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uk.tech.digital-tv (Digital TV - General) (uk.tech.digital-tv) Discussion of all matters technical in origin related to the reception of digital television transmissions, be they via satellite, terrestrial or cable. Advertising is forbidden, with no exceptions.

Freeview channel frequencies



 
 
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  #1  
Old October 5th 05, 05:47 PM posted to uk.tech.digital-tv
Michael Chare
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Posts: 835
Default Freeview channel frequencies

My Topfield Freeview PVR says that it is receiving channel 25 at 506 Mhz.

Should this not be 503.25 Mhz?

There is a similar discrepancy of 2.75Mhz on all other muxes/channels


--

Michael Chare


  #2  
Old October 5th 05, 06:41 PM posted to uk.tech.digital-tv
André Coutanche
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Posts: 620
Default Freeview channel frequencies

Michael Chare wrote:
My Topfield Freeview PVR says that it is receiving channel 25 at 506
Mhz.

Should this not be 503.25 Mhz?

There is a similar discrepancy of 2.75Mhz on all other
muxes/channels


*****

Others can probably give a more erudite answer, but I suggest one
possible one is 'it all depends' ;-) .

A UHF channel covers a certain bandwidth, not a single frequency. What
it all depends on is whether you consider the vision carrier, the
sound carrier (for analogue), the mid-point or whatever. So it almost
certainly doesn't matter.

André Coutanche


  #3  
Old October 5th 05, 07:23 PM posted to uk.tech.digital-tv
Paul Ratcliffe
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Posts: 1,911
Default Freeview channel frequencies

On Wed, 5 Oct 2005 19:41:26 +0100, André Coutanche
wrote:

Michael Chare wrote:
My Topfield Freeview PVR says that it is receiving channel 25 at 506
Mhz.

Should this not be 503.25 Mhz?

There is a similar discrepancy of 2.75Mhz on all other
muxes/channels


*****

Others can probably give a more erudite answer, but I suggest one
possible one is 'it all depends' ;-) .

A UHF channel covers a certain bandwidth, not a single frequency. What
it all depends on is whether you consider the vision carrier, the
sound carrier (for analogue), the mid-point or whatever. So it almost
certainly doesn't matter.


The analogue vision carrier is not centred in the channel. It is 1.25 MHz
above the lower band edge. As the channel is 8MHz wide, the centre of the
band is 4MHz above the band edge, thus 2.75MHz above the vision carrier.
So the box is entirely correct in its display.
  #4  
Old October 5th 05, 08:12 PM posted to uk.tech.digital-tv
Jasmine
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Posts: 5
Default Freeview channel frequencies

The analogue vision carrier is not centred in the channel. It is 1.25 MHz
above the lower band edge. As the channel is 8MHz wide, the centre of the
band is 4MHz above the band edge, thus 2.75MHz above the vision carrier.
So the box is entirely correct in its display.


The channels are not 8MHz wide, they are spaced 8MHz apart (in the UK) -
there is a huge difference between channel width and channel spacing.
So whilst on the right track, the above in not quite accurate.


  #5  
Old October 6th 05, 06:01 AM posted to uk.tech.digital-tv
Stephen
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Posts: 481
Default Freeview channel frequencies

"Michael Chare" wrote in message
...
My Topfield Freeview PVR says that it is receiving channel 25 at 506 Mhz.

Should this not be 503.25 Mhz?

There is a similar discrepancy of 2.75Mhz on all other muxes/channels


It is correct. The carrier frequency for DTT on Channel 26 is 506 MHz. DTT
has symmetrical sidebands so the carrier frequency is at the centre of the
TV channel. Analogue TV has asymmetrical sidebands so the vision carrier
frequency is offset below the centre by 2.75 MHz.

Sometimes people confuse the issue by saying that there are 2000 carriers in
a DTT channel, however there is only one TV transmiitter, not 2000, and it
has a single carrier frequency at the centre of the TV channel. This single
carrier is quadrature amplitude modulated, and supressed leaving only the
sidebands. The 2 signals which modulate this single carrier frequency, and
produce the sidebands, are coded in such a way that they produce a frequency
spectrum which is equivalent to about 2000 seperate narrow bandwidth
channels. There is infact however only one carrier (supressed), and two
modulating signals (I and Q axis), each with a bandwidth of about 3.5 MHz.


  #6  
Old October 6th 05, 11:12 AM posted to uk.tech.digital-tv
Andy Wade
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Posts: 263
Default Freeview channel frequencies

Jasmine wrote:

The channels are not 8MHz wide, they are spaced 8MHz apart (in the UK) -
there is a huge difference between channel width and channel spacing.
So whilst on the right track, the above in not quite accurate.


That's being a bit pedantic, I think, in this context. The DTT channels
are 7.61 MHz wide, so almost fill the 8 MHz slot. UK DTT transmissions
can have a +/- 167 kHz (= 1/6 MHz) offset or zero offset. When an offset
is present, one edge of the signal is within 30 kHz of the channel
edge. Analogue System I with NICAM more than fills the channel since
the NICAM signal encroaches into the channel above (by ~ 200 kHz, IIRC,
in the zero offset case).

The answer to the original question is that 506 MHz is correct. For
digital it's conventional to refer to the channel centre frequency,
which is 8*N+306 MHz, where N is the channel no. (21=N=69).

503.25 MHz is the nominal analogue vision carrier frequency and has no
particular significance for DTT (other then being where you'll find the
analogue CCI).

--
Andy
 




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