[mythtv-users] HD-PVR strange behavior - solved!

[Stephen Worthington]

On Thu, 6 Jun 2019 00:10:49 -0400, you wrote:

>Hi folks,
>
>I've got a strangely-behaving HD-PVR 1212 where I haven't been able to
>pinpoint the problem (until just recently, and I'm sharing this story in
>case it helps anybody else here).  The symptoms are:  When I turn my A/V
>equipment (TV, HDMI Audio receiver) on or off, the HD-PVR stops recording.
>
>The simplest test is to run:
>cat /dev/video0 > /dev/null
>
>The light on the front will turn red to indicate recording.
>
>At this point, if I turn on *or* off my TV *or* my AV receiver, the red
>recording light on the front of the HD-PVR goes off.  The cat
>/dev/video0 process isn't interrupted, and there's no indication of any
>trouble in kern.log.
>
>
>Things I've tried to no avail:
>- Wall wart PSU for the HD-PVR rather than the 5V-rail cable coming from
>my PSU (I'd heard enough horror stories about the wall warts dying, so I
>figured I'd preemptively set mine up this way).
>- Different USB cable
>- Different USB port on the PC
>- Different power outlets for the various components
>- Different Component Video cables
>
>Just as I was about to give up and ask for advice here, I figured out
>the problem!
>
>My cable box (from Verizon FiOS) has both HDMI output going to the AV
>receiver *and* component output going to the HD-PVR.  The purpose here
>was in case I had the very-rare occasion where I'd want to bypass the
>entire MythTV system to see the output directly from the cable box. The
>symptoms I noticed were that if I turned on the AV equipment while an
>HD-PVR recording was going on, I'd notice the red light on the box go
>off for a couple seconds, and eventually recover.  The recordings were
>mostly OK, except for a drop-out around the time that the light went
>off.  In some cases, the corrupt video was enough to crash mythfrontend!
>
>Anyway, as a last resort I tried unplugging the HDMI cable from the
>cable box.  Bingo!  No amount of turning the receiver and/or TV on/off
>would interrupt the HD-PVR recording!
>
>Theory: When an interconnected HDMI device (TV or AV receiver) is power
>cycled, it sends some sort of signal (or glitch?) along the HDMI "bus".
>Since the cable box is connected via HDMI to the AV receiver, this
>signal makes its way to the cable box.  At which point, the component
>video output is temporarily glitched.  The HD-PVR apparently doesn't
>like this.  If you're currently doing a cat /dev/video0, this never
>recovers.  MythTV appears to notice trouble and restarts the video
>recording when trouble is encountered.
>
>It's a long shot, but I suppose that if you've noticed that your HD-PVR
>occasionally stops recording temporarily when power cycling any
>HDMI-connected components, check to ensure that the HD-PVR is the *only*
>thing connected to your video source.
>
>
>-WD

Good catch, but I think the cause of the problem is likely something
like a ground loop.  That is providing a path for signals between the
boxes, and when you turn on or off a box, the power surge is causing a
pretty big signal along the ground loop path, which is glitching the
electronics in other boxes.  The HDMI cable is unable to do anything
that could cause this sort of problem via any data that can be validly
sent along it.  But a ground loop can cause "signals" that are well
out of range and can upset the entire ground plane of a device (by
raising or lowering where the 0 volts level is).  It is possible for a
ground loop of the worst sort to cause physical damage to the
electronics, so it is best to avoid them.  It is common for a big
pulse on a ground to cause a device with a CPU to restart, or to lock
up completely until it is turned off and all its capacitors allowed to
decay before it is turned on again.

The easiest way to avoid ground loops is to use optical fibre
connections (SPDIF for the audio, for example).  But with the need to
use aerial, video and audio connections in multimedia systems (and
possibly also Ethernet), ground loops can be pretty hard to get rid
of.  I have one in mine that manifested as a really bad 50 Hz audio
hum.  And I am using SPDIF from my MythTV box to my DAC.  The only way
I found to get rid of it was to isolate my class A audio amp from its
mains earth - not a safe thing to do.  So I have put it on an RCD that
should trip if it ever has a fault that shows up on its earth, but it
is still a bit of a worry.

If you would like to still have your HDMI connection, you can now buy
HDMI cables that send the signal over fibre.  They are intended for
long distance very high bit rate 4k video connections, and are not
cheap.  I think it would be difficult to find a short one.