[mythtv-users] Humming Noise coming from LCD tv

[Mike Thomas]

On Mon, 31 Mar 2014 22:55:57 -0400
Philip Isaacs <showcasefloyd at gmail.com> wrote:
> Okay, so this is slightly off subject, but I've hooked up my Myth PC
> to my LCD TV via a DVI to HDMI adapter. I then ran a single audio
> cable from the sound card on the PC into the audio port in the back
> of the TV.  The picture looks great but there's a annoying hum coming
> from the TV now. It's pretty loud too. I tried switching out the
> cables but I'm having no luck. Any suggestions on how to fix this? I
> know it's not exactly Myth related but I'm guessing maybe others have
> run into this issue.

Dear Floyd,

Let's diagnose the problem.

1. Does the "hum" disappear when you disconnect the audio cable?
   If yes, it's a genuine "hum" problem, so see 2.
   If no, it's not "hum" at all, but something inside the TV set, so
   see 5.

2. What is it's pitch?
   If you can't hum low enough, it's a proper "hum", most likely
   50/60Hz mains hum, so see 3.
   If you can hum that low, it's probably more of a low-pitched
   whistle, so see 4.
   If it's more of a hiss, see 4.
   If you can't whistle that high, it's not a hum at all, see 5.

3. A mains hum comes about because mains electrical current is flowing
   between two pieces of electrical equipment when it shouldn't be.
   There can be many causes. Like one of the other poster's said, it
   could be because there is an earth loop between your computer and
   your TV, but is is most likely just pick-up.

   To detect pick-up, try to re-route the audio cable between the
   computer and TV, extending it if necessary. If the hum's
   volume changes when your cable approaches certain other cables
   (think about your house wiring too) as you do this, it's pick-up.
   You might find the same things happen if you move the other cables
   around. There is nothing you can do about this, except use the HDMI
   audio signal (if your video drivers support this without crashing) or
   use a very short, screened audio cable. I recommend hanging a USB
   sound dongle from the TV on a very short lead and attaching it to the
   computer with a quality screened USB cable (they are thicker than
   the cheapo USB cables). It's cheap and effective.

   If that doesn't work try moving the TV and computer to the same
   electrical outlet using an extension adapter. Also try disconnecting
   the TV aerial and all other inputs and outputs from the TV. If the
   hum stops and stops when you disconnect one of the cables, you have
   found the culprit. The solution is probably to use HDMI or a USB
   sound dongle.

   I cannot recommend earthing the TV through the "chassis" because
   Class 2 electrical equipment doesn't expose enough genuine chassis
   to make a satisfactory low impedance bond, except perhaps through the
   metal aerial connector. The currents are tiny, and are easily
   dissipated through the screen of the HDMI cable; if they weren't,
   they would damage your computer's motherboard.

4. This whistle or hiss is probably down to currents flowing through the
   computer motherboard's audio circuits. It will probably alter as you
   run different programmes on the computer. This is a design
   limitation of the circuit layout on your motherboard. There is no
   solution other than using HDMI or a USB dongle.

5. A high-pitched whistle or buzz is most likely the metalwork in the
   TV resonating in response to the line frequency of the computer
   signal. An easy way to diagnose this is to tap the TV on the back
   (not too hard) to see if it changes its pitch or goes away for a
   second or two. The other way is to adjust the brightness of the LCD
   panel's fluorescent back-light from one extreme to the other. LED
   back-lit monitors might be immune to this phenomenon.

   If it is this, either live with a different back-light setting or
   twiddle your X server display resolution or mode lines to arrive at a
   different scan rate. There are many guides to this, but be warned,
   TVs have an appalling reputation for supplying bogus EDID, so the
   only sure way to work around the problem is to get a large list of
   mode lines and try them all. It is a slow process, but very
   worthwhile.

I hope this helps.

Yours,

Mike.