How signal stregth, quality and bitrate relate

Published on 1st September 2006 at 16:44 by Brian Butterworth on UK Free TV

I have been asked about the relationship between the "signal strength", "signal quality" and the pixilation and stuttering he got when trying to record from various Freeview channels onto a personal video recorder (PVR), computer (via a USB Freeview adaptor) and a DVD-R recorder.

In summary, the power of the transmissions has nothing to do with it at all.  This simply affects the distance from the transmitter that the services can be received. Each multiplex provides either 18Mb/s (16QAM, mux 1, B, C and D) or 24Mb/s (64QAM mux 2 and A), which is used to provide up to 8 TV channels by using "statistical multiplexing".  This means each TV channel is provided using around 3Mb/s, which is the value you found. (Mb/s is megabits per second, so 1Mb/s is a million bits per second and so on. Computers usually measure file sizes in bytes, which are 8 bits. So to record one second of a 8Mb/s transmission requires 1MB or megabyte.)

When the signals are increased in strength at switchover they will all be changed to 64QAM providing 24Mb/s.

Trial recordings

Trial records were made 8km from the Angus transmitter.

Let me explain why these results happen.

There is a huge amount of difference between the way analogue and digital transmission systems work. 

Analogue and digital

On analogue, the picture is transmitted by a system that scan from left to right, row by row down then up the TV screen.  At the start of each scan line, a "very high" signal is transmitted.  This means that even if the signal gets some interference, the TV can recover what it was doing at the start of the next line.

The digital TV pictures are first broken into three components, broken into 8x8 blocks, the blocks are then encoded using fast Fourier transformations, scaled by a compression value, and run-length encoded to create a stream of 1s and 0s.  Then up to eight of these streams of 1s and 0s are multiplexed together to produce no more than (but as close as you can get by dynamically adjusting the compression value) 18Mb/s or 24M/s.

These are then transmitted using a system called COFDM, which uses a system called "forward error protection" to hopefully deliver the bitstream without a single error to the receiver.  However, if a SINGLE error occurs, it can be quite some time before it can be corrected for.  This is because a single bit can represent many different values: the TV channel, a value of 1, 2, 4, 8, 16, 32, 64 or 128 in a single byte, the number of repeats of another value and so on.

This means that if you CAN get an error-free reception of the MPEG-2 (as it is known) bitstream, to record or playback this bitstream is a simple matter of copying it to storage and then reading it back again.

However, if you do not get PERFECT reception, then if you record this bitstream onto a DVD or the hard disk drive in a PVR, then it will pixelate and stutter at points where the bitstream was corrupted.

The "standard" picture size for MPEG-2 (called Main Profile at Main Level, MP@ML) is 720x576.  This can represent a picture in the old-fashioned 4:3 TV screen, or the modern 16:9 widescreen layout.  On a 625-line analogue TV system, only 576 lines are used for picture, the rest is for Ceefax/Teletext and other signalling.

Of the 720x576 pixels only 702 horizontal pixels are designated for viewing, the other 18 being a "bleed" so the recording can be scaled for broadcaster purposes.  ITV (and Channel 4 sometimes) use an "off-standard" system of using only

544 horizontal pixels (ie three-quarters width) that provides additional data compression benefits, but with a loss in the picture quality.

Bit rate

The bitrate is determined by several factors.

Firstly, there are some channels that cannot be statistically multiplexed, these being BBC ONE, BBC TWO, itv-1 and Channel 4.  This is because they are different on each transmitter, so these channels have a "reserved" (ie, maximum) bandwidth.

Secondly, there is the nature of the material.  Static pictures and cartoons compress much better than unpredictable material.  Strobe effects are the worst, followed by panning shots of crowds watching football.

Thirdly, material that is live and has to be compressed in real-time produces more data than archive material that can be compressed with more computing time.  This is why Films on DVD look better for the same bandwidth than something you might record.

Fourth, the amount of bandwidth the broadcaster wishes to use, because higher compression results in more choice of channels, but with reduced picture quality.

Fifth, the ratio between three types of frame: the "full picture" frames that are broadcast periodically (every second frame to several seconds apart) and the "forward predication" and "backward prediction" frames that consist of the difference from the "full picture" frames.

