Digital TV Interference Problems
4G TVI Filter
Digital TV interference problems (often referred to as "TVI"), can be difficult to diagnose.
This self help guide identifies common causes of television interference and provides troubleshooting tips and advice to solve TVI from various sources.
4G masts can interfere with terrestrial digital TV reception, or overload a Sky Digital distribution amplifier, Freeview box or VCR - read advice on 4G interference.
Television Interference can take many forms but on Freeview reception, TVI problems often result in picture freezing or breaking up giving picture 'blocking'. On analogue TV, interference problems often result in picture patterning.
TVI can often be solved by fitting an appropriate TVI filter, but TVI filters are available in many different types so it is important that the right filter is used to completely cure the digital TV interference problem. This article will help you pinpoint the interference source, so you can buy the most appropriate filter.
Common types of TVI filter include:
- TV Bandpass Filters - typically used to pass UHF TV Ch21 - Ch68 with low signal loss, but rejects other interfering signals both above or below the passband.
- Notch Filters - often used to reject one of two strong interference sources. The frequency of the interfering signal must be known in order to design or select the right notch filter. Typically used to solve Tetra, 2m or 70cm amateur radio TVI.
- Bandstop Filters - a Band-Stop Filter is a filter that passes most frequencies with little attenuation, but attenuates those in a specific frequency range.
- Low Pass Filters - Occasionally used to reduce interference from strong interfering signals above the UHF TV band (Ch21 - 68).
- High Pass Filters - Frequently used to solve TV interference where the interference source is a lower frequency than the UHF TV band. These types of TVI filter are particularly good at rejecting VHF interference to UHF TV.
Causes of TVI
Television interference (TVI) can be the result of co-channel UHF television interference due to abnormal weather, high pressure or unusual atmospheric conditions leading to a 'temperature inversion' - particularly during the summer time.
Patterning interference on analogue television or can also result from the poor installation of a VCR, Sky Digibox or Freeview receiver. Each of these set top boxes has a built in UHF modulator, which must be carefully adjusted to avoid TV interference. So, it's important to determine accurately whether patterning interference is coming from the prevailing weather conditions, an external source or whether it emanates from your Sky Digibox, Freeview box or VCR modulator.
Analogue TV Interference Help
Read our useful tips for solving analogue television interference problems:-
Permanent Patterning Interference
Where permanent patterning interference is seen on one or more TV channels, try connecting your television directly to your aerial, so that the aerial isn't feeding the TV via all your peripherals (video recorder, Sky Digibox or Freeview receiver). Then check whether the TVI is cured. If the TV interference is eliminated, it could be due to either:-
- Interference pickup by poor quality coaxial cable interconnects like this one, between your video recorder or set top boxes --> read more
- The UHF modulator of your peripherals --> read more
If interference is still present after running this test, then it is likely to be caused by a nearby radio transmitter, phone or Tetra mast which is overloading your TV distribution amplifier or signal booster if you have one.
Where interference is from a nearby PMR or Tetra radio transmitter, a suitable TVI filter may help. If your aerial has a signal booster, curing the problem will be more difficult. Read more
Where intermittent interference affects digital TV reception, it is likely to be from a nearby PMR radio transmitter mast, taxi base or amateur radio station. Note the times of the interference and contact the BBC for help, if the problem persists. Find out more about the new BBC TV and Radio Interference Investigation Service.
Interference from PMR radio masts can usually be cured by fitting a high pass or notch TVI filter or by improving the aerial installation and replacing any unscreened high gain aerial amplifiers. Screening the aerial from the source of the problem (if identified) can also help.
Causes of Freeview Interference
In areas which suffer poor Freeview reception, aerial amplifiers (also known as signal boosters) are frequently used to improve terrestrial digital TV reception (DVB-T).
Unfortunately, high gain aerial amplifiers (signal boosters) overload easily when strong radio signals are present nearby, often leading to serious TVI problems. In many cases, the nearby radio transmitter that is blamed for the digital TV interference is actually not at fault.
The real culprit is the aerial amplifier which is being overloaded by the very strong signal of the nearby radio transmitter or Tetra mast. A single stage signal booster (with a 10dB to 15dB signal gain) is a much less likely to suffer TVI than a extra high gain type (25db - 50dB) - avoid these at all costs!
Many older TV signal boosters comprise twin stage wideband amplifiers, with a high gain of 25dB or more making them particularly prone to overload. To make matters even worse, many aerial amplifiers are totally unscreened like the one pictured here from Fringe Electronics which, when tested, had very poor interference immunity!
When aerial amplifiers suffer strong signal overload from a nearby radio transmitter mast, you'll need to fit a suitable TVI filter between the aerial and the aerial amplifier input, not at your TV to reduce the level of the interfering signal.
