How Video On Demand Works (long and technical!)
 

With the software required for VOD now being rolled out in Glasgow, it seems Digitalspy's rumours of a January launch on NTL are quite possibly accurate.

With this in mind, there's been some speculation about how VOD works, whether it will have the same facilities as Sky Plus, what sort of picture quality we can expect, etc etc. So I thought I should spend a little time explaining things :)

At the moment, the Front Row service operates using what's called "Near Video On Demand", or NVOD. What this means is that there are many video streams coming to your house, each showing the same movie with staggered start times. When you order a film, the software selects the appropriate stream and shows it on channel 0. (On NTL Bromley and Telewest, this has always been invisable to the end user. Langley users will remember having to tune to channels in the 200s for Front Row, prior to the CR3 update.) This is also the way Sky Box Office operates, and how the Sky Movies "multistart" function works (there is more digital bandwidth available on satellite than cable, and so it's relatively easy for Sky to use 10 or so different streams).

True Video On Demand works rather differently. Rather than receiving scores of video steams, and selecting the appropriate one, the box takes notice of just one stream. The difference is that this steam is for you, and only you. So you can pause, rewind, fast-forward and stop playback, all without affecting anyone else. How is this possible?

Well the first thing to point out is that, contrary to what some people have suggested, the box is not downloading any video content via the in-built cable modem. I don't even know whether this is possible, and it certainly wouldn't be very efficient. So forget scare stories about web-quality pictures, because it's completely untrue. Your box will be displaying a multiplexed DVB MPEG2 steam, the same as it does at the moment.

VOD works its magic by changing the source of the video. At the moment, all the steams are prepared at NTL's DTV HQ (try saying that when you're drunk) in Langley, from where they're forwarded on to your local "headend", and then on to your house. In this way, every house in the country receives the same video streams. VOD changes this by housing the programming on servers at your local headend, instead of centrally. This means that it becomes possible for your box to send "rewind" commands (for example) to the server, which will then start playing the stream backwards.

The eagle-eyed amongst you will have spotted a problem here however. Say there are 100 Front Row streams at the moment (I don't know the exact number, but it's a ballpark figure). If you replace this by local content at the same quality, you've still only got room for 100 streams. This isn't enough for the 10,000 subscribers who might be connected to a particular headend (I've no idea how accurate this figure is!) to each have their own stream. So how do you do it?

The answer is by using the "localness" of cable. For the greater part of its journey, all data travels around on fibre-optic connections. It's only in the last few hundred metres -- at the local cabinet level -- that this is transfered to coaxial (copper) cable and into your house. If you can then send different data to different street cabinets, then you can re-use the same frequencies on the coax cable in different areas. I beleive this is already done with cable modem connections.

In this respect, it's possible to share the 100 "spaces" you've got between relatively few properties. How many you share it between will be a tradeoff. The lower the figure, the more equipment you need, and the more it costs to set up. The higher the figure, the greater the contention, and the greater the risk that there won't be a stream free when someone wants to use the service.

So that's how it works. But what will it do? Well, the first and most obvious application is to replace the NVOD Front Row system that's currently used. That's a no-brainer. But past that, you're really only limited by server disk space. A survey published last year suggested that VOD will offer a large library of old and not-so-old films, probably as a rival to Sky Movies. There are also things like classic programmes and sports. On the existing (DSL-based) VOD systems operating in the UK, Homechoice and KIT, the BBC have offered a large number of programmes -- so if you missed last week's Eastenders, you can watch it again on VOD. Channel 4 have a similar deal with Homechoice. As the first large-scale VOD deployment in the country, I suspect the On Demand Group will have been trying to make deals left, right and centre. We'll have to wait and see what they come up with though.

People have been wondering whether VOD will be cable's version of Sky Plus. It's true that some of the functionality doubles up, but really they're quite different beasts. For example, the only way you can see last week's Eastenders on Sky Plus is if your box recorded it in the first place. Likewise, pausing and rewinding films etc requires that these things are on your box's hard drive. Sky Plus also requires a new STB, whereas VOD will work on any cable box with a working return path. On the other hand, VOD will not allow subscibers to pause live TV, nor plan for future future viewing (though it's technically possible to do both these things on VOD, setting aside 80GB of disk space per subscriber is a very big ask). Both NTL and Telewest are said to be working on PVR devices, so it will be interesting to see how these work in relation to VOD.

