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<p class="MsoNormal" style="margin-left:.5in">As I mentioned in a
previous post, I have Heterodyne connected to Metis via a two-hop
wireless link. In this configuration, Heterodyne seems to work
well at 48,000 sps. At either of the two higher rates, the audio
is mostly unintelligible. This surprised me, as the theoretical
bandwidth of the link (27 Mbps) is far above what should be
required to handle a single Metis stream even at the highest
sample rate. (The routers are each rated at 54 MBps, but one of
them is splitting its bandwidth as it receives Metis packets and
forwards them to the MacBook laptop).<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.5in">I took a look at the
Metis protocol description, and came up with a rough estimate of
max throughput for each sample rate. I assumed one I&Q stream
at the selected sample rate, with 24 bits/sample. I also factored
in a band scope stream at 4096 sps with 16 bits per sample. I
added 1.5% overhead for HPSDR protocol (based on header sizes, but
ignoring padding), and 6% overhead for Ethernet/IP/TCP|UDP, which
is a standard benchmark estimate (although perhaps it should be a
bit higher for wifi Ethernet). Based on these assumptions, I get
the throughput estimates shown below. I’ve also added a column
indicating the actual bandwidth utilization measured by my router
for each sample rate.<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.5in">Sample Rate
Estimated Throughput
rate
Measured bandwidth (Mbps)<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.5in">48,000
1.3
Mbps
~3 Mbps<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.5in">96,000
2.5
Mbps
~4 Mbps<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.5in">192,000
5.0
Mbps
~6 Mbps<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.5in">So the utilized
bandwidth was twice the value expected at 48,000 sps, and
significantly higher (but at a lower ratio) for the other sample
rates. I’d like to know why the measured values are so much higher
than the predicted values? Is there something wrong in my
assumptions and/or calculations? I’d also like to know why the
audio was unusable at the higher sample rates. Clearly the router
is capable of higher bandwidth, so perhaps the buffer sizes need
to be adjusted. However that doesn’t appear to be possible with
Heterodyne; at least there is no preference setting for it. I also
looked inside the app bundle, and I see only plist files for
setting defaults of the properties that can be configured in the
UI.<o:p> <br>
</o:p></p>
<p class="MsoNormal" style="margin-left:.5in">Another curious thing
is that the router showed a packet stream at ~2.8Mbps from the
MacBook to Metis whenever Heterodyne was operating. Sounds like
audio incoming to Metis, but I never had Heterodyne in transmit
mode because there doesn’t appear to be a transmit mode. I see
some preferences related to TX, but no controls to actually
transmit.<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:.5in">Any light that can be
shed on any of this would be appreciated.<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
Mark – K4XML
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