The TVI filter will need installing close to the aerial if a masthead amplifier (signal booster) is used. In these cases always waterproof the TVI filter connections with Self Amalgamating Tape if outdoors.
If the signal booster is an older unscreened type, it should be upgraded to a newer fully screened masthead amplifier which offer much better interference immunity.
In addition, always inspect the TV aerial and down lead for deterioration and water ingression before fitting TVI filters. Sometimes, corrosion of aerials and feeders can exaggerate, or even be the sole cause of TV interference problems.
Good Freeview reception requires a decent aerial...
To get the best signal in poor reception areas, always fit a high gain digital compatible TV aerial (such as a Triax UNIX52 Aerial, Unix 100 or similar aerial) fed with double screened CT100 coaxial cable mounted above roof height.
This may increase the digital TV signal sufficiently for any masthead amplifier to be removed altogether. Where an aerial amplifier is still needed, always fit a screened masthead type for best results, with a gain not exceeding 16dB.
Interference to Sky Digital Distribution Amplifiers
It is not uncommon for a Sky TV distribution amplifier to suffer TVI and picture patterning. In some cases interfering lines may move across the screen giving a poor picture.
A Sky distribution amplifier is usually powered from the Sky Digibox (which provides 12 volts to the coaxial cable). UHF distribution amplifiers are often mounted in the loft of a house, feeding all TV points around the home.
A Sky TV interference problem can occur on one or more distributed UHF TV channels, including the Sky TV UHF channel itself.
To diagnose the exact cause of the problem, it is wise to read all of this page to get a better overall understanding. Interference to a Sky distribution system can be caused by a number of different issues.
Common causes are Sky TV distribution amplifier overload due to being too close to a TV transmitter, or by interference from a nearby radio transmitter. However, in some cases re-tuning the Sky Digibox UHF modulator to a more appropriate (totally clear) UHF channel resolves the issue.
If interference is caused by a nearby radio transmitter, the solution is to fit a suitable TVI filter to the "Aerial In" of the Sky Digibox (assuming your TV aerial isn't fitted with a signal booster). If your TV aerial has a signal booster, then the filter must be fitted at the aerial input of the signal booster. The signal booster may be in the loft or on the roof for masthead amplifier types.
Bursts of electrical interference appear as bright bursts of white lines or dots on an analogue TV picture and usually lasts less than one minute. This can cause momentary Freeview picture break up on one or more terrestrial digital TV channels.
Common causes of impulse interference are a faulty central heating thermostat, boiler thermostat, intermittent plugs, switches or sockets around your home which are arcing. Check for this by temporarily turning off your central heating or hot water at the time the electrical interference occurs to see if the problem is solved.
It is also possible that a neighbour's property is the source of the electrical interference. Commercial garage premises and MOT Centres can have arc welders for example, which can create high levels of electrical interference. The BBC Investigation Service can help diagnose this kind of interference too.
Electrical interference to Freeview reception will always be worse with a loft TV aerial and even more of a problem where the coaxial cable down lead used is not double screened and run close to house electrical wiring.
To reduce electrical impulse interference to Freeview reception, always use double screened WF100 coaxial cable as your aerial down lead and mount your TV aerial well away from 240V domestic wiring and loft pipe work. We offer a dedicated page with tips for installing loft aerials.
Ignition Interference from Vehicles
Impulse interference from car ignition systems should have been largely eradicated as all modern automobiles, mopeds and motorcycles incorporate resistive ignition leads which provide suppression.
However, there are always exceptions and poorly maintained old vehicles can still cause significant impulse type interference which can cause problems with Freeview reception - freezing the picture and bright white dots on analogue pictures. The problem can also affect FM and DAB reception.
When vehicle impulse TV interference is experienced, always ensure that the coaxial cable used to feed your aerial is the double screened WF100 coax for the entire run. It may also help to raise the height of your TV aerial and to reposition it as far away as possible from the source of interference (I.E. nearby roads). Also try earthing the braid of the coaxial cable at the TV.
Reducing Mains Borne Interference Pickup
When tidying up the cables to your TV installation, it's fine to use cable ties to bunch the cables but just make sure that you don't cable tie power cables to your TV aerial cable as mains borne interference can then be passed into your TV aerial feeder much more easily.
Symptoms of Freeview TV Interference
Freeview TV interference is likely to cause total picture loss or the picture freezing. Such digital TV interference can be the result of electrical noise from a defective central heating thermostat (particularly in areas of the UK where there is a poor Freeview signal) and where a loft aerial is used.
It is also possible that interference from nearby radio transmitters or a Tetra mast could result in Freeview signal loss, a frozen picture with 'blocking' or break up. In these cases, it is often a good idea to check your analogue TV picture at the precise time of Freeview picture break up, to see how that is being affected.