Any questions, please ask. Hope this is of some use,

Tristan

PS: Usual disclaimers apply. I don't work for NTL, Telewest, the On Demand Group, Sky, Homechoice, KIT, or anybody else for that matter. I have no qualifications in the subject. I could be completely, massively wrong.

Re: How Video On Demand Works (long and technical!)
 

Very very accurate - Ntl's chief executive, Simon Duffy told us, VOD was launching in January.

http://www.cableforum.co.uk/article/...ary-vod-launch

But can I just say if what you say is right on how it all works - this is one hell of a post and I would just like to personally thank you Tristan for taking the time to post it up. Thread stuck. :tu:

Re: How Video On Demand Works (long and technical!)
 

Hear! Hear!

Re: How Video On Demand Works (long and technical!)
 

I'll second that - nice explanation, Tristan - just a shame I've got to spread the love before repping you again!

Re: How Video On Demand Works (long and technical!)
 

Can someone please green rep Tristan for me :)

Re: How Video On Demand Works (long and technical!)
 

done :)

Re: How Video On Demand Works (long and technical!)
 

repped as well, but you say it is getting deployed in glasgow, does this mean we will all get a wee booklet again to explain what is happening?

ik

Re: How Video On Demand Works (long and technical!)
 

Excellent post.... pos rep given :D

Re: How Video On Demand Works (long and technical!)
 

Thanks Tristan

Re: How Video On Demand Works (long and technical!)
 

Another excellent post Tristan. Your contributions to the DTV part of these forums in particular are immense and are greatly appreciated. Just don't go leaving like proppin :p:

Re: How Video On Demand Works (long and technical!)
 

Thats a very good description Tristan, I don't work for ntl either but would just like to add that the VOD kit feeding the Nodal cabinets will be located in the current "Hubsites" The VOD streams will probably use DWDM technology to combine with the existing Analogue/Digital Tv and cable modem downstream. The current TV and cable modem services are fed to an optical transmitter (DFB) to feed generally 4 nodes in a ring of 8, the output of the DFB in the original architecture fed a 4 way optical splitter, the DWDM diplexer to add in the VOD service would be located between the DFB and optical splitter. It is likely they will follow a similar route as they did with cable modems, ie: use the existing architecture and further split down nodal areas as and when demand for the service increased. (The 4 way optical split could become a 2 way, and double up on DFB's for existing services etc or use a 2 way split to feed a 4 way with 2 DWDM's) The optical receiver at the node location has a photodiode that converts the RF modulated optical signal back to RF, because this is a wideband device it has the capability of accepting the different wavelenghts and demodulating them to their original frequencies.

Sorry if thats a bit boring, I thought there may be some who were interested in how that part of the network works. Most people appreciate what goes on in the headend and at local distribution level, but theres not usually much mentioned about the bits in between.

Re: How Video On Demand Works (long and technical!)
 

:Yikes: :Yikes: :Yikes:
You're just showing off now. Hands up who got that. Is it even English?! ;)

Re: How Video On Demand Works (long and technical!)
 

It's all greek to me!

Re: How Video On Demand Works (long and technical!)
 

Very good post.
Thanks. It has taught me things i didnt know before and was a good read.

Re: How Video On Demand Works (long and technical!)
 

Having read Tristans description of how VOD works it got me thinking. 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. At the moment all 200 or so channels are sent to every STB taking up a hell of a lot of bandwidth. If only the selected channel was sent to your STB from the headend or hubsite there could be plenty of bandwidth available for other uses.

Re: How Video On Demand Works (long and technical!)
 

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..."

Re: How Video On Demand Works (long and technical!)
 

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??

Re: How Video On Demand Works (long and technical!)
 

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.

Re: How Video On Demand Works (long and technical!)
 

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.

Re: How Video On Demand Works (long and technical!)
 

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.

Re: How Video On Demand Works (long and technical!)
 

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.

Re: How Video On Demand Works (long and technical!)
 

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.

Re: How Video On Demand Works (long and technical!)
 

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 :)

Re: How Video On Demand Works (long and technical!)
 

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.

Re: How Video On Demand Works (long and technical!)
 

when is it being rolled out in bromley ? :tu:

how will we know we have got it ? :tu:

will it rearange my channel list ? :tu:

Re: How Video On Demand Works (long and technical!)
 

Don't know

Everything will be blue (again)

No

:)

Re: How Video On Demand Works (long and technical!)
 

why that little smile at the end ?

im feeling nervous now

Re: How Video On Demand Works (long and technical!)
 

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 :)

Re: How Video On Demand Works (long and technical!)
 

