Re: [Discuss-gnuradio] Problem with 8-bit option and dropped samples

[Dan Halperin]

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Lisa Creer wrote:
> On 8/20/07, Eric Blossom <address@hidden> wrote:
>> On Mon, Aug 20, 2007 at 11:15:36PM -0400, Lisa Creer wrote:
>>> I'm testing the -8 option in various example programs, and haven't been
>>> successful in receiving valid data.  The following test resulted in a
>> very
>>> brief, flat line being displayed, then absolutely no data showed on the
>>> graph, but the program continued to run.
>>>
>>>  ./usrp_fft.py -f 100M -d 4 -8
>>> format = 0x288
>>> set_format = True
>> Have you tried increasing the gain?
> 
> 
> Yes, to no avail.
> 
> What daughterboard are you using?
> 
> 
> Basic Rx.
> 
> Do you have an antenna connected?  If so, what kind?
> 
> 
> Yes.  My antenna ignorance will become apparent here, it's what I'd call a
> basic antenna (non-telescoping), vertical antenna 14 inches tall  with a
> strong magnet on the bottom.

This sounds like it's the wrong length to pick up FM stations.

>> I decided to try decimating by 4 without specifying the 8-bit option, and
>> I
>>> received the 'uO' indicating that samples were dropped from the USRP to
>>> host, but data was displayed on the graph, and it was 16MHz
>> wide.  What's
>>> going on?
>>>
>>> ./usrp_fft.py -f 92.7M -d 4
>>> uOuOuOuOuOuOuOuOuOu
>> 64 MS/s / 4 * 4 bytes/S = 64MB/s.  This won't fit across the USB,
>> hence the continuous overruns.
> 
> 
> Wait, I thought the -8 option provided 8-bit  I and 8-bit Q across the USB.
> It sounds like you're saying the samples are still 4 bytes total (2 byte
> each I and Q). So the samples are converted to shorts AFTER they get to the
> host? Then how can we decimate by 4 when using the -8 option? How can we
> cram more data through the USB if it can only handle 32 MB/s?

This calculation is in reference to your immediately preceding run
without -8. With -8, the samples are indeed 2 bytes total, not 4.

- -Dan
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