Problems with sound interference

L31mr0d

I accidentally posted this over in the BB section. It should be here instead.

So last week I got Digital TV set up at my apartment that I recently moved into. Before this I had been using just the coaxial cable out of the wall and getting the base 15 cable channels (MTV, Cartoon Network, TV5...etc)

Once the digital box was setup I started to notice this ringing noise in the background of the sound. When I turned down the digi boxes sound and turned up the TV's sound it could be clearly heard. It was loud enough to be always heard in the background of shows where the people where just talking with no music.

At first I thought the problem was the Digi box as I wasn't getting this sound problem with the base 15 cable channels. So I tried the box over at a friends house who also has the same subscriber (i.e. Chorus) and the sound problem wasn't happening. I further narrowed it down to being directly related to the coaxial cable coming out of the wall as the sound could be heard on the TV even when the digi box wasn't plugged in, only connected to the TV via scart.

When inspecting the coaxial cable I realised that when I went to connect to the cable connecting from the wall there was a small spark off the metal housing of the connectors when they touched. I stripped back the connector and touched only the copper cores together. From this I was able to get a picture and sound on the TV through the digi box with no interference. Eureka I thought, so the metal sleeving around the coaxial cable must be shorting somewhere along the line. So I stripped back the metal sleeving about an inch, reattached the connector and turned on the TV. I now have no interference, and a clear picture.

BUT... some channels are now NOT showing up at all. I'd say 90% of the channels are still coming through except for a handful like MTV, VH1, Living +1...etc. They are showing up as "there must be a problem in the connection to the decoder"

Also I have my PC hooked up to the TV also. When I turn on the PC I lose ALL of my digital channels?... ***?

So my 2 problems are:
1. Missing channels when not having the metal sleeving connected.
2. All channels missing under this setup when PC is turned on.

My questions are.
1. What is the purpose of the metal sleeving? Does it actual carry any signal or is it simple a ground signal? Could I hook it up to the "earth" or "negative" signal on a plug and accomplish the same thing.

2. Other than not allowing the metal sleeving to connect, would there be another way of eliminating the sound interference noise. Note: I am renting an apartment so can't go ripping holes in walls tracking down the source of this short/interference in the cable.

3. Why would turning on the PC be interfering with all the TV channels? I've tried have the PC on the HDMI and VGA connecter of the TV but with the same results.

All help is appreciated.

tapfit2004

by dissconnecting the screen of the coax cable you are allowing any interferance in the apartment to affect your tv reception. the sound you are describing sounds like "mains hum". the mains electricity in your building is supplied at 50 hz and this can get amplified in tv's and radios. the usual way elimate this is to earth the system but this could be dangerous so you would probably be better off calling an electrician or aerial installer and letting them sort it. or better still tell the able provider and let them sort it.

byte

It could be mains hum alright, though I notice that on analogue channels, sometimes you'll also notice white bars rolling on the TV screen. Usually, cableco's use a fully isolated FM/TV diplexer which should block any mains leak.

As it seems you're on Cable and not satellite, I'm moving this to the Cable forum.

L31mr0d

byte wrote:

It could be mains hum alright, though I notice that on analogue channels, sometimes you'll also notice white bars rolling on the TV screen. Usually, cableco's use a fully isolated FM/TV diplexer which should block any mains leak.

As it seems you're on Cable and not satellite, I'm moving this to the Cable forum.

Hey i've noticed the white bars also (well they aren't block white, more very transparent, but on scenes where the background is a block colour I can see them rolling up the screen). Although changing to a better scart cable rectified this problem to a degree, although the lines are still there but less noticeable.

What i'm wondering is does the metal sleeving perform any sort of grounding? Also, how can mains hum be fixed?

Also how would I go about "earthing the system"?

L31mr0d

does nobody have any ideas?

Does anybody know a reason why turning on my computer would cause me to lose all of my cable channels? It only happens when I connect the PC to the TV. When its on and not connected I still have the channels. I connect the PC via the HDMI connection.

L31mr0d

Ok just in conclusion if anyone comes across this and has the same problem. I've pretty much solved the issue with the sound interference and white scrolling bars on the TV.

I reconnected the shielding connection coming from the wall port and got a spare wall socket plug I had lying around. I then soldered the earth cable off this plug (removing the prongs for negative and postive of course) onto a female to female coax adapter that was linking the wall coax cable to the cable coming out of my chorus digi box. Doing this has grounded any interference or "mains hum" that was getting induced into the coax shielding before it can get to the digi box. I'm now a happy camper

PauloMN

I'm really not sure that earthing your cable through the mains earth (if I've read what you did correctly) is a good idea. It sounds like a faulty cable. I'd ask the landlord to ring your cable co. and get them to come out and take a look.