He's just a friendly guy ;)

Re: How Video On Demand Works (long and technical!)
 

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

Re: How Video On Demand Works (long and technical!)
 

this could be a really good advertising ploy for ntl,

sky+ uses the line "create your own tv channel" when effectivly its just recording programs then watching them later.

If ntl programmed a customised scheldular and got together with content providers, a list of program could be made availiable and you just add them to your schedule for when you want them. so you really have got your own tv channel.
9:00 Ultimate Force episode 1
10:00 Battlestar gallacticer episode 11
11:00 slot free --click here to add a programme to your channel --
12:30 Ultimate force episode 3

etc

Re: How Video On Demand Works (long and technical!)
 

VOD is coming but not as soon as you think.

Up until the last week before xmas the contract for who would be doing the upgrade work on the head ends had not been awarded, and i believe if i remember correctly that the project was due to start sometime around march/april.

Re: How Video On Demand Works (long and technical!)
 

which will take how long ?

Re: How Video On Demand Works (long and technical!)
 

Er - Acropolis, which area is this? Actual pukka working VOD did launch in Glasgow a couple of weeks ago, you know!

Re: How Video On Demand Works (long and technical!)
 

this may well just be a test area, or i may be mistaken, but that was the last i heard on it.

Re: How Video On Demand Works (long and technical!)
 

18/1/05 was the full commercial launch of the first wave of VOD - several tens of thousands of live paying customers. The tests were a bit before that, unusually for ntl ;)

Re: How Video On Demand Works (long and technical!)
 

VOD sounds really cool!!! I hope its doesn't cost the world though.

Is there any free stuff??

Re: How Video On Demand Works (long and technical!)
 

... with more areas coming Real Soon Now[tm]

Re: How Video On Demand Works (long and technical!)
 

So the "Headend" - the server containing loads of pre-recorded programs - is in the green box in the street, you know the one with the doors that open with a bit of yob-abuse?

Re: How Video On Demand Works (long and technical!)
 

no I dont think the green box in your street is the head end.

Re: How Video On Demand Works (long and technical!)
 

I would hope it's in a nice, secure, temperature controlled data centre with redundant power supply (doubtful knowing NTL).

Re: How Video On Demand Works (long and technical!)
 

A head end is a building, generally one per region, where people work, with racks full of equipment. I think some headend buildings double up as depots for installation engineers, etc.

Re: How Video On Demand Works (long and technical!)
 

Well, it was wishful thinking... :D

Re: How Video On Demand Works (long and technical!)
 

I've been pretty impresed with VOD so far, although am slightly disappointed by how few porgrams are currently available.

Re: How Video On Demand Works (long and technical!)
 

How do I access VOD. I live in Huddersfield and if im correct we should have been enabled the other week. But I cant seem to find any other options on my box. I have an old Pace box if thats any help.

thanks in advance

Re: How Video On Demand Works (long and technical!)
 

just press the "on demand" button on your remote to get access...it seems at the moment that it is just some of the BBC programs you can watch again, but there is a lot of other pay for stuff such as music videos, films, cartoons etc.

Re: How Video On Demand Works (long and technical!)
 

Just press the On Demand button and select what you want from the menus. Or go to channel 106 or 107 and press Red.

If it doesn't work, power your STB off for 30 seconds then try again.

Re: How Video On Demand Works (long and technical!)
 

Have any Tivo users/Pace STB owners (Langley) noticed that after watching VOD, any programmed recordings which involve channel changes rarely work ? You have to switch the channel manually before the recording starts to ensure the recording happens okay.

Re: How Video On Demand Works (long and technical!)
 

So technically, the way VOD works... couldnt ntl implement some sort of PVR functionality remotely using existing boxes with new software with all the recording done at the head end and streamed in the way VOD currently does?

Im guessing it would be cheaper too?

Re: How Video On Demand Works (long and technical!)
 

Some American cable companies a trialling this idea. Forgotten the term for it though. So it is possible :)

Re: How Video On Demand Works (long and technical!)
 

Cablevision: http://www.multichannel.com/article/...=SUPP&nid=2226

Others have played with it in the past but dropped it due to legal concerns, namely that the content producers will sue them. I believe a lawsuit is in the process against Cablevision.

Re: How Video On Demand Works (long and technical!)
 

In cities where there is both a headend and a Hub where does the VOD come from ? I know my broadband comes from the hub but not sure about the tv.

Re: How Video On Demand Works (long and technical!)
 

