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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, 15 February 2012
Nikki Halsall
8:24 PM

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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Nikki Halsall's 4 posts GB
Thursday, 16 February 2012
Dave Lindsay

11:55 AM

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

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Dave Lindsay's 5,724 posts GB
Tuesday, 1 May 2012
8:41 PM

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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steve's 1 post GB

9:11 PM

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 ?

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NICK ADSL UK's 72 posts GB
Tuesday, 15 May 2012
Andy Peat
3:51 PM

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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Andy Peat's 1 post GB
Friday, 18 May 2012
Saturday, 19 January 2013
Gordon Elliott
3:33 PM

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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Gordon Elliott's 9 posts GB
Dave Lindsay

5:46 PM

Gordon Elliott: Your postings are on another page:

Freeview reception - all about aerials | Installing | - 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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Dave Lindsay's 5,724 posts GB
Sunday, 29 September 2013
George Buchanan

11:18 AM
Fort William

Does anyone else regard the poor quality of many of the digital channels to be an issue? Most have reduced resolution and bit rate which seems to me to be an issue as TVs mostly now have significantly larger screens. Maybe the poor quality was passable a few years ago but it's so apparent and intrusive with large screens. Our TV services should surely not be simply judged on how many channels we have. Watching some channels on a 42" screen is like watching an old videotape.

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George Buchanan's 43 posts GB
Friday, 21 March 2014
8:45 PM

I look after a distributed TV system in a Nursing Home located in the Mumbles Swansea. Signals are received from Wenvoe in Cardiff. Since the chimney that the aerial was attached to has been removed the reception has got worse. The aerial is now temporarily attached to scaffolding fairly near it's original position. I have a signal meter (a Satlink 6909) and looking at the signal level from the aerial which is around 60% but the quality is continually changing between 10 and 50%. Same effect with a test of a DAT45 aerial Any clues please? Thanks

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Chris's 1 post GB
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