When you see a Sky Plus box with a 160Gig hard drive you must be careful to check how much of it is actually available for you to record on. With all later models of Sky Plus the inbuilt system takes away and reserves 80Gig for services such as "Anytime TV".
So why is this important?
If you see a Sky+ box for sale on eBay, or indeed if you upgrade your home box with a larger hard drive yourself you will probably wonder why you end up with far less recording time than your maths would suggest.
For example if you take a standard Sky+ you presently have 40 hours recording time but you may not know that there is an 80 hour (160Gig) hard disk already inside. (In fact the available time is very variable as the raw stream from the satellite transponder is recorded and there is nothing that you, I or anyone else can do about how much each program occupies, it is decided by the broadcaster. A Fuller explanation is at the end of this article.)
This is why when you replace the 40 hour hard drive in your Sky+ with a 160Gig drive you get precisely nothing extra by way of viewing time! The box automatically reserves and hides away the other 80Gig and you cannot see or use it! In fact ALL new "80Gig" Sky+ boxes have a 160Gig hard drive fitted already!
If you want to increase your recording time then you must get a larger hard drive say a 240Gig model. This will give you 160Gig to record on yourself while the system reserves the other 80Gig for itself! There is nothing that you can do about this, it is "hard wired" for the box's operating system to take 80Gig away from ANY hard drive that is fitted. Fit an 80Gig drive and you end up with nothing!
There are some engineers who have managed to sidestep the restriction by altering one of the chips inside the box however we don't recommend that you buy one of these modified boxes as you run the risk of a future operating system upgrade taking the space back for Sky!
Fortunately there IS a way to get the whole of the disk for yourself and that is to buy a reconditioned model that does not have the 80Gig reservation built into the box. This is completely legitimate thing to do and provided the box has been properly reconditioned this would seem to be the way to go to get a reliable large recording time.
Incidentally there are some early Sky+ boxes that are VERY fussy which hard drives they will accept, some will not take any drive larger than 200Gig. Check before buying by asking the hard drive seller if the one on sale is suitable for your box. If he does not know then buy somewhere else!
This hopefully explains why your Sky+ box upgrade will not necessarily give you any extra viewing time when you put in a new hard disk - the new disk may be exactly the same size as the one already inside the box!
As promised there now follows an explanation of why it is not possible to say exactly how much time you can record on any particular Sky+ box...
The Sky+ box records on hard disk the whole signal that comes from the satellite. It neither adds nor takes away any data and waits until you actually watch the programme to unscramble it and put the picture on your screen. This is why you have to have a current subscription to the channel that you have recorded to view an old recording. So if you recorded a Sky Movie and a couple of months later decide to watch it while in the mean time you have stopped subscribing to the movie channels you will NOT be able to watch it!
There is nothing that you can do about the quality of the programms that you record (as for example you CAN do with a DVD recorder). Take for example a film shown late in the evening on BBC1 when the transponder isn't busy with "red button" inserts etc.
The BBC do not compress their transponders very much at this time so a two hour film may easily occupy 4Gig of a Sky+ disk (2 hours at 2 Gig per hour). Record exactly the same film broadcast on say one of the more compressed film channels such as "Movies 4 Men" and you will find that it has occupied much less space, probably just over 1Gig per hour which equals about 2 Gig of your hard disk.
This is because the free to air movie channels are more heavily compressed to "squeeze" more programms onto a single transponder (so making the space per channel cheaper to rent). This means that a film recorded from a free to air more compressed channel is more "squashed up" so takes up less room on your Sky+ hard disk.
At home you may notice that the picture quality isn't as good on the more compressed channels as on BBC1 but you can record more of their programmes onto your Sky+ disk. Incidentally there is nothing that you the viewer can do about this compression, it is all worked out by the broadcasters' computers before transmission. The broadcaster decides the picture quality - not you, nor I, nor your hard pressed Sky engineer!
On this occasion the Sky engineer is absolutely right, there is "nothing that he can do", the rest of the time we are not so sure. ;-)