What is TVI?
Simply, TVI is the interference of TV reception by transmissions from another source. In ham radio, we are specifically talking about transmissions from an amateur station, although it is not uncommon for an amateur to be blamed when the interference is coming from somewhere else.
TVI can take a number of forms:
- interference with picture only
- interference on certain TV channels
- interference with sound only – this is more common in situations where the audio system is separate from the TV
- interference with both sound and picture
- interference when operating on certain bands
On analogue TV stations, TVI will typically result in “patterning” in which either
- a pattern is superimposed on the TV picture with little actual distortion
- the superimposed pattern causes significant distortion to the picture
On digital TV stations, TVI typically causes picture freezing or breaking up.
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 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 filters are particularly good at rejecting VHF interference to UHF TV.
Curing TVI – practical suggestions
Interference with picture only
Interference with sound and picture
Interference with sound only
First, make sure that the amp has good earth ground. At the back of the amp, attach a .01µF disc ceramic cap between each speaker terminal and the amp chassis. 12 speakers, maybe 24 caps – whatever happened to good old mono?!?!?
Another thing to try is using a shielded speaker cable. Two conductors plus shield – shield grounded on amp end only.
An AC line filter on the power connection for the amp might help if the RF is getting in from the AC line.