Derating factor.....

In article ,
says...

On 28/07/2019 16:06, Terry Casey wrote:

Interesting point but, with some obvious exceptions, DTV muxes
were grouped into a similar bandwidth to the 11 channel
analogue groups, so there wouldn't be much difference in
levels across each group.


The response variations with frequency of most aerials is a much bigger
factor.


Agreed, Bill, but not something I've ever had to bother about
on the CATV systems I've worked on!


--

Terry

---
This email has been checked for viruses by AVG.
https://www.avg.com

In article ,
Terry
Casey wrote:
In effect, if they are all packed together you end up with a SNR
determined by all the 'other' channels having the same power level as
the one carrying the Mux you want to pick up!

Interesting point but, with some obvious exceptions, DTV muxes were
grouped into a similar bandwidth to the 11 channel analogue groups, so
there wouldn't be much difference in levels across each group.

Must admit the comment only came to my mind because I'm currently writing
about a project a few decades ago to 'mask' a low-power transmission by
spreading it over a 1GHz bandwidth and use this to present any
'evesdropper' with a SNR that was well below unity.

The result was that you needed to know 'key' info to even tell the signal
was being transmitted. Steganography rather than crytography. :-)

In that case the assumed background was 'natural'. But this has set me
thinking about something similar for bands that are stuffed with 'normal'
signals like DVB muxes...

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics https://www.st-andrews.ac.uk/~www_pa...o/electron.htm
biog http://jcgl.orpheusweb.co.uk/history/ups_and_downs.html
Audio Misc http://www.audiomisc.co.uk/index.html

On 29/07/2019 10:25, Jim Lesurf wrote:
Must admit the comment only came to my mind because I'm currently writing
about a project a few decades ago to 'mask' a low-power transmission by
spreading it over a 1GHz bandwidth and use this to present any
'evesdropper' with a SNR that was well below unity.

The result was that you needed to know 'key' info to even tell the signal
was being transmitted. Steganography rather than crytography.:-)

In that case the assumed background was 'natural'. But this has set me
thinking about something similar for bands that are stuffed with 'normal'
signals like DVB muxes...

You might like to look at GPS. It's a bit like that.

Andy

In article , Vir Campestris
wrote:
On 29/07/2019 10:25, Jim Lesurf wrote:
Must admit the comment only came to my mind because I'm currently
writing about a project a few decades ago to 'mask' a low-power
transmission by spreading it over a 1GHz bandwidth and use this to
present any 'evesdropper' with a SNR that was well below unity.

The result was that you needed to know 'key' info to even tell the
signal was being transmitted. Steganography rather than crytography.:-)

In that case the assumed background was 'natural'. But this has set me
thinking about something similar for bands that are stuffed with
'normal' signals like DVB muxes...

You might like to look at GPS. It's a bit like that.

Bit late for me now as the work was 20 years ago. :-) That said, GPS was
one of the factors we looked at back then.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics https://www.st-andrews.ac.uk/~www_pa...o/electron.htm
biog http://jcgl.orpheusweb.co.uk/history/ups_and_downs.html
Audio Misc http://www.audiomisc.co.uk/index.html