Re: [Discuss-gnuradio] WBFM modulation. Bad sound and aUaU

[Martin Braun]

See also
http://gnuradio.org/redmine/projects/gnuradio/wiki/FAQ#I-have-a-receiver-with-acoustic-output-and-keep-getting-aUaUaU-errors

-- M

On 03/05/2017 05:30 PM, Marcus D. Leech wrote:
> On 03/05/2017 06:47 PM, Fernando wrote:
>> Yes, this USRP has "very strange" sample rates, like 2.631579 MSps the
>> are far from rounded values, every single sample rate I try to use, it
>> change it to a slightly different one. 2.64Msps came from using a
>> sample rate over 2Msps, with a 44Ksps in audio and interpolating in
>> two stages one the FMTransmit (QuadratureRate/AudioRate =6 ) which
>> must be an integer and a second stage in the resampler (10), so
>> 44K*60=2.64M Using a UHD USRP source instead of osmocom source for the
>> USRPit works fine (it sound okay), altough some aUaU keep appearing
>> (maybe for mismatch between in sample rates?) regards 
> 
> Not that strange--a USRP N-series device has a master clock of 100MHz. 
> Sample rates are constrained to be integer fractions of the
>   master clock rate.  The sample rate you show is exactly 100e6/40, but
> is not a multiple of 44Ksps.
> 
> The osmocom block simply calls UHD, so in terms of sample-rate, there'll
> be no difference.
> 
> Since 44Ksps is not an integer fraction of 100Msps, you'll need to use
> an arbitrary resampler.   This is exceedingly common in DSP.
>   Audio subsystems are sample-rate constrained, and SDRs are sample-rate
> constrained, and it's very rare that their constraints
>   "mesh" in useful ways.   Doing fractional resampling in the software
> part of the flow is MUCH easier than having the hardware do
>   arbitrary resampling.
> 
> 
> 
> 
> 
> 
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