I have just been watching news reports here in the UK sent by TV reporters in the USA, and when the news presenter in the UK says ’ over to our reporter in Washington’ there is a time lag before the US correspondent hears what the presenter has said.
You can phone anywhere in the world without experiencing a time lag.
Why is it different?
The news crew is using a satellite link to talk to the reporter. Even at the speed of light, it takes the better part of a second to bounce up and down again. Add in a little delay for digital signal processing, too.
Phone calls don’t use satellites (anymore.) There are wire and microwave links to just about everywhere you need to call. But those don’t work so well for HD video.
How fast are satellite links and wire and microwave links?
Ask anyone who tries to use Sat internet for gaming, where pings are important.
Unlike the space shuttle and ISS that orbit a few hundred miles above earth, Geo Sync satellites are just over 22,000 miles above earth (that is roughly 1/12 the distance to the moon).
Add to this multiple relays and perhaps multiple hops to and from the satellite and even at the speed of light (roughly 186k miles per second), there are delays that are fractions or even whole seconds.
All this means never to play an FPS over Satellite :dubious:
At 22,000 miles, the round trip to the satellite takes just under a quarter of a second. So the delays mostly have to be in switching (or simply delayed human response from the remote newscaster) I’d think.
In addition , the encoding used in digital broadcasts introduce another delay.
As fast as light!
If this is a serious response, wire transmissions don’t travel at the speed of light.
Exactly. The difference is that you can go around the world almost twice with a cable as long as it takes to get up to a geostationary satellite and back down again.
I’m confused. How are signals sent to satellites?
They’re sent to the satellite via an uplink antenna, and retrieved via a satellite dish.
Here’s a picture of a truck for a TV station, with an uplink antenna on the back.
And, here’s a picture of some of the BBC’s satellite dishes (presumably located at or near a BBC production facility).
So, your UK reporter in the US is at a location somewhere, with a camera truck, containing one of those uplink antennas. The antenna on the truck has been oriented to point at a particular satellite in geosynchronous orbit. His signal goes into the truck, through the antenna, and up to the satellite. The signal bounces off the satellite, and is picked up by the BBC’s antenna back in the UK. That one-way trip, as noted above, is 44,000 miles.
Not true in my experience. When my sister was living on the other side of the world, her phone conversations had a noticeable lag.
Right. Just to elaborate a little, many digital transmissions involve “store and forward” technology. A packet of data is 100% received, then re-transmitted. This may happen several times from one end to the other. That’s what happens with most Internet traffic. For a browser page, it doesn’t make much difference; for real-time stuff like phone calls, it does, and multiple hops add up to noticeable times.
In contrast, most analog signals (pre-digital days) were sent at nearly the speed of light, and nothing was stored. In one end and out the other as simultaneously as possible. This kind of transmission is limited only by the speed of light and that is a factor only over long distances, like to/from geo-sync satellites.
This is why the BBC time signal on digital radio lags behind that on FM/AM by a second or so:-
In the US if you watch a game on TV and have the radio on at the same time, the radio is normally 1 or 2 seconds ahead. For example in BB you hear the radio guy say “it’s good” while the shot is still in the air on TV.
They have some sort of store and forward in NYC buses too BTW. The driver speaks into his handset, and a full half-second later it sounds out on the bus. I have no idea why.
Not disagreeing with you, but I think Rayne Man might have been talking about digital compression latency of the content stream itself - for video to be encoded in MPEG (or whatever), there’s a processing overhead.
I’ve noticed that as well. It’s very disconcerting when you’re sitting near the front. No idea why they do it that way.
Actually, I forgot about that. The store & forward might indeed include unpacking, processing and re-packing each packet, adding even more delays. Some other engineer will have to estimate how much each process contributes to the total delay, but it certainly can add up.
[QUOTE=kenobi 65]
So, your UK reporter in the US is at a location somewhere, with a camera truck, containing one of those uplink antennas. The antenna on the truck has been oriented to point at a particular satellite in geosynchronous orbit. His signal goes into the truck, through the antenna, and up to the satellite. The signal bounces off the satellite, and is picked up by the BBC’s antenna back in the UK. That one-way trip, as noted above, is 44,000 miles.
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More likely, the remote truck sends its signal up to a satellite, the satellite returns the signal to earth to be received by the TV network, processed and made ready for broadcast, then they kick it up to a satellite, it’s sent back down to earth again, received by your satellite TV provider, and bounced up off another satellite, making for a roughly 130,000 mile journey from that live reporter to your TV.
