How Video On Demand Works (long and technical!)

[spiderplant]

Originally Posted by ianathuth

Would it be possible to supply the normal digital channels using the same method as VOD but without the pause, rewind, etc. that VOD has? That is just transmitting the live pictures from the selected channel to your TV.

Yes, it's known as Broadcast On Demand (BOD). See here for a presentation. (Although that's not about cable, the same principle applies).

"There goes Bod..."

[Hom3r]

Forgive my if I'm being stupid but to rewind/pause VOD doesn't the STB use a hard drive in it recording the last 30 mins or so??

[bob_builder]

Originally Posted by david.ewles

Forgive my if I'm being stupid but to rewind/pause VOD doesn't the STB use a hard drive in it recording the last 30 mins or so??

No, it just requests that video stream is rewound or paused and requests the play is started again from the point when play is pressed in much the same way as a Real Video stream works on the internet.

What you are thinking about is a PVR, which records live TV broadcasts on a hard drive.

[etccarmageddon]

I think some freeview boxes record the last 30 mins or seconds (cant remember which) into internal memory - perhaps that's what he's thinking of.

[bob_builder]

Originally Posted by etccarmageddon

I think some freeview boxes record the last 30 mins or seconds (cant remember which) into internal memory - perhaps that's what he's thinking of.

There are a number of Freeview PVRs available, some with dual-tuners like Sky+, from around £150 that offer "Pause Live TV" as an option. These work by buffering up to 30 minutes of video from the currently viewed channel.

They also offer the recording to hard drive feature of all PVRs. Hopefully, the new NTL PVRs will offer the same features when they are released at the end of the year.

[wir3d]

Originally Posted by bob_builder

No, it just requests that video stream is rewound or paused and requests the play is started again from the point when play is pressed in much the same way as a Real Video stream works on the internet.

What you are thinking about is a PVR, which records live TV broadcasts on a hard drive.

When a movie is loaded on to a VOD server for the first time, "trick-play" files are generated. One is just an MPEG of your movie being played fast-forward at 6-7x speed, and another is an MPEG of your movie going backwards. These files, and the main movie file, are all cross-indexed together with respect to time.

When you REW or FF, the server stops playing out the main movie, and switches to playing out the appropriate trick play file, from the appropriate index point.

This saves a huge amount of CPU load on the server, freeing up more capacity for normal playback: generating the FF/REW image on the fly is a fairly expensive process.

[etccarmageddon]

Originally Posted by bob_builder

There are a number of Freeview PVRs available, some with dual-tuners like Sky+, from around £150 that offer "Pause Live TV" as an option. These work by buffering up to 30 minutes of video from the currently viewed channel.

They also offer the recording to hard drive feature of all PVRs. Hopefully, the new NTL PVRs will offer the same features when they are released at the end of the year.

no, I've actually seen a freeview box in a shop which isnt a PVR but which has a short 'pause' facility via a small in built buffer (unless it was labelled incorrectly in the shop!). perhaps that's what this bloke was thinking of.

[Tristan]

Originally Posted by wir3d

When a movie is loaded on to a VOD server for the first time, "trick-play" files are generated. One is just an MPEG of your movie being played fast-forward at 6-7x speed, and another is an MPEG of your movie going backwards. These files, and the main movie file, are all cross-indexed together with respect to time.

When you REW or FF, the server stops playing out the main movie, and switches to playing out the appropriate trick play file, from the appropriate index point.

This saves a huge amount of CPU load on the server, freeing up more capacity for normal playback: generating the FF/REW image on the fly is a fairly expensive process.

I was told about this after I posted the big long thing above. Decided not to include it as I couldn't think of a way to put it consisely, but you've done a grand job there

[bob_builder]

Originally Posted by etccarmageddon

no, I've actually seen a freeview box in a shop which isnt a PVR but which has a short 'pause' facility via a small in built buffer (unless it was labelled incorrectly in the shop!). perhaps that's what this bloke was thinking of.

Ah! I have not seen those myself, but I guess the technology is the same as the PVR just using a memory buffer rather than a hard drive. However, you would need a lot of memory to store 30 minutes of video so I guess it would be a smaller buffer period.

[paul11974]

when is it being rolled out in bromley ?

how will we know we have got it ?

will it rearange my channel list ?

[Tristan]

Don't know

Everything will be blue (again)

No

[paul11974]

why that little smile at the end ?

im feeling nervous now

[russellelly]

Originally Posted by bob_builder

Ah! I have not seen those myself, but I guess the technology is the same as the PVR just using a memory buffer rather than a hard drive. However, you would need a lot of memory to store 30 minutes of video so I guess it would be a smaller buffer period.

The Fusion FRT101 has a 30 seconds channel buffer for rewinding live TV.

Thanks for the post Tristan, I had always wondered where all this bandwidth was coming from, obviously localistation is the key. Cunning

[browneggsandham]

Originally Posted by paul11974

why that little smile at the end ?

im feeling nervous now

He's just a friendly guy

[dd2k]

great post tristan. cheers
Any news on content for launch?