So, this means:

• Signal strength has nothing to do with the bitrates;
• Channels on the same multiplex can use different bitrates;
• File size is directly proportional to the bitrate (eight bits in a byte and so on)
• Momentary interference that is not correctable by the forward error correction (say from a GSM phone or two-stroke engine) takes at least one frame and often as many a few seconds to correct for.
• A single bit error can occur without any change in the signal strength, as it is function of the interference level, not of the original transmission.

Conclusion

If you conclude that the "claim that switching to digital TV is an improvement in viewing quality is not substantiated" is quite correct.  If you had prefect or near-perfect analogue reception, digital TV will decrease the picture quality.  This is because digital TV provides more services using the same transmission frequency.

However, many people are unable to get PERFECT analogue reception, but will get PERFECT (uninterrupted bitstream) Freeview reception.  For these people, the picture quality will improve.

But, Freeview reception where the "bit error rate" is high will result in pixilation or picture freezes.  In this situation, you should:

• improve the quality of the digital signal received by the aerial by using a Class I CAI-marked digital aerial;
• then, use a masthead amplifier to boost the level of the digital signal, not the background interference;
• then, use satellite grade co-axial cable to connect the aerial to the set-top box;

So, in conclusion, Freeview reception for recording will only work if you can receive the bitstream with no errors.  This depends more on the level of interference, rather than the signal strength.

I hope this explains everything.

First comments Earlier comments  ◊ 

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George Buchanan
Sunday 17 October 2010 1:49PM Fort William

'When the signals are increased in strength at switchover they will all be changed to 64QAM providing 24Mb/s'

Can you expand on this? Is this the total bit rate transmitted for all channels, a group of channels or what? After the first phase switchover, there doesn't seem any change in bit rate in the BBC channels at least.

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Mike Dimmick
Sunday 17 October 2010 5:22PM

George Buchanan: it's the total bitrate available from one multiplex.

The original onDigital service used 64QAM 2/3 mode, 24 Mb/s, for all six multiplexes. To expand the coverage available on low-power services, after the failure of ITV Digital, multiplexes 1 and B (BBC) and C and D (Crown Castle, now Arqiva) were changed over to 16QAM 3/4 mode, for 18 Mb/s. At switchover, the high-power services go back to the original 64QAM 2/3 mode to get more bits from the same space.

The BBC have been forced to give up their second multiplex (Mux B) for the high-definition services; as a result, the services on Mux B before switchover - BBC Four/CBeebies, BBC Parliament, additional Red Button video content - move to BBC A and it's believed that the BBC have had to reduce bitrate for their channels slightly to cram them in.

To keep parity between the services available before and after switchover, the BBC have actually killed a number of the Red Button streams that used to be available on Mux B, and use the capacity to broadcast services that are in the extra capacity available on ArqA and ArqB after switchover. The first two slots went to BT which chose to broadcast Sky Sports 1 and 2 in them, and a further slot on ArqA is now being made available.

See  Freeview multiplexes | ukfree.tv - independent free digital TV advice for the line-up of which channel is on which multiplex. (RG47SH)

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George Buchanan
Monday 18 October 2010 10:50PM Fort William

Thanks for that! It's all quality reduction for the sake of often dubious quantity. I have noticed that BBCHD bit rate is quite a bit less than ITV HD judging by the file size.

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George Archer
Monday 8 November 2010 7:57PM

I live close to the Darvel xmtr - while most channels have high strength, I am suffering from poor/variable signal quality which I am assuming is the reason I get interrupted pictures on itv1,3, channel 5 - bbc1 is almost 100% ok. I have already added a signal booster ( not mast head) but still got issues. How critical is aerial alignment to the mast? Thoughts on how to remove pixelisation ?

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Brian Springthorpe
Wednesday 10 November 2010 6:03PM Penmaenmawr

I live in Penmaenmawr, North Wales. We are now fully digital and the analogue signal is no more!

I believe our transmitter is on Anglesey, (Llandonna).

