Dealing with Radio Frequency Interference, OR "Hiss, the Villain!"

In my previous post on Ground Loops and Power Line Interference Hum, I mentioned another common complaint in Home Theater setups:  "Hiss" from your speakers.  Hiss comes from Radio Frequency Interference (RFI) -- i.e., higher frequency signals emitted by something in or near your Home Theater setup.  If these emissions get past the shielding of your electronics they can result in audible Hiss from your speakers, even when you are not playing anything!

If you'd like  more info on what's going on, and suggestions for how to deal with it, this post is for you!

The first point I should make is if you are hearing Hiss *DURING* playback, it is quite possible that's simply how this particular content was authored.  Audio recording, mixing, and compression errors can result in audible Hiss.  Typically this would not be common across all the things you play.  Indeed, you may even discover movies where the dialog from just one or two actors exhibits Hiss, while other audio in the movie is clean.

Audio tracks for the oldest sound films were also recorded with an intentional boost applied to treble -- called "Academy Equalization" (for the Academy of Motion Picture Arts and Sciences).  This treble boost was intended to compensate for the fact the speakers were placed BEHIND the movie screen -- during a period when movie screens were made of much heavier material than today:  Less transparent to sound.  When creating modern discs of such movies for Home Theater use, the Studios usually -- but not always -- remove that Academy Equalization.  However if they FAIL to do so the audio from the movie will sound distinctly brassy and hissy.  Indeed, many people mistakenly think this is just the "natural" sound of movies from that era.  Not so!

TECHNICAL NOTE:  Some Home Theater electronics will offer a sound processing mode for removing Academy Equalization.  That's for dealing with older movies like this which still happen to have their original, Academy Equalization.

You can also get content-specific Hiss if you've been overly aggressive in your application of sound processing "features" in your electronics.

But the topic of THIS post is not content-specific Hiss -- whether authored into the content you  are playing, or due to how you've tried to gussy it up with sound processing features.

Rather, we are concerned here with a more BASIC level of Hiss which happens INDEPENDENT of the content you are playing.  So in particular, TRY THIS:

Pause what you are playing:  NOW do you still hear the Hiss?

If so, it's time to track down your source(s) of Radio Frequency Interference!

Tracking down RFI sources can be a pain in the neck -- even more so than tracking down a source of Ground Loop Hum.

Like the garbage current that flows due to a Ground Loop, RFI *CAN* travel on the cable shields of the cables connecting your devices.  But it can also penetrate the shields of those cables and get into your electronics as part of the normal signal being carried by the cable.  It can ALSO travel on power cords.  And it can ALSO get into your electronics as direct, radio frequency emissions, which overwhelm the shielding designed into your electronics.

On the positive side, all of the electronics in your Home Theater will have had to pass emissions certification tests (by the FCC in the US, or other regulatory agencies elsewhere).  These tests limit the amount of emissions which can escape from the device in normal use.  If the shielding design of the device doesn't pass the test, it's back to the drawing board!

That is, each of your devices is designed not only to protect itself from external emissions, but also, not to be a SOURCE of emissions which can screw up OTHER devices.  This is why you don't have problems ALL THE TIME with the collection of electronics in your Home Theater.

Nevertheless, a faulty device may still leak emissions.  And emissions may also escape on the cabling leading from a device.  And narrowing down the culprit can be a chore.

BUT, you may not even need to DO that!

And that's because the single most common source of RFI resulting in Hiss in Home Theater speakers is....

Wait for it....


Yep, it's not the high tech gear which causes the most grief; it's lighting fixtures and their power switches!

Fluorescent lights can be bad.  Halogen lights are even worse.  But by far the biggest culprit is room lighting DIMMER switches!

And determining whether any of THESE are the source of your problems is, of course, easy!  Just turn them Off and see if the Hiss goes away.

For the Dimmer switches, this does NOT mean dropping them down to their dimmest setting, even if that setting shuts the light shuts off.  Instead it means finding the little On/Off mechanical switch on the Dimmer and turning THAT Off to disconnect the Dimmer from power.  All Dimmer switches have these -- usually with some Caution in their installation literature advising you to turn that to Off before changing any bulbs controlled by the Dimmer.

If you can't find the mechanical Off switch on your Dimmers, turn off the circuit breaker that powers them so they are truly Off.

Note also that this problem of RFI from Fluorescent or Halogen light fixtures -- or from Dimmer switches controlling ANY style of light fixture -- may even be coming from fixtures and switches in an ADJACENT room.  So check those too!

Of course if you find one or more light fixtures or Dimmer switches are indeed causing your Hiss, the fix is easy.  Replace them.

And remember again, we are testing for this by listening to the speakers while all your electronics are On, but no content is playing -- for example when you've Paused playback.

With any luck the above solved your Hiss problem.  If not, it's time for some real sleuthing.

The first step is to turn Off devices one at a time to see if you can isolate the problem to just one device being On.  Of course you need to have your speaker amp and preamp on so audio can get to the speakers, but try turning off your other devices.

If you find a culprit device, you could try moving it further from the rest of your electronics, or replacing it.  This test is simple, but usually you won't find the culprit so easily.

The next step is to look to your cables.  If you have any UN-shielded cables in your system now is high time to replace them with shielded cables.  Unshielded cables provide an EASY pass for RFI to get into your electronics.

You may also have a faulty cable -- a cable that's still good for passing the signal you have on it, but which happens to have a failed cable shield.  This can happen for example if the connection breaks between the shield and the plug at either end, or if there's a break of the shield somewhere along the length of the cable (inside it's protective wrapper).  Cables that have been bent too sharply or which have been stepped on too heavily might be damaged in this way.

Before you go about simply replacing cables willy nilly, try the simpler test of "Re-Dressing" the cables.  This means moving them about so they follow a different path.  And in particular, try to change which OTHER cables they cross or run close to.  NOTE:  You should include Power cords in this exercise, as they may be carrying the RFI.

You may find that the nature of the Hiss changes as you shift some of your cables in this fashion.  That's an important clue!  It confirms the cables are carrying the RFI!  And what's more, that the RFI is passing from one cable to another cable by proximity!  By approaching this logically -- i.e., which cables you move, in combo with which devices are still powered On -- you ought to be able to nail down the likely culprit here.  Hopefully it is just a faulty cable -- easy to replace.  But you may end up isolating the source of the problem to one of your devices.

And if no ready culprit emerges, you may still find a particular, new layout of your cables (including Power cords) produces satisfactory reduction of your Hiss problem.

But again, before you spend time going down THIS path, look to your light fixtures and Dimmers!