Spectral subtraction

"Rune Allnor" <allnor@tele.ntnu.no> wrote in message 
news:1146219830.185098.304600@g10g2000cwb.googlegroups.com...
>
> Clay S. Turner wrote:
>> "thom" <soniceric@hotmail.com> wrote in message
>> news:svKdna87uJkuTM3ZnZ2dnUVZ_tmdnZ2d@giganews.com...
>> > Hi,
>> >
>> > I would like to know if you think it's possible to implement a noise
>> > reduction system in every receivers with spectral subtraction method 
>> > (for
>> > example could it reduce noise in a signal with a bandwith of 10MHz at a
>> > frequency of 500MHz)?
>> >
>> > Thank you
>> >
>> > Thom
>>
>> Hello Thom,
>>
>> Basically all receivers employ noise reduction via spectral subtraction. 
>> The
>> preselect and IF filters do exactly this.
>
> Hmmm.... I may be splitting hairs here, but I think this at best is a
> gross
> oversimplification; at worst it is plain wrong.
>
> A filter does not *subtract* noise from a frequency band. It
> *mutliplies*
> the noise inside that band wit a factor H(w) << 1. The difference is
> that
> with multiplication, you don't need to know the amplitude of the
> noise, whaile if you want to subtract in spectrum domain, you both
> need to know the amplitude and the phase, or you will de facto
> *add* noise to your signal.
>
> So spectrum subtraction is not a very good way to do things.
>
> Rune
>


Hello Rune and others,

The interpretation of my answers depends of what exactly "spectral 
subtraction" means. Certainly in receiver design one places filters which 
either absorb or reflect incoming energy based upon frequency. So if a 
receiver has a bandwidth at any point wider than the channel of interest, 
then one admits extra noise.  This can be common Johnson noise or man made 
noise. So my saying that using filters for noise reduction based upon 
spectral subtraction is correct in than the receiver's bandwidth gets 
reduced to the channel of interest so out of band noise is excluded. Hence 
the out of band spectrum's energy is subtracted from the total.  Imagine 
putting wideband noise into a cavity filter. The power coming out the other 
end is reduced mostly because out of band power is reflected back towards 
the input. The inband portion is barely affected. For example standard 
cavity filters have an in-band insertion loss of around 1dB. Some super 
conducting ones are around 0.1 dB insertion loss.


Now once someone has zoomed in to just look at a channel of interest, then 
further filtering in that channel while removing noise will also likely 
remove information.  So this is generally not done although some spread 
spectrum methods will do this with dynamic filtering. This works by having 
the signal not occupy the whole channel at any one time.  And this is likely 
the situation the OP referred to, but his question was quite general so I 
gave a quite general answer hoping he would father refine his question.

Clay

"thom" <soniceric@hotmail.com> wrote in message 
news:K-2dnYtDIPVclM_ZnZ2dneKdnZydnZ2d@giganews.com...
> So if spectral subtraction isn't the right solution, do you know other
> methods to reduce white noise in a "broadband" signal?? (BW of several
> MHz)
No.

Best of Luck - Mike