Re: [Discuss-gnuradio] Choppy Audio from uhd_rx_nogui on a VM

[Tom Rondeau]

On Wed, Jun 6, 2012 at 8:47 AM, Shepard Siegel
<address@hidden> wrote:
> This is a simple question with a long prelude.
>
> We have a handful of N210/WBX that work great with UHD with which we
> are familiar. Until last night, I had not spent more than ten minutes
> with GNU Radio. This morning I installed a fresh Fedora-16 guest VM
> running on VMware Workstation on a strong Win7-64b host. I used
> build-gnuradio to install GNU Radio 3.6.0 . And in twenty minutes with
> no issues whatsoever, I’m feeding our RF signal generator into a USRP2
> and making sane GUI comparisons to our spectrum analyzer. Brilliant!
>
> address@hidden ~]$ uhd_fft -a addr=10.0.209.194  -f 935M -s 2M
> linux; GNU C++ version 4.6.3 20120306 (Red Hat 4.6.3-2); Boost_104700;
> UHD_003.004.002-133-gb61a1103
> -- Opening a USRP2/N-Series device...
> -- Current recv frame size: 1472 bytes
> -- Current send frame size: 1472 bytes
> Using Volk machine: avx_64
>
> I then experimented with the uhd_rx_nogui program and found that the
> audio is discontinuous (though almost intelligible), as is shown by
> the stream of audio underflow indications that feed to stdout:
>
> address@hidden ~]$ uhd_rx_nogui -a addr=10.0.209.194 -f 99.5M -m FM
> linux; GNU C++ version 4.6.3 20120306 (Red Hat 4.6.3-2); Boost_104700;
> UHD_003.004.002-133-gb61a1103
> -- Opening a USRP2/N-Series device...
> -- Current recv frame size: 1472 bytes
> -- Current send frame size: 1472 bytes
> UHD Warning:
>     The hardware does not support the requested RX sample rate:
>     Target sample rate: 0.256000 MSps
>     Actual sample rate: 0.255102 MSps
>>>> gr_fir_ccf: using SSE
> Using gain:  19.0
>>>> gr_fir_ccc: using SSE
>>>> gr_fir_fff: using SSE
> aUaUaUaUaUaUaUaUaU … aUaUaUaUaUOaU
>
> I’m uncertain if this is my clueless-ness of audio with the Fedora VM
> under VMWare; or something more GNU Radio related. Audio seems work
> fine in the VM. And GNU Radio seems to work fine (no errors, good
> data) writing files with uhd_rx_cfile. Is this issue likely a VM audio
> device issue? Am I “off in the weeds” running GNU Radio audio on a VM?
> This isn’t my day job; but any guidance on bisecting this issue is
> appreciated.
>
> -Shep

Shep,

I think the problem is a mismatch in sampling rates between what
you're sending to the audio sink and what the audio sink is expecting.
Notice that there's the lines that say "Target sample rate" and
"Actual sample rate". So if you are basing any rate changes on the
target sample rate to get to the audio rate, things are going to be
off a bit and the audio samples are going to be wrong.

The easiest thing to do is just use a blks2.pfb_arb_resampler(rate)
where rate=(target/actual). In reality, you'll probably want to put
that resampler as close to the audio device as possible so it's
running at the lowest sample rate that it can.

Tom