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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.

channel filters continued



 
 
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Old January 28th 18, 12:40 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 2,304
Default channel filters continued

On 27/01/2018 15:48, Ian Jackson wrote:

Regardless of whether it's a bandpass or bandstop, the most important

requirement of any filter is that it should adequately reject the
signals you that DON'T want, rather than to pass the signals you DO want.

Err well... Excessive through loss can lead to additional amplification
which probably, in moderation, isn't too much of a problem. However I do
try to minimise end-to-and amplification.
More important is the flatness of the response within the passband. This
can be a problem with passive single-channel pass filters. It's a matter
of tuning the stages (normally three) very carefully to achieve steep
sides but a flat top. A variation of 6dB across a mux is not good.

In some applications, instead of individually filtering each channel

before combining, it may be perfectly in order (and more convenient) to
combine a group of channels that are fairly near neighbours, and then
filter them together 'en bloc' before combining them further with other
channels. [One obvious example of this is what an aerial diplexer does -
but there are far more situations where this principle is in operation.]
This has the advantage that, without any change or addition to the
filtering, any of the channels in a given block can moved in frequency -
and also other channels can be added at a later date. Provided the final
result meets the spec for signal-to-**** ratio - job done.

In essence I agree. However much depends on the signal sources. The
expensive UHF modulators of years ago, and the very cheap ones of
nowadays, do produce a raised noise floor on nearby channels, so such a
technique might cause problems. At the least it would reduce s/n ratios
which I think is always to be avoided, even then they are 'adequate'
having been raised. Modern analogue modulators, such as the Vision ones
at about £34, are much better and for a domestic job you could easily
get away with combining three (say) on channels 21, 25, and 28, then
using a Gp A bandpass filter, then a combiner. Personally I wouldn't do
it because there's little cost advantage over using a three-way one
channel pass filter block, and you'd lose the individual level control,
but yes you could do it if you wanted to.
DVB modulators are much better with very little in the way of spurious
noise, so you can take more liberties. Not that I would, but our work
isn't cost-sensitive.

Bill
 




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