To clarify, I am interested in making my file sampling rate at 125 k S/s and
running the USRP at 250 kS/s. I am using a resampler block in between the file
source and usrp sink. The file source reads data from a fifo file and external
modulator code writes to that file.
Thanks, Tom
________________________________
From: Tom Hendrick <[email protected]>
To: Tom Rondeau <[email protected]>
Cc: "[email protected]" <[email protected]>
Sent: Wednesday, February 6, 2013 2:54 PM
Subject: Re: [Discuss-gnuradio] Changing Nout_items and Buffer Settings for
GNURadio 3.6.2
Hi Tom,
I appreciate your response and information on setting noutput_items for version
3.6.2. I am not sure if changing the noutput_items or buffer size is going to
help at all.
Basically when the data rate is lower than the transmission rate (purposely to
eliminate latency), I'm noticing that the USRP is outputing random gaps in my
packet of data during some of the underruns. Some of the underruns generate a
wider gap between packets (this is good) but some of them generate a gap inside
the packet data blocks causing problems. This happens when I am using a 125kHz
sampling frequency and the USRP's rate set lower than 1M S/s. However with the
USRP at 1M S/s it works fine. My goal is to run the USRP at 250k S/s to
reduce processor load.
For the 125k S/s case and lower
than 1 M S/s USRP rate, if I increase my packet size by a factor of two, the
gaps caused by underruns happen only between packets not inside them. The
problem is the increase in packet size is not desirable for my application.
The behavior I observed makes me think the issue could be fixed with changing
the Nout_items or buffer settings. Any idea if that might be true?
Thank you, Tom
________________________________
From: Tom Rondeau <[email protected]>
To: Tom Hendrick <[email protected]>
Cc: "[email protected]" <[email protected]>
Sent: Wednesday, February 6, 2013 2:36 PM
Subject: Re: [Discuss-gnuradio] Changing Nout_items and Buffer Settings for
GNURadio 3.6.2
On Wed, Feb 6, 2013 at 1:34 PM, Tom Hendrick <[email protected]> wrote:
Hello,
>
>
>I think its possible some of the underrun issues I mentioned in my other posts
>(where random gaps occur inside a block of data sent to the USRP when set the
>USRP sampling frequency any lower than 1 MHz,) may be solved by tweaking
>nout_items and buffer related settings.
>
>
>For GNURadio 3.6.2, Is there a simple way to modify the python code generated
>by GRC in order to change those settings? I had seen instructions for doing
>this with 3.6.3 but I think they don't apply to the 3.6.2 version. I can
>consider upgrading to 3.6.3 if there isn't an easy solution.
>
>
>Thank you, Tom
You can tweak the max noutput_items in 3.6.2. You can pass a value to the top
block object when you run start/run that will set the maximum value
noutput_items can be.
In 3.6.3, you now have the ability to change the maximum noutput_items on a per
block basis with block.set_max_noutput_items (this might be doable in 3.6.2; I
forget when we added it). You can also change the buffer sizes of a block
before you start the flowgraph with set_max/min_output_buffer.
There should be very little concern going between 3.6.2 and 3.6.3. We maintain
API compatibility between those versions.
But, having said all of this, I'm not sure this is really going to help you.
Are you thinking of decreasing the buffer sizes or the max noutput_items?
That's actually going to hurt you computationally. It will lower latency, but
it means less time inside a work function and more overhead in the scheduler,
moving things around, and context switching. Larger buffers mean more time in
work for each block, which is when you get the most power.
Tom
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