I am currently experiencing a problem with my digital recorder. It is a basic, (i.e. non-HD) Panasonic DVD/HD recorder. Occasionally it asks the question when it claims that new channels are available etc. "Do I want to retune", (or words to that effect.

Well over the last few days or so I have noticed that it is not picking up the BBC channels, i.e. BBC1, BBC2, BBC3 and BBC4.

Its strange because all of the other channels are OK and also the TV which it is attached to, which is also a Panasonic which has its own tuner, is picking up the BBC channels.

The little TV in the bedroom which is a cheap and cheerful Tevion from Aldi is also picking up the BBC channels without any problem.

I have noticed some pixellation on ITV4 but I don't know if that is related to the BBC problem.

Does anyone know why this is happening?

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Brian Souter
Wednesday 15 December 2010 10:40AM

I was getting good picture quality with no break up using an LG digital tv. I have now connected a Humax pvr and have lost ITV, Channel 4, 5 and several others. Should I consider a new aerial?

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Briantist
Wednesday 15 December 2010 1:52PM

Brian Souter: Perhaps, but first please see Single frequency interference | ukfree.tv - independent free digital TV advice .

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charles
Wednesday 2 March 2011 7:16PM

I have a new Samsung LED TV.
I am getting vertually no viewable channels(very very poor pixalation). I am however getting good reception from another Pioneer TV using the same ariel socket and lead.
The TV has been back to Samsung who say there is nothing wrong with the TV, it must be the ariel.
Do you have any suggestions

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Briantist
Wednesday 2 March 2011 7:41PM

charles: I would phone Samsung and ask them to explain how your other TV is fine with exactly the same aerial and cables.

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Richard
Wednesday 16 March 2011 3:17AM

Charles,
I also have Samsung LED TV, I paid quite a bit for it. I have also noticed that channels on Muxs C, D, & 2 pixel out & freeze, and a re-tune will often not even pick up the channels on these muxs. My old Sony CRT TV never had a problem, and my Sony PVR box is fine.

I think certain components have suffered in order to get an ultraslim TV.

Sad thing is they'll never admit this, nor will they attempted to right it. They'll just be concentrating on making an even thinner TV for next year.

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NICK ADSL UK
Wednesday 16 March 2011 7:59AM

Hi Charles
The Samsung TV's need a strong signal only they don't work in my house where my panasonic is perfect

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Robert Sprigge
Sunday 17 April 2011 1:53PM

I had expected Freeview reception to improve when the 'Switchover' was completed whereas it's disappeared on my recorder. I was expecting they would be on higher power now.

I've been able to recieve Freeview Sandy Heath signals without problem previously on both TV and digital recorder however now the TV signal breaks up and the digital recorder merely gives the channel details without picture.

Given this aerials alighnment the Sandy Heath transmitter has a lower power than the London region transmitter it's aligned to and hence channels are listed from 800 onward.

I've retuned twice since Switchover part two without sucess :-(

Any thoughts appreciated.

3 posts

Briantist
Monday 18 April 2011 6:08AM

Robert Sprigge: Please do this My Freeview box has no EPG, is blank on FIVE, ITV3, ITV4, ITV2+1, has no sound or the channel line up is wrong | ukfree.tv - independent free digital TV advice .

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Keith Fox
Saturday 19 November 2011 3:45PM

I have a Samsung tv. Just recently and on some days [all] BBC channels including the radio channels freeze intermittently . At other times they work fine. At the same time all other channels work normally. As the BBC picture freezes the signal strength goes to 0 and the bit date level shoots up to the maximum of 2000. Is this a TV problem or is something up with the BBC transmission?

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Martyn Orchard
Tuesday 22 November 2011 9:11AM

Hi
During foggy, damp weather, like now, we suffer from a poor quality TV signal that breaks up yet the strenght is strong. We live in the SO45 Area near Southampton

This tends to happen primarily during the Autumn/ Winter months and only seems to affect the ITV Channels ie ITV1,2, Channel 4 and all derivatives.

It is very frustrating as we can no even record programs

Is there anything we can do please? And will things improve when our area goes fuly digital?

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Briantist
Wednesday 14 December 2011 6:01PM

Keith Fox: Please can you see Single frequency interference | ukfree.tv - independent free digital TV advice ?