If patterning interference is also noticeable on your Sky distribution system, then it is likely that a nearby radio transmitter may be the cause. Where bright bursts of white dots or lines are on the screen, suspect electrical interference.
Popular Types of TVI Filter
There are several different types of TVI filters as described earlier in this article. The two most common types are as follows:-
- High Pass TVI Filters - These pass UHF TV frequencies with low loss, but attenuate frequencies below the UHF TV band. High pass filters and braid breakers are particularly useful for solving interference from HF and VHF radio transmitters. Tetra High Pass Filters like the AKD HPF-6 (available from Garex) have a very sharp cut off frequency are also suitable for reducing UHF interference from amateur radio transmissions for example.
- Notch TVI Filters - Notch filters are effective at reducing TVI on one or two different frequencies. Different types are available depending on the frequency of the interfering signal.
- 4G TVI Filters - Designed specifically to reject signals from 4G cellular telephony masts.
TVI filters will not help reduce to the effects of electrical interference on your TV. They will only help when interference is due to a strong signal from a nearby radio transmitter.
The vast majority of 4G TV interference problems are caused by the use of 2 stage aerial amplifiers (high gain 25dB signal boosters) and to a lesser extent by Sky TV distribution amplifiers which inherently have less gain and are less susceptible to Tetra interference.
Some aerial amplifiers (particularly 2 stage high gain signal boosters used in extreme fringe reception areas) can get overloaded by a nearby 4G mast which operate in the 800MHz frequency range.
If you are experiencing 4G interference problems visit at800 who will provide you with one 4G TVI filter free of charge. Additional filters (should you need them) will be available to buy.
Where TVI is experienced from 4G masts, Ofcom state that most TV interference problems are caused by aerial signal boosters.
There are a variety of filters available to solve 4G interference - some are notch filters and others are low pass filters. Amazon sell at800 approved 4G filters for around £12.
Good advice on choosing the best filter and curing Tetra TVI can be found on Your Book.
If you use a TV signal booster, any TVI filter must be fitted between the aerial and the booster.
Garex HPFS - High Pass TVI Filter
The HPFS is a high pass TVI filter for reducing interference from both HF (short wave) and VHF PMR radio transmitters. These TVI filters are available from Amazon.co.uk.
Garex 6 Section High Pass TVI Filter
The HPF6 filter is highly effective at reducing HF, VHF and UHF aerial borne digital TV interference below 450MHz with a particularly sharp cut off making it ideal for solving Tetra interference. It is available from Garex. Garex also supply a range of notch filters to help eliminate interference from amateur radio frequencies.
Build a TV Notch Filter for 2m
Notch filters are effective at reducing TV interference from a particular frequency band. For example, where TVI problems are experienced from the 2 metre amateur radio band, a 2m VHF notch filter design can prove effective. It is possible to build a tuneable notch filter by using a simple series tuned circuit or by connecting an open circuit quarter wave coaxial cable stub in parallel with your TV feeder and cutting it to length to minimise the interfering signal.
I made the tuneable 2m amateur radio notch filter pictured above using an old 35mm film container. Please take a look at the TVI filter design and the circuit diagram. It is highly effective at solving TV interference from 2 metre amateur radio transmissions. In addition, you can read another useful article on building low loss band pass TVI filters by clicking here.
Sky TV Reception Problems
Read our new Sky TV troubleshooting guide for comprehensive help and tips to solve Sky Digital picture break up and pixilation issues. For more advice on improving terrestrial digital TV reception, read our Freeview troubleshooting guide.
Ofcom Radio Investigation Service (RIS) - Discontinued in 2010
Ofcom used to offer a comprehensive Radio and TV Investigation Service aimed at helping members of the public solve difficult interference problems affecting both analogue and digital TV reception as well as interference to FM broadcast services, but have now discontinued this service.
The BBC are now responsible for investigating interference problems to UK radio and TV services. To assist viewers the BBC offer help with reception problems.
The Ofcom website provides more information about the new BBC TV and Radio Interference Investigation Service. This replaces the previous Ofcom Radio Investigation Service - RIS.
Before completing a BBC interference report, please try to ascertain whether the interference is caused by faulty equipment inside your own property (frequent causes being your own central heating thermostat or fridge freezer for example). This will avoid wasting the BBC engineering team's time.
In addition, be particularly careful if you have a signal booster fitted to your TV (or masthead amplifier) as these products can directly contribute to digital TV interference issues and the BBC engineer may deem the cause to be your own equipment. In these circumstances it is worthwhile fitting a better outdoor aerial and dispensing with the booster first to see if this provides a solution to your interference problem.