Re: [Discuss-gnuradio] audio underrun when bad signal quality

[Marcus Müller]

Hi Markus,

maybe just a CPU hiccup? You can try to reduce CPU load by removing the
decimating rational resampler, and integrate the decimation into the Low
Pass (it does the same thing, with one filter less).
By the way, there might be a slight mistake in your flow graph, as it is:
After the first rational resampler, your sampling rate is already 2MS/s
/ 4=500kS/s, but then you feed it to a Low Pass filter that you
calculate for 2MS/s; that means that in fact, you're cutting off all
signal above 25kHz instead of the 100kHz you specified. Probably even a
good thing, in your case :)
I love playing around with filters, so maybe it's something you want to
try later on, too: To optimize SNR, fire up gr_filter_design, and try to
build a filter that best selects your channel -- the only limit is your
CPU speed; at 2MS/s and with a decimation of 4, you can probably use a
lot more taps than your current filter has, so you can make the
transition between pass- and stopband much steeper.

Best regards,
Marcus

On 23.09.2015 11:24, Markus Heller wrote:
> Hi Marcus,
>
> ok, here is the sketch:
> http://www.dl8rds.de/index.php/Datei:2015-09-18-gnuradio_rtlsdr1.png
>
> There is no squelch in the data path. But thanks a lot for explaining
> the effects of a squelch. 
>
> Maybe there is another reason for my observation, because I unplugged my
> antenna from the rtlsdr stick for a while and there were no audio
> underruns today. 
>
> Let me observe some more time and I will come back on this if I have new
> insights. 
>
> best73
> markus
>
> Am Dienstag, den 22.09.2015, 21:28 +0200 schrieb Markus Heller:
>> Hi Marcus,
>>
>> I have been reading from you more frequently on this list than you've
>> been reading from me :-)
>>
>> I have my setup in my office computer, so I will send the setup to this
>> list tomorrow.
>>
>> Thanks ahead!
>>
>> vy73
>> markus
>> dl8rds
>>
>> Am Dienstag, den 22.09.2015, 19:56 +0200 schrieb Marcus Müller:
>>> Hi Markus,
>>>
>>> nice to be hearing from you!
>>>
>>> I'm making assumptions about your RX flow graph here:
>>>
>>> osmocom source --> squelch of some sort --> FM demod/ AM demod of sorts
>>> --> Audio sink
>>>
>>> If the squelch is set to stop the flow of samples as long as there's not
>>> enough signal, then downstream, the audio sink doesn't get any samples,
>>> though it always needs the same amount of samples per second. Hence,
>>> Underflows.
>>>
>>> Hopefully, that explains the issues you're having; if not, please
>>> explain your receiver in a little more detail,
>>>
>>> Best regards,
>>> Marcus
>>>
>>> On 22.09.2015 18:34, Markus Heller wrote:
>>>> Hi,
>>>>
>>>> playing around with my RTLSDR stick (RTL2832U / Rafael Micro R820T
>>>> tuner) I observed that whenever the reception (signal quality) is bad, I
>>>> am getting Audio Underruns and glitches in the accoustics. 
>>>>
>>>> How can that happen? I always thought the sample rate was constant? When
>>>> designing the sketch I took good care that the sample rates were fine.
>>>> Whenever I get good signal quality I have no underruns at all. 
>>>>
>>>> Thanks in advance,
>>>>
>>>> br
>>>> markus
>>>>
>>>>
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