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Briantist
Wednesday 14 December 2011 6:03PM

Martyn Orchard: The most likely cause is that water is getting into the cables that connect your box to the aerial. The best thing to do is to replace the cable, preferably with satellite-grade coaxial cable.

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Ian
Sunday 12 February 2012 11:19AM Bognor Regis

Have just set up freeview Humax box for my parents in Bognor Regis area. ITV all fine but BBC channels breaking up which appears to be due to signal quality issue. Is the view this might correct at switchover in March or an issue with the areial. Thoughts welcomed

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Dave Lindsay
Sunday 12 February 2012 11:54AM

Ian: Yes, the power increase at switchover should hopefully change reception from being "iffy" to being solid.

If the transmitter that the aerial is facing is Rowridge on Isle of Wight then it will transmit signals horizontally (as now) and vertically. However, the power of the commercial (COM) multiplexes will be lower for horizontal polarisation and will match that of the Public Service Broadcaster (PSB) multiplexes for vertical polarisation.

The PSBs will be the same power horizontally and vertically and this will be to provide coverage equivalent to the current analogue (and so that aerials shouldn't need adjusting for reception of PSB services).

So if reception issues are found picking up the COMs, then switching the aerial from horizontal to vertical might help.

It will not be until 18th April that the COMs will move to their final channels at their final powers (and when their vertical signal will be introduced), so if issues are found before then, it might be best to wait until that date before considering whether the aerial needs adjusting (or replacing).

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Nikki Halsall
Sunday 12 February 2012 12:31PM

hi with regards to a caravan set up which works perfecly with my philips freeview tv, i have just bought a humax 9300t pvr that shows 100% signal strength but the quality ducks and dives from 100% to 40% causing loss of picture does a pvr need a better quality signal than freeview

4 posts

Dave Lindsay
Sunday 12 February 2012 12:50PM

Nikki Halsall: The TV and PVR use Freeview signals in the same way.

100% signal strength could indicate that the strength is too high which is causing quality issues. This could be caused by an amplifier/booster and/or an OTT aerial pulling in too much signal.

See here for a further explanation and possible solutions:

 Freeview signals: too much of a good thing is bad for you | ukfree.tv - independent free digital TV advice

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Nikki Halsall
Sunday 12 February 2012 6:18PM

thanks dave for quick response have removed booster and turned off booster in built in the pvr signal strength is down to 71% and holding steady (super) but the quality is jumping about each couple seconds from 100%, 40%, 50%, 100%, 60%, 40% etc it just wont settle, any ideas?

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Dave Lindsay
Monday 13 February 2012 1:31PM

Nikki Halsall: Different equipment has different levels of sensitivity and tolerances for poor signals. There can be strong "poor" signals just as there can be weak "poor" signals.

You could check that both the Philips and the Humax are tuned to the same UHF channels (frequencies). This information is usually given on the signal strength screen whilst a programme channel is selected. The following services are all on different channels (multiplexes): BBC One, ITV1, BBC One HD, ITV3, Pick TV, Yesterday. For each of these on the Philips, see which UHF channel they are on. Then cross-reference with those that the Humax is tuned to. This will establish that the signals to which the Philips is tuned to are the same as the Humax.

You could be in an area served by more than one transmitter, and that your receiver has tuned to the one which your aerial does not face.


Some thoughts on TV reception for a caravan in general are that, if it's a conventional direction TV aerial, such as the "Yagi" shown in the top picture here:

 Freeview reception - all about aerials | ukfree.tv - independent free digital TV advice

then:

1. Try turning it and check that it isn't sloping downwards. For example, if it's on a pole where one end rests on the ground, ensure that it isn't causing the aerial to slope downwards. The angle of the aerial could be slightly off (i.e. to one side of the main beam of the signal) and turning it slightly might correct this.

2. Due to the fact that a caravan tends to be low down, the aerial is going to be relatively close to the ground. Try adjusting the aerial so that it slopes upwards slightly, if the fixing allows. It is a "try it and see" sort of thing.

