[Discuss-gnuradio] Question about "extra" header

[Bennett, David S. (Scott)]

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