Multiplexes

There are 6 multiplexes on DTT and 4 of them use the modulation system 16QAM, the remaining 2 use the 64QAM modulations system.

Does anyone know the capacity of either modulation (eg. how many TV channels can fit on 16QAM and how many TV channels can fit on 64QAM?)

Through all the speculation about CBM being the 'last' slot, I was wondering how many more channel slots we would have on Freeview if all Muxes used the 64QAM modulation system.

I realise that its almost 100% unlikely, due to power and reception problems, but would there be a possiblity of many more channels on DTT?

16 = 4 channels

64 = 6 channels
You can also squeeze some other bits and bobs on it.

The more channels, the lower the quality of the picture you receive.

They've used the 64QAM before in the ON/ITV Digital era's

That's it in a nutshell.

The "onDigital information site" (Can't remember the URL) states that one of the onDigital multiplexes had many more than 6 channels on it... what's the maximum they can squeeze on?

Well in theory you can fit as much or as little as you want to onto a multiplex, it's just it depends what quality you want it to come out it!

Once you've taken part of the data stream out for error correction, you then have to decided how much data you want to apply to each channel... the higher the the value (kbps i think) the better the quality - so at least 128kbps for Radio and about 3500 - 4000kbps for TV.

For the 64QAM Multiplexs the standard is 6 channels, perhaps 6 radio stations, the EPG data etc. As for the 16QAM ones it's generally 4 channels, along with the rest of the gubbins.


Hope this is of some help...
Stu

Remember that the signal from a digital MUX is just a stream of data, you can transmit anything you want over it from TV channels to Microsoft Word documents (if you so wish).

The bandwidth will probably stay the same, but compression techniques do and will change as technology improves. Soon a lot more TV channels might be able to be carried

To put it into a bit or reterospect, your Freeview Receiver is a PC with a Network Card. (Without all the bits and bobs it doesn't need)

You receive the data via your aerial through your box, just like you could if you sent data on a network from PC to another.

and by watching Eurosport you could tell that fact. It's compression levels made it look like one of the high quality RealMedia files (400k).

Does anybody have a screen capture of a channel with heavy compression on it?

In the UK, with the broadcast channels and error correction deployed :

16QAM have 18Mbs capacity
64QAM have 24Mbs capacity

It is not possible to give exact figures for picture quality vs data rate as it depends on the source picture quality, the algorithms used in the encoding, whether the individual streams are fixed bit rate or statmuxed (i.e. sharing data dynamically based on picture content)

For example one form of MPEG2 requires ** 50 Mbs ** for broadcast quality - however this is a special form called "I Frame only" - where no motion compression is used.

DVDs max out at about 9.9Mbs - and the BBC use 8Mbs for distribution feeds between national centres.

Most UK broadcasts go out at between 2.5 and 6Mbs - with the Beeb running BBC One at about 4.2Mbs un-statmuxed for the English regional feeds on satellite. I think ITV1 goes out at about 2.8-3Mbs on satellite - so the quality is lower on some fast moving material.

It should begin to look like a low compressed JPEG I believe.