More than likely both are in the same building, but you can get virtual headends in major citys. All TV and BB traffic goes to a headend where genarally the telephone switch is located also.

Re: How Video On Demand Works (long and technical!)
 

No in my city the headend and hubs are in two seperate parts of the city so do i presume the tv comes from the headend while the internet comes from the hub

Re: How Video On Demand Works (long and technical!)
 

Is there a simplified english version of that?

Re: How Video On Demand Works (long and technical!)
 

Think of it this way. Have you ever watched a video clip from the BBC website? When you double click on them, your PC communicates with the server at the other end and requests that it start sending your video clip. In this way, it's like downloading the clip. Now, the major difference between streaming and downloading is that with streaming, the player downloads a very small section of the clip, then plays that, while downloading the next section in the background. It carries on like this until the clip is finished. That's basically the priniciple behind web streaming.

VOD uses the same priniciple (you select a programme, click, the STB then streams it in the same way as the video clip above), but you can pause it, rewind and fast forward the video as if you were watching it on tape.

The reason it is taking a long time is that NTL actually require an awful lot of hardware to make it work (mainly servers for the Set Top Boxes to stream from). The user will never see this hardware, so it's easy to think NTL is doing nothing, but the hardware is being installed at each headend (where your TV comes from)., and takes a while to install.

Note: STB = Set Top Box.

Re: How Video On Demand Works (long and technical!)
 

great info folks, keep up the work

Re: How Video On Demand Works (long and technical!)
 

Anyone know if they plan to change whats on tv on demand

Re: How Video On Demand Works (long and technical!)
 

a new section has been added to tv hits, warner...............has about 3 pages of stuff.

war at home, cold case veronicas closet, veronica mars just to name a few.
get smallvile on there and i'll be a happy bunny

Re: How Video On Demand Works (long and technical!)
 

It won't let me rep you. :(

But very good post anyway, thanks. :)

BTW What's happened to Bill C?

Re: How Video On Demand Works (long and technical!)
 

Simplified version:

Press button. See what you want. :D

Re: How Video On Demand Works (long and technical!)
 

Am I right in thinking there's only a certain number of streams available at any one time, and that if too many people want to use the on demand service there won't be enough streams available for everyone?

Tonight TV was really rubbish so we had to resort to watching some stuff on on demand. We watched the first half hour show fine with no problems. When starting the second one I got the message about on demand not being available but tried again and it worked. After watching that half hour show I tried to start a third one. I was getting the message about on demand not being available popping up all the time but persevered and eventually got as far as the part where the programme is about to start, only it never started. Now effectively the box had crashed. I was unable to press the "tv" button to abort and go back to the tv (had to pull the plug on the STB). Whoever wrote the on demand software forgot to program for the possibility of the stream not being available and to allow you to press the tv button to abort and go back to the tv, so the box crashes. I'd hope that the software will be improved on in the near future.

Re: How Video On Demand Works (long and technical!)
 

Re: How Video On Demand Works (long and technical!)
 

I kid you all not but i just went onto the statues page because i cant get video on demand and i swear it said the following...

Date Issue raised:
September 14 2010, 15:46
Estimated repair time:
January 01 1970, 01:00
Fault reference:
F001372805.

:confused::shocked::confused::shocked::confused:

Re: How Video On Demand Works (long and technical!)
 

Another thing coming soon LOL :D We need the Delorean to get that.

Re: How Video On Demand Works (long and technical!)
 

@ the OP,

An interesting and accurate post.

Re: How Video On Demand Works (long and technical!)
 

And your reason for bringing up an old thread was?

Re: How Video On Demand Works (long and technical!)
 

Sorry I'm new to the site and read the thread for the first time then commented as that's what I do on forums. Give a comment and kudos or equivalent on a good thread.

Now I see the thread is old.

Re: How Video On Demand Works (long and technical!)
 

There is no need to give a comment or 'kudos' on every thread you read.

However to find this thread you would have had to go looking for it so I don't believe you not noticing it was old.

Re: How Video On Demand Works (long and technical!)
 

Its a sticky on the TV section.Go to the TV section there is 3 stickied topics.
This is one lol.

Re: How Video On Demand Works (long and technical!)
 

Since this is stickied, there's no harm in mentioning that the 1970 date above is because UNIX systems count the number of seconds since Jan 1st 1970 to store dates. If the date field isn't set to anything, or is set to zero, then it will be Jan 1st 1970.

If this thread is still active in 2038, you get to see it wrap around to 1970 again.