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How signal stregth, quality and bitrate relate

What is 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.

What is  the relationship between the  signal strength ,  signa
published 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.


Signal strength (max 6)

Average bit rate Mb/sec

5 minutes file size MB


Multiplex and mode





720 x 576

1, 16QAM





720 x 576

1, 16QAM





704 x 576

2, 64QAM





704 x 576

2, 64QAM





720 x 576

A, 64QAM





720 x 576

1, 16QAM





544 x 576

2, 64QAM

Film 4




720 x 576

D, 16QAM

More 4




544 x 576

2, 64QAM

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.


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.

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Wednesday, 2 March 2011
7:16 PM

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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charles's 1 post GB

7:41 PM

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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Briantist's 38,844 posts GB
Wednesday, 16 March 2011
3:17 AM

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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Richard's 1 post GB

7:59 AM

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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NICK ADSL UK's 72 posts GB
Sunday, 17 April 2011
Robert Sprigge
1:53 PM

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.

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Robert Sprigge's 3 posts GB
Monday, 18 April 2011
Saturday, 19 November 2011
Keith Fox
3:45 PM

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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Keith Fox's 1 post GB
Tuesday, 22 November 2011
Martyn Orchard
9:11 AM

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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Martyn Orchard's 1 post GB
Wednesday, 14 December 2011

6:03 PM

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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Briantist's 38,844 posts GB
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