Virgin Cable help please

Figment

The VM STB in our bedroom has started having problems displaying some channels - the picture is pixelated and it often looks like two channels are being mixed together. All other channels work perfectly.

The problem started after I swapped a 2-way splitter for a 3-way to add an extra TV point. I suspect the splitter may be at fault, but testing has been inconclusive. I've tested for shorts and continuity through the different cables, and have also checked that all connections are tight. I've also eliminated the STB as being the cause by swapping with another. The fault remains in the room.

The splitter has:

• Outlet 1 going to the cable modem where it splits again to feed this STB.
  
• Outlets 2 and 3 go to (as yet) unused TV points (terminated on F plugs).

If I disconnect either of the cables from outlet 2 or 3, (or both at the same time) the STB works.

If I swap 2 & 3 around and the affected channels work but others stop working.

I can't understand how two lengths of cable that lead to nothing more than F plugs can cause problems. (If it's relevant, cable 2 and 3 are 'shotgun' cable)

Can anyone suggest what else to check before I make a call to VM to arrange an engineer to check signal levels?

Figment

Anyone ?

espresso

You have effectivly added extra loss into the system and this will vary at different frequencies.

Exactly what type of 3 way splitter are you using?

There are two types of 3 way splitters available: A balanced 3 way and an unbalanced 3 way. A balanced 3 way splitter is designed to equally divide the signals from the input port to three output ports. This results in approximately 6.0 dB of loss between the input port and each output port.

An unbalanced 3 way splitter will have one port with approximately 3.5 dB of signal loss, and two additional ports with approximately 7.5 dB of signal loss - like using two 2 way splitters.

Remember that each 3dB halves the signal and you can't just keep splitting the input signal without it causing problems.

Figment

Thanks for replying.

The splitter isn't marked, but taking the worst case scenario I'm assuming it's unbalanced. I understand what you're saying about the signal loss, but could you please explain how plugging a length of unused coax into the splitter can cause this loss. At the moment I'm using the splitter with just one output cable connected and the TV picture is fine on all channels, but this isn't the case if I connect the other coax leads as well

espresso

You are trying to drive a signal of insufficient level down these lengths of co-ax, which I assume are not electrically terminated? and therefore some signal will be reflected back from the distant end and this will very likely cause interference to the input signal.

spike7451

Normal shop bought coax & splitters are not suitable for cable services.Splitting a cable service isn't like splitting a normal TV service.For one normal Coax has a higher loss of signal over distance,the F connections (and I'm guessing the OP is using Sky type screw on F's) are not designed for cable.Shotgun cable is NOT suitable for cable services.
By introducing a non cable & splitter,the OP has altered the forward & return path levels as well as reducing the signal strength.These can only be checked by a SLM & checking the levels at three points.Not only that but the would also be signal egresss & ingress,as expresso stated.This is a signal level issue & not one the OP can fix himself.
The OP also risks having his services from VM terminated as he's made a change to VM's network that has broken the T&C's,if a service tech comes out to fix it,he'll be charged £50.

Figment

Thanks for the explanation.