Why don't PVRs have DVD burners?


A question often asked on various digital TV web sites is "Why aren't there any twin tuner PVRs with DVD burners." I've answered the question so often that it's becoming tedious typing out the same information over and over; so here's a summary of the reasons why you're unlikely to find such a beast on the shelves just yet, based on a posting I originally made on Digital Spy.

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Although DVD Video and DVB-T video both use MPEG2, it's in a different format; the former uses a Programme Stream, and the latter a Transport Stream, which is designed to be robust over systems like wireless, while DVD knows it's going to get the info off the disk more easily.

To record digital TV to DVD, you have to convert the Transport Stream to a Programme Stream; this is actually not too hard, and there's no quality loss, since it's really just rearranging the existing info in a slightly different way.

BUT - and it turns out to be a big one - the DVD Video standard is quite strict on the range of parameters that can be used for the MPEG2 video, with regard to things like the picture size and the number of frames between complete pictures; it's more strict than the standards for DVB-T.

Some of the channels on Freeview use settings that, while working fine for broadcast, don't fall within the allowable range for DVD Video. They have a resolution that's wrong, or too many intermediate frames between pictures - good ways of saving bandwidth, sometimes at the cost of picture quality. But not acceptable for DVD.

That means that, to maintain compatibility with every DVD player, a DVD recorder will have to re-encode the images. (As an aside, I imagine that if a DVD recorder were to create discs that couldn't be played on everything with the DVD logo, there would be licensing issues - just like some copy protected CDs in the past haven't been allowed to have the 'Compact Disc Digital Audio' CD logo, because they don't work in some players.)

Even on a modern PC, re-encoding can be a pretty time consuming thing to do; it can take much more than the running time of a programme. Bear in mind that the processors in typical set top boxes and DVD recorders are much slower, too.

So, instead, these boxes use a dedicated MPEG encoder chip; even when they have a digital tuner, they effectively convert DVB-T to analogue and then re-digitise it - which the encoder chips can do in real time. It's a bodge, but it ensures that everything that's saved to the hard drive or DVD of these machines is in a format that complies with the DVD Video specs.

Next thing to understand is that MPEG is a standard that has license fees, and fees are payable on encoding chips. Those aren't huge amounts - but in the world of consumer electronics, even shaving a few cents off the cost of components is important.

If you have two tuners, there are two things you can do.

a) you can have one encoder chip, save things in their native format, and then encode later when you want to save them to DVD.

or b) you can have an encoder chip for each tuner, and make sure everything is saved on the hard drive in DVD Video format.

Option b is the simplest from the end user point of view, but you'll end up with a higher materials cost; and if it's a Freeview PVR, most people will be just timeshifting, and not making that many DVDs, so it's simplest to just cut out the encoders altogether, and if someone wants to make DVDs, they'll buy a separate recorder.

Option a opens up a big can of worms for the user interface; will people really want to wait hours to burn a DVD from something on the hard drive, when they know a PC can do the job quicker? It's going to be a real-time re-encoding job, followed by the burn time. Users would be frustrated, and, not understanding the technical aspects, would just complain that it's slow. Or they'd have to choose when recording whether or not they might want to burn to DVD later, and only be able to do that with one recording at a time.

One day, there might be cheap silicon without many royalties that will encode video to DVD-Video faster than real time. But until then, I don't think we're that likely to see a dual tuner PVR with DVD burning.

End of article.

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