[Lowfer] Audio recording of a PLC

[craig wasson]

I found a half way decent presentation with a few useful bits of info about
plcc signals here:

http://docslide.us/documents/plcc-overview.html

Slide 19 has a spectrum chart of sorts showing how the 4khz channel is
divided up between an ssb voice channel, multiple relay carriers and a 300
or 1200 baud data channel.  (bell 303 or 202?)

I think the relay carriers have the option to shift (unknown amount) to
indicate a positive on/off command while loss of carrier indicates a
failure.

It also seems that for long haul lines they prefer frequencies under 140khz
with the higher frequencies being used for shorter paths due to increased
loss/mile.

Reading up on this - some lines see received noise levels as high as -7dbm
!   Given the loss per mile, a 1 watt signal would have a hard time being
that loud at the other end of a line even if you used the power line itself
as your antenna.  That presentation says the receiver has a sensitivity of
-24dbm so they are not doing weak signal work!

Craig


On Thu, Apr 30, 2015 at 1:55 PM, JD <listread at lwca.org> wrote:

> Which pitch of whistle would you prefer to hear? :)
>
> That's all most PLCs sound like, most of the time.  The vast majority of
> the
> continuous background carriers you hear at 1750 meters, and nearly any such
> at 2200 meters, are PLCs.  They tend to be clustered around steps of 100
> Hz,
> and at night you can often find multiple carriers in a cluster separated by
> a hertz or so on Argo at QRSS3 or 10 speed.  Most of the signals simply
> mean
> "I'm still here, everything is OK," with a fault being indicated if the
> signal drops out.  Every so often, some of them do on/off keying to
> indicate
> status in more detail or to issue a command...sometimes sounds a little
> like
> a letter or two in slow Morse, except for the dit-dah timing not being in
> the right ratios.  Some of the more advanced systems utilize brief
> intermittent bursts of FSK, reportedly, but I've never encountered one of
> those myself.
>
> John
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