L31mr0d

PauloMN wrote:

I'm really not sure that earthing your cable through the mains earth (if I've read what you did correctly) is a good idea. It sounds like a faulty cable. I'd ask the landlord to ring your cable co. and get them to come out and take a look.

Why not? I'm just bypassing the process of it being earthed through the digi box or the TV's mains plug, thus canceling the interference. All the electronic devices will share a common ground, regardless of what link in the chain they do so. This sort of problem can't be easily fixed anyway, as there must be a mains line running along beside the coax cable inducing this current in the shielding which causes the interference. To fix it would mean either increasing the shielding level of the coax cable or completely rerouting it.

zilog_jones

L31mr0d wrote:

All the electronic devices will share a common ground...

They may not. All the TVs in my house anyway are double insulated, i.e. they don't use the earth pin on the plug. A lot of electronics are like that. Probably your STB too. I have somechronic ground loop problem with my cable - it's grand when plugged into the TV, but if I try using it on my PC (capture card) I get horrible interference on the monitor and buzzing sound, and if I plug it directly into my hi-fi (for cable FM radio) I also get this AC buzz. I'm sure Chorus will never fix the problem :rolleyes:

watty

Fit a braid breaker isolator on the cable TV coming in. You can buy them on some Amateur Radio sites under TVI / RFI reduction sections. The problem is high common mode 50Hz AC on the cable TV system.

DIY
A plastic box with two F-connectors and connect the connectors to each other via 1nF 1000V rated capacitors. Earth the outer of connector to STB box. Don't let cable TV shield touch earth.

L31mr0d

watty wrote:

Don't let cable TV shield touch earth.

What do you mean by TV shield? What are the possible problems with the way i've set it up?

To clarify, what i've done is soldered the earth off a socket plug to one of these then attached the f connector coming out of the wall to it and then the f connector coming out of my STB to the other side. This has fixed the problem of interference when I turn on my PC and the loss of some of my digital channels.

You've been giving me alternatives but nobody has outlined why it is bad to earth the coax braiding directly into the wall socket?

watty

You could make every equipment in your house live if you have an earth fault, or make cable live, or if lightning hits within 200m of cable anywhere you get lots of blown electronics.

It's contrary to safety practice. You need to ISOLATE the "earthed" outer screen of cable company from your "earth"!

L31mr0d

watty wrote:

You could make every equipment in your house live if you have an earth fault, or make cable live, or if lightning hits within 200m of cable anywhere you get lots of blown electronics.

It's contrary to safety practice. You need to ISOLATE the "earthed" outer screen of cable company from your "earth"!

I don't understand how doing this will cause all the equipment in my house to go live? Wouldn't any earth fault accomplish the same thing?

Maybe answering this question will clear things up for me. The braid around the copper core of the coax cable is used to shield against interference right! Is it not also grounded? How is grounding it via the cable different to it going through my STB and getting grounded through the earth connection on the STB?

lucernarian

watty wrote:

Fit a braid breaker isolator on the cable TV coming in. You can buy them on some Amateur Radio sites under TVI / RFI reduction sections. The problem is high common mode 50Hz AC on the cable TV system.

DIY
A plastic box with two F-connectors and connect the connectors to each other via 1nF 1000V rated capacitors. Earth the outer of connector to STB box. Don't let cable TV shield touch earth.

By earthing, you mean getting a wire from the case of the STB box to the outer part of the F-connector? Isn't the STB box itself not earthed, merely shielded?

watty

Only the cable company should earth the screen. Screens should not be earthed at both ends in different buildings

Many STB do not have earth. Many TV sockets in the past isolated at inner core and outer screeen via 1kV capacitors due to chassis was connected directly to mains neutral.

There is a small leakage from each TV or setbox of AC to the outer screen and inner core (common mode AC voltage). This should only be earthed by the cable company. Tens or more of these can add up.

To avoid earth loop hum, ensure only the RF can get from your cable TV cable to your house by a "braid breaker" or homemade box where the inners of two F-connectors connected via 1nF / 1000pF @1000V rated and the two outers F-connectors connected via 1nf/1000pF @1000V, thus the screen is not low frequency (AC mains) or DC connected to set top or TV outer screen.

Then you can safely earth the screen (outer) on your side of the briad breaker/ stb.

Any other arrangement is unsafe and even possibly breaking wiring regulations.

If you are EVER going to earth an incoming aerial cable it should be to a dedicated earth spike, never your household top pin of plug. Then it must have AC mains quality isolation from the outer screen of any equipment via a "braid breaker" to avoid problems in event of a fault with ESB earth spike.

