How many digital channels can you have within the bandwidth of one analogue channel?
 

Now I probably have this all wrong, in fact I know I do but I'd really like
to gain some understanding of this.

As far as I can tell uncompressed video 720*576*25fps*24-bit colour is
approx 250Mbits per second and from a very simple viewpoint I would think if
MPEG2 compresses 25:1 shouldn't we be able to fit 25 channels in the
bandwidth assigned to that channel?

I await any answers with great interest!

Thanks all
J

"John Muir" wrote in message ...
Now I probably have this all wrong, in fact I know I do but I'd really like
to gain some understanding of this.

As far as I can tell uncompressed video 720*576*25fps*24-bit

10 bit colour is used, but you need 1440 samples per line (720 Y, 360
Cr, 360 Cb)
giving about 210Mbps

colour is
approx 250Mbits per second and from a very simple viewpoint I would think if
MPEG2 compresses 25:1 shouldn't we be able to fit 25 channels in the
bandwidth assigned to that channel?

The channels used were completely analogue - with a Luminance
bandwidth of about 7.61MHz (with a notch to allow for the Chrominance
info to be inserted). Using COFDM and 16QAM you can get about 18Mbps
through this channel - therefore you can fit 2 channles in (if your
25:1 is correct)


I await any answers with great interest!

Thanks all
J

In article , John Muir
writes
Now I probably have this all wrong, in fact I know I do but I'd really like
to gain some understanding of this.

As far as I can tell uncompressed video 720*576*25fps*24-bit colour is
approx 250Mbits per second and from a very simple viewpoint I would think if
MPEG2 compresses 25:1 shouldn't we be able to fit 25 channels in the
bandwidth assigned to that channel?

But they weren't transmitting uncompressed digital video in the channel,
it was analogue video. So your 250Mbit/sec figure is not relevant. The
bandwidth used by analogue video is not in any way related to the bit
rate of an equivalent digital signal.
--
Tim Mitchell

The right question to ask would be how many digital channels in the frequency range
used by one analogue channel. A multiplex takes up one 'channel' but im not sure about
bandwidth.


"Tim Mitchell" wrote in message ...
In article , John Muir
writes
Now I probably have this all wrong, in fact I know I do but I'd really like
to gain some understanding of this.

As far as I can tell uncompressed video 720*576*25fps*24-bit colour is
approx 250Mbits per second and from a very simple viewpoint I would think if
MPEG2 compresses 25:1 shouldn't we be able to fit 25 channels in the
bandwidth assigned to that channel?

But they weren't transmitting uncompressed digital video in the channel,
it was analogue video. So your 250Mbit/sec figure is not relevant. The
bandwidth used by analogue video is not in any way related to the bit
rate of an equivalent digital signal.
--
Tim Mitchell

Cy wrote:
"John Muir" wrote in message ...

Now I probably have this all wrong, in fact I know I do but I'd really like
to gain some understanding of this.

As far as I can tell uncompressed video 720*576*25fps*24-bit


10 bit colour is used, but you need 1440 samples per line (720 Y, 360
Cr, 360 Cb)
giving about 210Mbps


colour is
approx 250Mbits per second and from a very simple viewpoint I would think if
MPEG2 compresses 25:1 shouldn't we be able to fit 25 channels in the
bandwidth assigned to that channel?


The channels used were completely analogue - with a Luminance
bandwidth of about 7.61MHz (with a notch to allow for the Chrominance
info to be inserted). Using COFDM and 16QAM you can get about 18Mbps
through this channel - therefore you can fit 2 channles in (if your
25:1 is correct)

Except they cram 4 in instead :-(

"Ben" wrote in message
...
Cy wrote:
"John Muir" wrote in message
...

Now I probably have this all wrong, in fact I know I do but I'd really
like
to gain some understanding of this.

As far as I can tell uncompressed video 720*576*25fps*24-bit


10 bit colour is used, but you need 1440 samples per line (720 Y, 360
Cr, 360 Cb)
giving about 210Mbps


colour is
approx 250Mbits per second and from a very simple viewpoint I would
think if
MPEG2 compresses 25:1 shouldn't we be able to fit 25 channels in the
bandwidth assigned to that channel?


The channels used were completely analogue - with a Luminance
bandwidth of about 7.61MHz (with a notch to allow for the Chrominance
info to be inserted). Using COFDM and 16QAM you can get about 18Mbps
through this channel - therefore you can fit 2 channles in (if your
25:1 is correct)

Except they cram 4 in instead :-(

Thanks, that was the answer I was expecting.

"nick" wrote in message
...
The right question to ask would be how many digital channels in the
frequency range
used by one analogue channel. A multiplex takes up one 'channel' but im
not sure about
bandwidth.


Ok,
from other answers above they fit 4, does 16v64 qam really make a large
difference on the number of available digital channels?

Thanks
John

John Muir wrote:
"nick" wrote in message
...
The right question to ask would be how many digital channels in the
frequency range used by one analogue channel. A multiplex takes up
one 'channel' but im not sure about bandwidth.


Ok,
from other answers above they fit 4, does 16v64 qam really make a
large difference on the number of available digital channels?

The 'payload' of a 16QAM DTT transmission is 18Mb/s, and 24Mb/s for 64QAM.
SI and other housekeeping data chops 1 to 2 Mb/s off these figures.

A bit rate for a TV channel at reasonable quality is 4Mb/s. So about 4 can be fitted
into 16QAM, 5 or 6 into 64QAM.

"Mark Carver" wrote in message
...
John Muir wrote:
"nick" wrote in message
...
The right question to ask would be how many digital channels in the
frequency range used by one analogue channel. A multiplex takes up
one 'channel' but im not sure about bandwidth.


Ok,
from other answers above they fit 4, does 16v64 qam really make a
large difference on the number of available digital channels?

The 'payload' of a 16QAM DTT transmission is 18Mb/s, and 24Mb/s for 64QAM.
SI and other housekeeping data chops 1 to 2 Mb/s off these figures.

A bit rate for a TV channel at reasonable quality is 4Mb/s. So about 4 can
be fitted
into 16QAM, 5 or 6 into 64QAM.




Mark
Thanks alot!

John