FFT length

Rick Lyons <R.Lyons@_bogus_ieee.org> wrote:

(snip)
>>> What 'noise' is increased when you average multiple FFT 
>>> magnitudes?  Why would computing ten 4096 FFTs take more 
>>> processing time than computing a single 40960-point FFT?

(snip)

>  Good gosh you're 'quick on the trigger'.
> I was hopin' to hear pierson's answers to my 
> questions.  In any case, thanks glen.

> By the way my question of "On what sort of signals 
> can you perform coherent averaging of multiple FFTs?"
> was a serious question.  Yes, I know that the 
> blocks of time samples must be 'phase coherent'
> for sucg coherent analysis.

> What I was wondering was *EXACTLY* what sort of 
> signals there are, which we might want to analyze, 
> that would be 'phase coherent'.  No real-world, 
> information-carrying, signal that I know of meets 
> that criteria.  The only such signals I can think of 
> that could be analyzed, spectrally, with coherent 
> averaging is the sinusoidal input signals used by 
> guys who are measuring the performance of A/D 
> converters using the FFT. 

(snip, I previously wrote, and moved down from above)

>>Consider, though, the way radio-astronomy is done. Signals
>>are collected from widely spaced dishes, along with exact timing.
>>The signals can then be combined with the appropriate phase,
>>to get the equivalent of a very large antenna. Without the 
>>timing, the signals would have to be considered incoherent.

I believe this is a good example. By adjusting the phase, 
they change the direction that they are looking. I believe
that phased array radar should also qualify.

But also important are the cases where you don't want 
them to average coherently. One interesting one is the effect
of multiple UTP ethernet cables in the came conduit. If they
count as incoherent, then the noise adds as sqrt(N), but since
all run at (about) the same frequency, that isn't so obvious.

-- glen

dbd <dbd@ieee.org> wrote:

(snip)

> There is better coverage in the IEEE Trans on
> Instrumentation and Measurement than in the Signal Processing
> literature. A good search term might be "synchronous sampling". Gas
> turbines in jet planes, helicopters and stationary power generation
> plants are monitored and maintained with instrumentation that uses
> synchronous (re)sampling. Helicopter rotor track and balance
> instruments will measure imbalance and tell the maintenance operator
> how much weight to put how far out on which blade to balance the
> rotor. Gas turbines are balanced in the same manner. Much large
> rotating industrial machinery gets such attention.

It is also done for car tire balancing, which is closer to
home for most of us.

Same principle, though.

-- glen

On Feb 14, 11:50&#4294967295;pm, glen herrmannsfeldt <g...@ugcs.caltech.edu>
wrote:
> dbd <d...@ieee.org> wrote:
>
> (snip) *Rick's question on coherent averaging*
>
> > There is better coverage in the IEEE Trans on
> > Instrumentation and Measurement than in the Signal Processing
> > literature. A good search term might be "synchronous sampling". Gas
> > turbines in jet planes, helicopters and stationary power generation
> > plants are monitored and maintained with instrumentation that uses
> > synchronous (re)sampling. Helicopter rotor track and balance
> > instruments will measure imbalance and tell the maintenance operator
> > how much weight to put how far out on which blade to balance the
> > rotor. Gas turbines are balanced in the same manner. Much large
> > rotating industrial machinery gets such attention.
>
> It is also done for car tire balancing, which is closer to
> home for most of us.
>
> Same principle, though.
>
> -- glen

Gee Glen, I wasn't aware of anyone who used coherent averaging to
balance car tires, Can you you give us an example of someone who does?
Or is this meant as an example of average incoherence?

Dale B. Dalrymple

dbd <dbd@ieee.org> wrote:

(snip)

> Gee Glen, I wasn't aware of anyone who used coherent averaging to
> balance car tires, Can you you give us an example of someone who does?
> Or is this meant as an example of average incoherence?

There is a device that spins the tire for a while, then stops,
and tells the user where and what weight to put on. 
It is called dynamic balancing, much preferred to static
balancing which doesn't spin the tire. Also, it knows how
much to put on which side, which you can't tell from static
balance.

Sounds just like your other examples, but I suppose it could 
also use incoherent averaging. As I said before, though,
for incoherent averaging you usually have to know that the
phase is random. If it isn't, you can get artifacts, pretty
much like aliasing. I have seen the machines, but never 
looked inside.

-- glen

On Feb 15, 10:05 am, glen herrmannsfeldt <g...@ugcs.caltech.edu>
wrote:
> dbd <d...@ieee.org> wrote:
>
> (snip)
>
> > Gee Glen, I wasn't aware of anyone who used coherent averaging to
> > balance car tires, Can you you give us an example of someone who does?
> > Or is this meant as an example of average incoherence?
>
> There is a device that spins the tire for a while, then stops,
> ...
> I have seen the machines, but never
> looked inside.
>
> -- glen

So where's the DSP? Rick's question was about FFTs:

On Feb 14, 5:53 pm, Rick Lyons <R.Lyons@_BOGUS_ieee.org> wrote:
> By the way my question of "On what sort of signals
> can you perform coherent averaging of multiple FFTs?"
> was a serious question.
> ...