L31mr0d

Ok, I think i'll make the "braid breaker". Do you have any links to tutorials or pictures of it? I'm having a hard time visualizing how it works and how there is an end free to connect to my STB if i'm connecting both ends in a loop?

watty

      Braid Breaker

TV cable                   Set-Box
          1nF 1000V cap
J1     \                    /  J2 
F      o-------||-----------o F
conn.  /                    \ conn.
       *---------||---------*
                1nF 1000V
        (connects outers of 2 x F-Conn.)


Use plastic box or if metal, then 
J1 outer must be on plastic washer 
to insulate from case.

1nF = 1000pF = 0.001uF 

o = Inner of F-Connector
/ = Outer of F-connector
* = washer with solder tag to connect wire to Outer

Connectors & box available Maplin. Not sure about capacitors. Similar ones on old Modems, PSU filters or network cards are usually blue and say 1kV.

Too low a capacitance will block the FM radio channels/ lower TV channels.
Too high a capacitance lets too much 50Hz hum through.
470pF may still be OK (as minimum) and 10nF/0.01uF/10,000pF may not be too high. Any voltage from 500V to 2,000V is really OK.

1nf @ 1kV is ideal ... probably.

Only earth setbox outer connector, if desired (optional).

lucernarian

Helpful info there. I presume the capacitor would be some high-quality polypropylene or tantalum-bead capacitor.

As for earthing, that would mean getting a separate earth spike and a wire to it etc? sounds very awkward for a double-brick wall house and surrounded on all sides by at least 2 metres of concrete paving/driveway.

L31mr0d

It might be simpler to just buy one of these "braid breakers" than try and source those caps.

Do you know where I could just buy one already made?

watty

To_be_confirmed wrote:

Helpful info there. I presume the capacitor would be some high-quality polypropylene or tantalum-bead capacitor.

As for earthing, that would mean getting a separate earth spike and a wire to it etc? sounds very awkward for a double-brick wall house and surrounded on all sides by at least 2 metres of concrete paving/driveway.

Ceramic only.
Tantalum are polarised and only high value (10,000 times) and low voltage (5V to 50V).
Polypropylene are too high self inductance as they are wound foil, usually for 10x to 100x higher capacitance. Usually less than 300V rating.

Earthing is entirely optional.



RF transformer based design off the shelf £9
http://www.nevadaradio.co.uk/acatalog/AKD_BB1_Braid_Breaker_Filter.html

The Braid breaker schematics on Internet are usually to block HF radio transmitters interfering with UHF (470MHz and higher TV). NTL uses 87..108 MHz for Radio on Cable, 5MHz to 60MHz for Broadband upstream and 110MHz to 500MHz for TV and Broadband downstream, so the capacitors need to be 470pf to 1000pf rather than the typical 4.7pF to 10pF shown in these circuits.


http://www.rsgb.org/emc/emcwebsite/filters.html
BB1 Braid breaker (RSGB order code Filter 1)
Pass band: Most amateur bands plus FM radio broadcast (88 - 108 MHz), and UHF TV.
Stop band: Some rejection below 10 MHz but the BB1 is primarily intended as a 'braid breaker' rather than a high pass filter.


I can't guarantee that any of these shop "braid breakers" will work as they are to reject unwanted RF on the cable and may interfere with Broadband. Some do not actually physically "break" the outer (braid) and thus will not reject mains hum.


Here is a more complicated design to block CB radio etc, rather than Mains hum:
http://www.uksmg.org/braidfilter.htm
(if the coils are not large enough, broadband would be affected, so these coils are too small)
The component values are for UHF TV, they do not suit NTL cable

zilog_jones

Thanks for the info - I've heard about these things before (but by a different name I think) but never bothered actually trying to make one.

Would there be much signal loss caused by this, and is there any benefit to using a metal box over a plastic one? Must try and look for some of those caps

watty

Give me a shout if you want the caps (I'm up the road).

Virtually no signal loss.

Only slight advantage to metal box and much harder as the street side connector has to be completely insulated from box.

L31mr0d

Is there any test I can perform to see what type of braid breaker i'd need?

I've noticed that even though i've earthed the coaxial braiding I can still hear a very faint hum coming out of the speakers. I'd like to eliminate this entirely but I don't want to lose any of my digital channels in the process.

I've found a page full of different types of filters here

Can you have a look at it and recommend which would be the best filter for my needs?

watty

Earthing the incoming cable is a bad idea and not as good as braid breaker.

Best kind is one that uses capacitors or transformer to physically "break" the braid, thus blocking 50Hz mains hum.


None of those filters are ideal as they are all to block CB, thus would interfere with Broadband on the cable.

The X07 is the closest, but has a highpass also. You only want the braidbreaker as it is a 50Hz, not RFI problem.