HackRF Shell: Conversion to Floating Point

As expected, the bottle neck has disappeared at the floating point conversion. At least, I can say that I didn’t have trouble pulling 8 million samples per second (msps), which with 32 bit floating point (and 2 floating point numbers per sample) is 64 MB/sec. (I am short on time this evening, so I haven’t had a chance yet to try 20 msps.) As before, I fed the data into a Gnu Radio FM demodulator and got clean FM radio station audio out of it.

GCC 6.3 with -O3 appears to do some SSE optimization on the byte buffer to float buffer conversion:

christopher@nightshade:~/Repos/hackrf-shell$ objdump hackrf-shell -x -D | less
    172c:       66 0f 6f c1             movdqa %xmm1,%xmm0
    1730:       66 0f 68 cc             punpckhbw %xmm4,%xmm1
    1734:       66 0f 60 c4             punpcklbw %xmm4,%xmm0
    1738:       66 0f 6f f1             movdqa %xmm1,%xmm6
    173c:       66 0f 65 e8             pcmpgtw %xmm0,%xmm5
    1740:       66 0f 6f f8             movdqa %xmm0,%xmm7
    1744:       66 0f 61 fd             punpcklwd %xmm5,%xmm7
    1748:       66 0f 69 c5             punpckhwd %xmm5,%xmm0
    174c:       66 0f 6f eb             movdqa %xmm3,%xmm5
    1750:       0f 5b ff                cvtdq2ps %xmm7,%xmm7
    1753:       66 0f 65 e9             pcmpgtw %xmm1,%xmm5
    1757:       0f 17 3c 24             movhps %xmm7,(%rsp)

The next thing, perhaps, should be to create a little demo program where it captures the data at certain times of day. Or I could work on the next stages of an FM receiver, i.e., frequency multiplication, a low pass filter, and the FM demodulator.

2 thoughts on “HackRF Shell: Conversion to Floating Point”

  1. Great! 8 msps is phenomenal! At 64 MB per second, I can see that the captured data can get pretty big, pretty fast.


  2. HackRF SDR will provide up to 20 msps, i.e., 160 MB/s in floating point values. The samples are actually provided by HackRF as unsigned 8-bit values (0-255) which is more space efficient than a 32-bit floating point value. But, I would suppose, it is more sensible to convert to floating point values, with a range of 0 to 1, so that downstream processing functions would not have to be re-written if the SDR later became a 12-bit device or a 16-bit device. Also, if you are feeding data to Gnu Radio, Gnu Radio expects floating point values.

    Edit: 80 MB/s -> 160 MB/s


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