[DaleT (OP)]

Whenever I use a hairdryer in the same room as my television, the signal appears to be disrupted. What is happening here?

It is to be noted this only happens with the 'freeview' box.

Best

[graham.d]

See the thread "Why does my digital TV Freeze".

Electric motors can produce a lot of Radio Frequency noise that is both radiated to the TV aerial and conducted via the mains wiring. If the aerial for your Freeview box is a simple set top box type aerial in the same room, it is probably that which is picking up the interference. The solution would be to use a proper outside aerial. If you are using an outside aerial anyway, then the interference is probably going via the mains supply. Using mains leads with suppressors (cylinder things on the leads) has some effect, but it may be that the freeview box electronics has not been adequately designed to reject mains noise. As for the hairdrier design, there is not too much that can be done apart from adding noise suppression.

You may notice this more with digital TV because of the nature of the noise. There are steps taken in the way the digital signal is comprised to make it not susceptible to various types of noise. Noise on analog TV makes it snowy or produces bright spots, but you can make out the picture OK. On digital TV the picture is perfect unless the noise reaches a certain level then will freeze or go blocky. This is the effect of the error correction schemes and of "interleaving" the data.

[lyner]

The main reason that the digital signal is worse affected it that the digital signal is at a much lower level.  There is also the factor that a single analogue channel takes the equivalent spectrum space of a lot of digital channels (it's simple but wasteful).  This can be achieved because the subjective impairment of  interference is  different with digital signals - they look almost perfect until they crash. If the analogue signal was at the same level as the present digital signal, it would look very noisy but just watchable. Until it freezes, the digital picture is OK.

[LeeE]

Exactly what sort of disruption is being seen?  Is the picture just noisy or is it dropping out/freezing (or getting 'blocky' as graham.d suggests)?

Digital signals are inherently resistant to induced electrical noise in the same way that FM analogue is resistant to it.  Digital and FM signals are not dependent upon the signal amplitude, unlike analogue AM, and it is the signal amplitude that is most affected by induced electrical noise.

Now I don't, and have never had a TV, but I'm guessing that the freeview box produces an analogue output that is then fed to the analogue tuner in the TV.

If this is so, then if you're getting 'noise' or 'snow' on your picture it may be being picked up between the freeview box and your TV and perhaps a better length of co-ax might help.  If frames are dropping or the picture is blocking or freezing, then the interference is more likely to be affecting the digital feed into the freeview box.

[techmind]

Digital signals are inherently resistant to induced electrical noise in the same way that FM analogue is resistant to it.  Digital and FM signals are not dependent upon the signal amplitude, unlike analogue AM, and it is the signal amplitude that is most affected by induced electrical noise.

Actually digital TV uses OFDM modulation which is a complicated sort of amplitude and phase modulation which is susceptible to impulsive noise interference. This is well-known.

For some in-depth technical info, see the BBC R&D White Paper 080 : http://downloads.bbc.co.uk/rd/pubs/whp/whp-pdf-files/WHP080.pdf


If you look closely at an analog TV picture when you're using the hairdrier you might see some horizontal bands which are a bit spotty (this may be less visible with the signal processing on a modern LCD TV than on a simple CRT TV).
Given the weaker digital signal, similar levels of interference could quite conceivably cause the picture to break up/freeze/whatever.

I've certainly seen the hairdrier spots on analog TV even with a rooftop aerial and a good signal from Crystal Palace, which implies that the interference is probably conducted through the mains electricity wiring. Hairdriers usually use motors with commutators so they spark a lot inside, which is what causes the interference.

You could try plugging the TV and set top box into a mains-filtering power-strip and see if that helps. You could also try plugging the hairdrier into a (different) filtered power-strip.

[graham.d]

Techmind, I would be interested in any references you have that suggests that OFDM is particularly susceptible to burst errors. All modulation schemes (on their own) are susceptible to burst errors which is why the overall schemes involve layers of error correction and (for correcting burst errors) use interleaving techniques so that the effects of a burst are distributed over time thus enabling the errors to be correctable. I wonder whether the interleave time employed is sufficient to cope with the length of signal blocking that may result from the spikes from a hairdrier motor. The problem could come via the aerial or via the mains and both possible routes of interference could be minimised by better design of the freeview box. Alas, these are built to an exacting budget and many of the designs are poor. I suspect that the front end receiver could be getting heavily overloaded and its recovery time exceeds the capability of the digital error correction scheme's ability to cope.

I agree with trying mains noise suppression though a roof aerial (if it is radiated noise) may also be the answer. I would bet that some freeview boxes are better than others but it would not be any guarantee that a more expensive one would be better - there is not always a relation between cost and quality of design. Not using the hairdrier whilst watching TV would also be a solution :-)

[lyner]

I think it's true to say that many of the cheaper set top boxes which are on the market are incredibly bad quality. The control software is really dire - response to the remote control is shocking - giving the impression of a total idiot on the other end! This would suggest bad design everywhere - including the RF and decoding circuits. Just thrown together, I reckon. I imagine they work better  under the specified conditions but that reception in many places is well below spec.
The integrated systems in good quality receivers are much better.
Hopefully, later generations of stb will be better.

Maybe the hairdryer is a bit of a rogue. They are all supposed to be suppressed to a reasonable standard. C.l.S.P.R. (Comite International Special des Perturbations Radio-electrique) have regulations for all classes of device.

[DaleT (OP)]

Thanks for all the responses guys....


I think I may do an experiment.