Hello Josh,
The wav file has float values in the -1 to 1 range. In the old GRC script I
had used a constant multiplier of (2**15-1) to get the correct amplitude.
In the newer GRC script I took that out since it was already in the correct
range.
When I check the output of the LFTX with an oscilloscope, they both look
identical except that I see the blip/noise (which I think is from underrun)
show up right at the start of the transmission. I have added zero padding to
the signal. This is for an audio recording project and the blip/noise causes
problems on the receive side because I am listening continuously for the
incoming signal and the blip causes unwanted effects.
Any other suggestions?
Thanks so much for your responses, - Tom
________________________________
From: Josh Blum <j...@ettus.com>
To: discuss-gnuradio@gnu.org
Sent: Tuesday, November 13, 2012 9:50 PM
Subject: Re: [Discuss-gnuradio] UHD Underrun with Wav File Source and USRP Sink
On 11/12/2012 02:10 PM, Tom Hendrick wrote:
>
>
> Hello Josh,
>
> Thanks for the suggestion. Yes I had zero padded the file with about 1
> second of zero signal prior to the signal I want to transmit. I had still
> seen the blip/noise when running the script even when there was 1 second of
> zero padded signal. Is there anything else I should try?
>
Well, then perhaps it not the result of an underflow.
What is your transmit amplitude? In the old libusrp1 floats were
+/-2**15, but now they are +/-1.0 fullscale. So you must have had to
adjust the amplitude of the baseband samples; but are they scaled small
enough to avoid truncation?
-josh
> Thanks, -Tom
>
>
> ________________________________
> From: Josh Blum <j...@ettus.com>
> To: discuss-gnuradio@gnu.org
> Sent: Monday, November 12, 2012 1:03 PM
> Subject: Re: [Discuss-gnuradio] UHD Underrun with Wav File Source and USRP
> Sink
>
>
>
> On 11/11/2012 01:21 PM, Tom Hendrick wrote:
>> Hello all,
>>
>> I am using a USRP1 and an LFTX daughtercard and Ubuntu 12.04.
>>
>> I am having an issue with a simple GRC script I made which has three
>> blocks. A WAV file source connects to a resampler block and then to
>> the UHD USRP sink.
>>
>> When I run the script I get a single underun right at the start of
>> running the script and no other underrun. When I run the script using
>> the sudo in front of it, I get two underruns right at the start of
>> running the script and no other underrun. I monitored the signal out
>> of the LFTX using an oscilloscope and noticed some blips/noise when
>> the script is run and I believe they correspond to the underruns. I
>> tried this with and without real-time scheduling enabled and didn't
>> see any difference.
>>
>>
>> I tried the same exact type of script with an older version of GRC on
>> a different laptop with Ubuntu 10.04. This uses the USRP sink block
>> not the UHD sink block. This script plays the file perfectly without
>> any underruns at all and I don't see the blips/noise at the beginning
>> of the signal.
>>
>> I am using the LFTX for an audio application and the blips/noise will
>> cause a problem for me. Does anyone know why I am seeing this
>> behavior with the newer UHD block? Is there anything I can do to
>> eliminate it?
>>
>
> Hi Tom,
>
> If I am understanding correctly, you are getting some initial underflows
> when the flow graph begins processing. This is causing some
> discontinuous stream interruption over on the receiver side.
>
> If thats the case, I dont know of anything specifically to cause this,
> so it might just be the issue of interrupt coalescing. That is the host
> isnt initially ramped up to push out USB packets at full speed. So,
> driver wise, there may have been a subtle difference thats brining this
> out, USB 1.0 vs .1 for example.
>
> I'd like to replicate it over on end. But if I have a quick suggestion,
> it may be helpful to zero-pad the beginning of the wavefile so those
> initial discontinuities are only lost in the padding.
>
> -josh
>
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