UKC

Why do all TV recorders only record one channel at a time? (I know sky+ and TiVo do but they have multiple decoders)

Why do they not record the input from the aerial/dish so that when you play back it then re decodes the signal? This would mean you could record an hour, and later watch any of the channels on in that hour.

In reply to balmybaldwin:

Because recording one program in a decoded and compressed data format lakes up relatively little space. Where as recording all un-decoded data over all transmission frequencies would take up huge amounts of memory.

In reply to balmybaldwin:

I'm no expert but surely an hour of however many hundreds of channels there are would fill up most recorders? An online TV seems the way to go to me.

In reply to balmybaldwin:

My folks have a digital recorder that records two programmes at once. No idea what make it is.

In reply to balmybaldwin:

Weird, cause I'm downloading 5 video files as we speak with no problem. Maybe look at downloading what you want to watch rather than recording it from a tv.

In reply to graeme jackson:


My computer has a TV card inside it where I can either record two TV shows at once or record one TV show and watch a different channel at the same time. All this is done using Microsoft's own Windows Media Center.

My TV card is no longer able to buy however here's a link to something similar.

http://www.amazon.co.uk/PCI-E-DVB-T2-Definition-Digital-Receiver/dp/B005JTA...

In reply to balmybaldwin:

Andy DB is spot on.

Remember when you first did 'channel installation' on your TV, and it spent ages 'scanning'? Well, it was scanning through the TV broadcast frequency band, looking for TV broadcast signals. Just like you might when tuning a radio. And your TV probably found quite a few TV broadcast signals, each of which resulted in a flurry of TV stations or radio stations being added to the 'channel list'.

So, if we wanted to record 'everything', we'd have to be able to record the entire TV broadcast frequency band, so that we could go through it later, and pick out the particular broadcast signal we wanted. Trouble is, that TV broadcast frequency band is about 500MHz wide. And a chap called Nyquist said we have to sample that at twice the maximum frequency in order to be able to reconstruct it perfectly. There's a bit of a trick we can do with digital sampling, by doing what's called quadrature sampling in the centre of the band, and sample +/-250MHz either side, but each quadrature sample needs two samples, so we end up with 500MSa/s. We probably need 16-bit samples, so that gives us 1GB/s.

So, for your one hour recording, you'd need 3.6TB of storage...

Now, an additional complication with digital TV is that each of those TV broadcast signals, or 'radio channels', carries what called a 'multiplex', which is a data stream containing a number of TV stations. The nearest equivalent in the radio analogy is the two stereo channels of FM stereo. Only a digital multiplex stream contains more than two TV stations... If we could record this entire multiplex data stream, we could at least go back and pick out a number of TV stations from it later.

Some digital TV recorders contain up to three 'radio tuners', so can receive three TV broadcast signals at once, each of which carries a multiplex stream. So, potentially, we could record three multiplex streams at once, which would probably record quite a lot of TV or radio stations. Sadly, I'm not aware of a digital TV recorder that does multiplex recording...

My Humax, on the other hand, contains two 'radio tuners', and it can record two TV stations at once, and, if I want to watch a third TV station, it will let me, provided that is in the same multiplex stream being received for one of the recorded stations. It won't let me do time-shift stuff on the TV station I'm watching, though (I don't think...).

In reply to captain paranoia:


Which model of Humax do you have? The HDR FOX-T2 certainly does let you time-shift the third channel (and you can use another little-known feature to capture the time-shift buffer as a recording, effectively allowing you to record three concurrent programmes).

IIRC, back in the dim and distant past, someone (possibly the BBC labs) built an experimental device which recorded all six (then) Freeview multiplexes. Subsequent to that, one of the multiplexes was changed to DVB-T2, which offers higher bitrates in order to carry HD channels. Right now there are (in some parts of the country) nine multiplexes, three of which are DVB-T2. So a six-tuner multiplex recorder now needs another three tuners...

A DVB-T multiplex has an available total bitrate of about 25Mb/s; DVB-T2 gives about 40Mb/s. Thus the current maximum of six DVB-T and three DVB-T2 multiplexes total about 270Mb/s. If all of that bandwidth was used for TV channels (which certainly isn;t the case on eg the Local multiplex, or the third DVB-T2 multiplex) then a hour's recording of the whole lot would require about 120GB. A 2TB HDD would fill up in less than a day.

There are 40 UHF channels available for digital TV. If all of them were in use, using DVB-T2, then the total bitrate available would 1,600Mb/s. That would still require less than 1TB for an hour's recording of the whole lot.

In reply to captain paranoia:


Oh, the shame...

+/-250MHz still requires us to sample at 500MHz for Nyquist. But each of those samples is quadrature, so we need 1GSa/s, each of which is 16 bits, thus giving us 2GB/s storage requirement...

That'll teach me to rush posts out...

In reply to Martin W:


The 9300. The only reason for being unsure is that I don't remember; I rarely use the facility to record two shows and watch a third.

I did recently pull all the recorded programmes off the device; it required pulling the SATA drive out, and connecting it to the PC, and running a little DOS shell program, but it worked perfectly... Search for humaxrw-1.15-win32.zip