[Discuss-gnuradio] Radio astronomy question

[Andrew Rich]

How does a radio telescope make a picture ? 

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Andrew Rich

On 14/11/2011, at 14:10, "Marcus D. Leech" <address@hidden> wrote:

> On 11/13/2011 11:00 PM, Josh Blum wrote:
>> It looks like the ofdm blocks are using a fair amount of multiply and
>> multiply const blocks:
>> 
>>> gr-digital$ find -name "*ofdm*" | xargs grep mult
>>> ./python/ofdm_sync_ml.py:        self.mixer = gr.multiply_cc();
>>> ./python/ofdm_sync_ml.py:        # The output theta of the correlator above 
>>> is multiplied with this correlation to
>>> ./python/ofdm_sync_ml.py:        self.mul = gr.multiply_ff()
>>> ./python/ofdm_receiver.py:            self.chan_filt = 
>>> gr.multiply_const_cc(1.0)
>>> ./python/ofdm_receiver.py:        self.sigmix = gr.multiply_cc()
>>> ./python/ofdm_sync_pn.py:        self.corr = gr.multiply_cc();
>>> ./python/ofdm_sync_pn.py:        self.square = gr.multiply_ff()
>>> ./python/ofdm_packet_utils.py:    to get to a multiple of 8.
>>> ./python/ofdm_packet_utils.py:    # pad to multiple of 8
>>> ./python/ofdm_packet_utils.py:    up being a multiple of 512 bytes when 
>>> sent across the USB.  We
>>> ./python/ofdm_packet_utils.py:    is a multiple of 128 samples.
>>> ./python/ofdm.py:        @param pad_for_usrp: If true, packets are padded 
>>> such that they end up a multiple of 128 samples
>>> ./python/ofdm.py:        self.scale = gr.multiply_const_cc(1.0 / 
>>> math.sqrt(self._fft_length))
>>> ./python/ofdm_sync_pnac.py:        self.corr = gr.multiply_cc();
>> I bet many of these multiply consts could be simplified out. But, doing
>> little things like replacing the multiplier implementation with one
>> optimized with the SIMD unit makes a big difference, especially on a arm
>> where NEON>>  FPU.
>> 
>> So, there is a multiply and multiply const for floats that has been
>> optimized in my gr-basic branch. You may want to try using those blocks
>> instead. http://gnuradio.org/cgit/jblum.git/log/?h=gr_basic
>> 
>> -Josh
> The main transform in an OFDM transmitter, though, is an inverse FFT, and I 
> assume those blocks use the built-in FFTW implementation,
>  rather than doing their own.
> 
> -- 
> Marcus Leech
> Principal Investigator
> Shirleys Bay Radio Astronomy Consortium
> http://www.sbrac.org
> 
> 
> 
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