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Digital TV's
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I have a regular analogue TV and a seperate Digital TV. When both are on, tuned the same channel, the digital TV is presenting the programme a few seconds after the analogue. Why? Surely then time checks presnted this way are inaccuate?
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The terrestrial tv network has been digital at the distribution level for some time. Analogue and Digital transmitters and distibution networks are not necessary the same for a given region. The digital delay that you have experienced arises from the distributed signal being stored on buffers during its path through the distibution network on its way to the various transmitters.
A further delay is inevitable when the signal arrives at the digital transmitter as the stream has to be converted again into the multiplex signal - rather similar to creating an MP3. This requires more buffering hence the delay. I don't think that the total real delay from studio to tv set can really make that much difference to your time-checks, as the latency can only be in the order of two seconds at most.
As an aside, you might notice that some peak time adverts on ITV1 are different on digital and analogue. This is because the digital signal is networked from a different regional centre and you are seeing adverts aimed at, say, Yorkshire when your loacl analogue is aimed at Northumberland. (It's a well known fact that Yorkshire people drink tea, whilst the Northumbrians are all gaga on Lindisfarne Mead!)
A further delay is inevitable when the signal arrives at the digital transmitter as the stream has to be converted again into the multiplex signal - rather similar to creating an MP3. This requires more buffering hence the delay. I don't think that the total real delay from studio to tv set can really make that much difference to your time-checks, as the latency can only be in the order of two seconds at most.
As an aside, you might notice that some peak time adverts on ITV1 are different on digital and analogue. This is because the digital signal is networked from a different regional centre and you are seeing adverts aimed at, say, Yorkshire when your loacl analogue is aimed at Northumberland. (It's a well known fact that Yorkshire people drink tea, whilst the Northumbrians are all gaga on Lindisfarne Mead!)
Classy, I based my explanation on the terrestrial set up, but the same conditions apply to a satellite system, it is just that the distribution path is slightly longer. However, it is not the length of the path in this case that introduces the noticeable delay, but the electronics and signal processing at stages along the way.
Consider the geostationary tv satellite high above our heads. To be geostationary (appear in the same place in the sky all the time) it has to exactly match the rotation of the earth. To do this using momentum and an equilibrium of forces requires the satellite to be 35,900 kilometres above the surface, so the signal from the earth to the satellite and back will be at least twice that, i.e. 71,800 kilometres. As the signal path will be unlikely to be exactly perpendicular to the Earth's surface, the path will be slightly longer than this, say in the order of 75,000 kilometres (very generous). The radio signal travels at a speed of 299,792.458 kilometres per second which for ease of calculation we will round to 300,000 kilometres per second. So the time taken for the radio signal to go to the satellite and back is 75,000 divided by 300,000 = 0.25 seconds.
A quarter of a second is hardly enough to be noticeable.
Electricity travels somewhat more slowly along solid cables so the delay is greater per kilometre, but the distances are considerably shorter. When chatting on the telephone to Australia in the days before satellites, and the signals were sent on cables for most of the trip, I noticed the delay appeared to be about 0.8 second for the 38,400 kilometre round trip.(average signal speed through cable and electronics on the way then being 30,720 kilometres per second)
The delay observed in the tv example by Martinf57 is almost wholly due to the electronic processing of the signal, and most of that is the multiplexing just prior to the main transmitter.
Consider the geostationary tv satellite high above our heads. To be geostationary (appear in the same place in the sky all the time) it has to exactly match the rotation of the earth. To do this using momentum and an equilibrium of forces requires the satellite to be 35,900 kilometres above the surface, so the signal from the earth to the satellite and back will be at least twice that, i.e. 71,800 kilometres. As the signal path will be unlikely to be exactly perpendicular to the Earth's surface, the path will be slightly longer than this, say in the order of 75,000 kilometres (very generous). The radio signal travels at a speed of 299,792.458 kilometres per second which for ease of calculation we will round to 300,000 kilometres per second. So the time taken for the radio signal to go to the satellite and back is 75,000 divided by 300,000 = 0.25 seconds.
A quarter of a second is hardly enough to be noticeable.
Electricity travels somewhat more slowly along solid cables so the delay is greater per kilometre, but the distances are considerably shorter. When chatting on the telephone to Australia in the days before satellites, and the signals were sent on cables for most of the trip, I noticed the delay appeared to be about 0.8 second for the 38,400 kilometre round trip.(average signal speed through cable and electronics on the way then being 30,720 kilometres per second)
The delay observed in the tv example by Martinf57 is almost wholly due to the electronic processing of the signal, and most of that is the multiplexing just prior to the main transmitter.
Solid answer Hippy...can I ask how you managed to notice a 0.8 second delay on a phone call though :>)
I have noticed that the delay changes depending on the set up....I have seen it at a max of about four seconds one time memorably watching football in a social club on a terrestrial set we were able to shout "he'll miss it" and annoy those watching on the bigger digital set.....For some real fun with this stuff try listening to radio on 1/an analogue set 2/ digital and 3/ streaming on the PC....it's like listening in the alps with all the delays!
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