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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.

Channel

Signal strength (max 6)

Average bit rate Mb/sec

5 minutes file size MB

Resolution

Multiplex and mode

BBC 1

3/6

3.9/3.28

154.5/128.6

720 x 576

1, 16QAM

BBC 2

6

3.21

125.5

720 x 576

1, 16QAM

ITV 1

4

3.25

124.8

704 x 576

2, 64QAM

C4

3/5

2.52/2.38

98.5/94.9

704 x 576

2, 64QAM

C5

5

2.52

100.7

720 x 576

A, 64QAM

BBC 3

6

2.83

111.5

720 x 576

1, 16QAM

ITV 4

3/5

1.94/1.95

77.3/80.2

544 x 576

2, 64QAM

Film 4

6

1.76

69.4

720 x 576

D, 16QAM

More 4

4

2.54

99.1

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.

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.



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Comments
Wednesday, 14 December 2011
Briantist
sentiment_very_satisfiedOwner

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
Sunday, 12 February 2012
I
Ian
11:19 AM
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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Ian's 1 post GB
Dave Lindsay
sentiment_very_satisfiedPlatinum

11:54 AM

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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Dave Lindsay's 5,724 posts GB
N
Nikki Halsall
12:31 PM

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

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Nikki Halsall's 4 posts GB
Dave Lindsay
sentiment_very_satisfiedPlatinum

12:50 PM

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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Dave Lindsay's 5,724 posts GB
N
Nikki Halsall
6:18 PM

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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Nikki Halsall's 4 posts GB
Monday, 13 February 2012
Dave Lindsay
sentiment_very_satisfiedPlatinum

1:31 PM

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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Dave Lindsay's 5,724 posts GB
Tuesday, 14 February 2012
N
Nikki Halsall
9:19 PM
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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Nikki Halsall's 4 posts GB
Wednesday, 15 February 2012
Dave Lindsay
sentiment_very_satisfiedPlatinum

4:35 PM

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's 5,724 posts GB
Dave Lindsay
sentiment_very_satisfiedPlatinum

4:38 PM

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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Dave Lindsay's 5,724 posts GB
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