Re: [Discuss-gnuradio] Question about "extra" header

[Tom Rondeau]

On Thu, Oct 10, 2013 at 9:39 AM, Bennett, David S. (Scott)
<address@hidden> wrote:
> I’m using the file_meta_sink to record time-stamped data, and I have a
> question about the implementation of this block.  I currently call it in
> Python as follows:
>
>
>
> self.blocks_file_meta_sink_0 = blocks.file_meta_sink(gr.sizeof_short*2,
> self.dat_filename, samp_rate, 1, blocks.GR_FILE_SHORT, False,
> 2*int(samp_rate), "", False)
>
>
>
> If I understand the parameters right, I have configured this block so as to
> not include extra header information.
>
>
>
> If I also understand correctly, there are (by default, if no extras) eight
> fields provided in the header.
>
>
>
> However, when I run my Python flowgraph and read the header information from
> the created file using gr_read_file_metadata.py, I get the following:
>
>
>
> HEADER 0
>
> Version Number: 0
>
> Sample Rate: 10000000.00 sps
>
> Seconds: 1381406414.235846
>
> Item size: 4
>
> Data Type: short (1)
>
> Complex? False
>
> Header Length: 171 bytes
>
> Extra Length:  22
>
> Extra Header?  True
>
> Size of Data: 80000000 bytes
>
>               20000000 items
>
>
>
> Extra Header:
>
> rx_freq:
>
> 8.99e+08
>
>
>
> As you can see, a ninth field is returned as an extra header, rx_freq.  As
> far as I can tell in my flowgraph, I’m not doing anything myself that would
> cause this field to be added.  I’d like to know where this field is added
> since I’d like to add more.  Specifically I’d like to add a GPS string or
> two to tag location and velocity.  I’m using a USRP N210 with GPSDO and
> querying the GPGGA, GPRMC, and XYZSP strings to get this information.  I can
> currently parse these strings and write what I want from them to another
> file, but if I can contain it all in one file it would be easier for me to
> deal with off-line.
>
>
>
> Thanks,
>
> Scott Bennett
>
> JHU/APL


The extra_dict argument to the constructor is just a way to add more
data to the extra part of the header; any more data you want to add
that wouldn't be in there normally.

The extra part of the header collects any tags observed, so the USRP
will generate tags like rx_freq that will go into the header.

The metadata section of the manual has a pretty good explanation of
how it works:

http://gnuradio.org/doc/doxygen/page_metadata.html

Tom