CE Pro December 2022 - 18

Networking+Home Control
HDMI CORNER by DPL Labs
Ins & Outs of HDMI That
Don't Get Much Attention
WITH SO MANY devices
from displays to sources,
AV receivers, and accessory
products all having to work
in harmony with the interface
specification, there are
more chances of something
going astray. Here is one
example of a system failure
turning into a nightmare by
arbitrarily trying unproven
techniques that exemplify
the problem rather than
curing it.
Take the system example
in Fig 1. Source device to
AVR to display by way of a
long-distance AOC (Active
Optical Cable). Pretty
straightforward, but the system
operated intermittently.
It all started off with decent
troubleshooting skills. When
that did not solve the issue,
it went on to abstract modifications
learned from some
industry forums.
One such idea was to add
a distribution amplifier (DA)
device between the source
and the AVR (Fig 2). This is
a Hail Mary move and can
sometimes cure an issue but
can also introduce others. It
turned out that the DA did
not fix anything; in fact, in
this case, it stopped working
altogether.
The first thing we did was
remove the DA addition and
put the system into a more
mainstream configuration.
We don't need any more peripherals
inserting variables.
18 | CE Pro December 2022
The second test that many
use is to bypass the AVR
and go direct to the display.
This would at least tell us
that something is working.
This proved positive taking
the AOC and connecting it
directly to the source device.
So, we now know the system
the AOC is bad? Not really.
Replaced the AOC and the
same thing happened again.
The reason why the
system would not function
in this configuration was
due to the way the interface
was designed (Fig 3). What
many don't know is that the
from the input of the AVR
and to satisfy the AVR's output
it comes from the input
of the display. Yep, there is a
DC output going backwards
on every HDMI signal input.
If this DC is off a certain percentage
the output of the
AVR can have problems. We
went in and measured the
amount of voltage coming
back from the display and
found no problem there.
But remember, an AOC is
works without the AVR. Does
that mean the AVR is bad?
Not really, we just know the
AOC and the source device
have reliable communication
and function. Now by adding
a replacement passive cable
between the AVR and the
display, the system came
back to life. Does that mean
output of the source device
cannot operate on its own. It
requires some supply voltage
(DC) coming back into its
output - yes, I said output
- to produce any video
data. The catch is that the
DC originates at the input of
the next stage. In this case,
for the source, it originates
a fiber and not copper so DC
cannot be sent backward up
the fiber as copper could.
Every fiber device MUST
self-emulate the same level
of DC on all 8 signal channels
that the display would.
Where does that come from?
The already tapped out
5-volts off the HDMI bus. In
this case, the AOC input was
not quite providing the level
of DC the AVR was happy
with. It was the AOC, but
how? It worked when we
jumped over the AVR, didn't
it? This is true but the source
output DC requirement was
a bit more robust and could
respond using the lower
voltage from the input of the
AOC. Now, depending on
which system this particular
AOC is connected to, it may
work or may not. Got it?
JEFF BOCCACCIO,
president of DPL Labs, can
be reached at info@dpllabs.
com, or visit dpllabs.com.
cepro.com
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CE Pro December 2022

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