[Discuss-gnuradio] Sample size and data rates

[Aadil Volkwin]

Hi,

I have modified the usrp_wfm_rc.py script to capture samples of an FM
signal and write them to disk, and a second script to demodulate and
playback the samples.

The scripts are available here: http://rrsg.ee.uct.ac.za/members/aadil/

I have 3 questions:

1) 1 000 000 samples writes a file 8 000 000 bytes (7.6Mb)
This suggests each sample is 8bytes; 4bytes I and 4bytes Q.
Is my assumption correct?
Considering that the ADC produces 12bit samples, I would expect that
each I and Q sample pair would be 4bytes. 2bytes I and 2bytes Q.
Please can someone advise where i've missed some understanding.

2) With reference to the above question, where can I find further
details regarding the function of the gr.sizeof_item identifiers, in
particular gr.sizeof_gr_complex.

3)If my file is 1 000 000 samples and my audio rate is 32KHz( I
understand this to be 32K samples/second) I expect my playback time to
be (1e6/32e3) = approx 30s, however when I play the file I get only
approx 3s playback time.
Im not sure where i've gone wrong.

Thanks for your time, and I look forward to and appreciate your advice.

Aadil