Your average still seems to be incoherent to the question.

Dale B. Dalrymple

On Tue, 14 Feb 2012 20:50:10 -0800 (PST), dbd <dbd@ieee.org> wrote:

>On Feb 14, 5:53 pm, Rick Lyons <R.Lyons@_BOGUS_ieee.org> wrote:
>
>>
>> By the way my question of "On what sort of signals
>> can you perform coherent averaging of multiple FFTs?"
>> was a serious question.  Yes, I know that the
>> blocks of time samples must be 'phase coherent'
>> for sucg coherent analysis.
>> What I was wondering was *EXACTLY* what sort of
>> signals there are, which we might want to analyze,
>> that would be 'phase coherent'.  No real-world,
>> information-carrying, signal that I know of meets
>> that criteria.  The only such signals I can think of
>> that could be analyzed, spectrally, with coherent
>> averaging is the sinusoidal input signals used by
>> guys who are measuring the performance of A/D
>> converters using the FFT.
>>
>> See Ya',
>> [-Rick-]
>
>Machine vibration analysis is a common application of synchronous
>averaging. In practice the sampling is performed synchronously with a
>shaft rotational rate or a tachometer signal is used to supply
>information to resample to a synchronous sample frequency that can
>follow speed variations. There is better coverage in the IEEE Trans on
>Instrumentation and Measurement than in the Signal Processing
>literature. A good search term might be "synchronous sampling". Gas
>turbines in jet planes, helicopters and stationary power generation
>plants are monitored and maintained with instrumentation that uses
>synchronous (re)sampling. Helicopter rotor track and balance
>instruments will measure imbalance and tell the maintenance operator
>how much weight to put how far out on which blade to balance the
>rotor. Gas turbines are balanced in the same manner. Much large
>rotating industrial machinery gets such attention.
>
>As to the application of averaging to windowed power data, the
>following documents are a good practical discussion:
>
>Windows to FFT Analysis (Part I)
>Technical review No. 3 - 1987
>This article demonstrates how the analogy between DFT/FFT (Discrete
>Fourier Transform/Fast Fourier Transform) analysis and filter analysis
>(analogue or digital) can be used to better understand the
>applications of different weighting functions used in DFT/FFT. The
>filter characteristics of the most commonly used weighting functions
>(also called windows) are illustrated and discussed with respect to
>their use in various practical applications of system and signal
>analysis. The mathematical formulations of the analogy as well as
>rigorous details of the article will be given in the Appendices in
>Part II of this article to be published in Technical Review No.
>4-1987.
>And p29 Signals and Units
>
>at:
>http://bruel.ru/UserFiles/File/Review3_87.pdf
>or
>www.bksv.com/doc/bv0031.pdf
>registration required at www.bksv.com, and worthwhile if you want to
>learn about applied dsp
>
>
>Windows to FFT Analysis (Part II)
>Technical review No. 4 - 1987
>Technical Review 1987-4 Use of Weighting Fuctions in DFT/FFT Analysis
>(Part II); Acoustic Calibrator for Intensity Measurements SystemsPart
>II of the article "Use of Weighting Functions in DFT/FFT analysis"
>contains the following Appendices referred to in Part I of the article
>A: Analogy between filter analysis and DFT/FFT analysis,
>B: Windows and figures of merit,
>C: Effective Weighting of overlapped spectral averaging
>D: Experimental Determination of the BT product for FFT-analysis using
>different weighting functions and overlap,
>E: Examples of User Defined Windows,
>F: Picket Fence Effect
>
>at:
>www.bksv.com/doc/bv0032.pdf
>registration required at www.bksv.com, and worthwhile if you want to
>learn about applied dsp
>
>Bruel&Kjaer have extensive application notes and technical papers on
>DSP algorithms used in their long history of marketing sensors and
>instruments.
>
>Dale B. Dalrymple

Hi Dale,

   Thanks for all the info!!

[-Rick-]

On Wed, 15 Feb 2012 12:42:25 -0800 (PST), dbd <dbd@ieee.org> wrote:

>On Feb 15, 10:05 am, glen herrmannsfeldt <g...@ugcs.caltech.edu>
>wrote:
>> dbd <d...@ieee.org> wrote:
>>
>> (snip)
>>
>> > Gee Glen, I wasn't aware of anyone who used coherent averaging to
>> > balance car tires, Can you you give us an example of someone who does?
>> > Or is this meant as an example of average incoherence?
>>
>> There is a device that spins the tire for a while, then stops,
>> ...
>> I have seen the machines, but never
>> looked inside.
>>
>> -- glen
>
>So where's the DSP? Rick's question was about FFTs:
>
>On Feb 14, 5:53 pm, Rick Lyons <R.Lyons@_BOGUS_ieee.org> wrote:
>> By the way my question of "On what sort of signals
>> can you perform coherent averaging of multiple FFTs?"
>> was a serious question.

>Your average still seems to be incoherent to the question.
>
>Dale B. Dalrymple

Hi Dale,
  For me, glen's posts are relavent because I was 
wondering about applications that require repeated 
capturing of time-domain signals where those 
various signals are phase-coherent. 

[-Rick-]