3. A classic mistake of caravanners, barge owners and the like is to have the aerial incorrectly polarised for the transmitter they are receiving. By this I mean that the aerial is horizontal when it should be vertical and vice versa. Copying off neighbours might be of no help because they may have it wrong too! If this is the case, then you might get, what on the face of it, seems to be a good signal, but which, when you wish to watch your favourite programme, turns out to break-up.



If you provide your location (preferrably in the form of a post code), then reception possibilities could be checked upon.

Or, you could use the Digital UK checker here:

 Digital UK - Postcode checker

Tick the box to say that you're in the trade and it will come up with a list of transmitters and their channels. It will also tell you the direction from your location. Whether you need your aerial horizontal or vertical is shown in the "Aerial Group" column, denoted as "H" or "V" respectively.

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Nikki Halsall
Tuesday 14 February 2012 9:19PM Banbury

hi dave; thank you so much for your help am going to keep experimenting with your sugestions will let you know how i get on, meantime postcode is ox17 1qu thank you

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Dave Lindsay
Wednesday 15 February 2012 4:35PM

Nikki Halsall: Oxford looks to be your most likely transmitter. It is at 162 degrees from your location which is therefore a bit anti-clockwise of south. The aerial should be horizontal. When you go on to the signal strength screen it should tell you that it is tuned to channel 53 (which is the BBC signal from Oxford).

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Dave Lindsay
Wednesday 15 February 2012 4:38PM

Nikki Halsall: I should also warn you that the commercial multiplexes from Oxford are on low power until 18th April. This includes ITV3, Pick TV, Yesterday and others. Thus, if you have issues receiving these, then that might be the reason why.

The Public Service Broadcaster multiplexes are on full power now. These include BBC, ITV1, ITV2, C4, E4, More4, C5, HD and a few others.

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Nikki Halsall
Wednesday 15 February 2012 8:24PM

hi dave

a massive thank you from a very happy pvr owner i have retuned tv now on same uhf channels as pvr all seems well i would never have thought this without your help what a brilliant site, thank you.

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Dave Lindsay
Thursday 16 February 2012 11:55AM

Nikki Halsall: I'm happy to help. Good to hear that you got it sorted.

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steve
Tuesday 1 May 2012 8:41PM

I HAVE COMPLETELY LOST FREEVIEW ON MY SAMSUNG LE 40C530 LCD TV. TV SAYS WEAK OR NO SIGNAL AND BIT ERROR LEVEL 2000, SIGNAL STRENGTH zero. I have tried unplugging everything and reconnecting - makes no difference. I have plugged my 32 inch samung in at same point with same aerial connection and it works fine. Any advice welcome. If i can't solve this will sky work on tv instead - i may have to get it.g

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NICK ADSL UK
Tuesday 1 May 2012 9:11PM

Hi Steve
Samsung can be a problem with a less then perfect signal quality
What signal quality are you getting with your 32 inch Samsung ?

66 posts

Andy Peat
Tuesday 15 May 2012 3:51PM

I'm having a total nightmare with my LG 50pk590 since the main switchover last autumn. I've had an engineer out twice and on the last visit for diagnostic purposes he put a splitter on the cable and monitored it using a spectro. Sure enough it pixelated with no errors coming in on the spectro. It was fine on digital channels before the switchover. Don't know what to do as it looks like the signal is good. maybe the tv decoder can't cope with the new type of signal (all firmware up to date).

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Briantist
Friday 18 May 2012 6:48PM

Andy Peat: I would suggest you see the Freeview signals: too much of a good thing is bad for you | ukfree.tv - independent free digital TV advice page.

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Gordon Elliott
Saturday 19 January 2013 3:33PM

Sorry, but can someone tell me why I cannot see posts later than 18th May 2012.

I actually have posted a couple of days ago and had a response, but now I cannot find them anywhere. Is it because I have cleared my browser cache?

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Dave Lindsay
Saturday 19 January 2013 5:46PM

Gordon Elliott: Your postings are on another page:

Freeview reception - all about aerials | Installing | ukfree.tv - 10 years of independent, free digital TV advice

In some cases you may find that you have to click on the "Page #" tab near the bottom of the page, because since you made your posting a new page has been started.

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