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 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 complex<float>.
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 gr.file_source(gr.sizeof_char,"file") -> gr.char_to_float() -> gr.float_to_short() -> gr.interleaved_short_to_complex() 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 Macs). > 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. No idea > 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 * scale?" > 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. Signed. Good luck! Dan -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFHTzlty9GYuuMoUJ4RAvDdAJ0ez/tzvBN4WYBM54dqsus2jSbo3gCgm3pv ot0SaQ5gcSorrETliLcaulI= =t79U -----END PGP SIGNATURE----- _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
