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# Discussion Groups | Comp.DSP | DFT & other basic questions

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# DFT & other basic questions - manishp - 2012-10-13 22:47:00

```Sirs,

Few basic questions on DFT:

1) what is the signifiance of -ve amplitude in frequency domain
2) what is the significance when cosine component has -ve and sine has +ve
amplitudes
3) In some literatures, the frequency domain is shown with magnitude and
phase. Shouldn't this be magnitude and phase for real and imaginary parts
(so totally 4 components)?
4) How is the energy calculated using Parseval's equation in the complex
plane. Does it use real & imaginary and their phase counterparts for
calculation also?
5) what is the purpose of windowing and whether this is done in time or
frequency domain?

Thanks, Manish
```
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# Re: DFT & other basic questions - robert bristow-johnson - 2012-10-14 00:22:00

```On 10/13/12 10:47 PM, manishp wrote:
> Sirs,
>
> Few basic questions on DFT:
>
> 1) what is the signifiance of -ve amplitude in frequency domain
> 2) what is the significance when cosine component has -ve and sine has +ve
> amplitudes

boy, my ignorance again slaps me in my face.

i have no idea what "-ve" or "+ve" is.  never read those terms before
regarding the DFT.

> 3) In some literatures, the frequency domain is shown with magnitude and
> phase. Shouldn't this be magnitude and phase for real and imaginary parts
> (so totally 4 components)?

the phase of the imaginary part is always +90 degrees.  the phase of the
real part is 0.

> 4) How is the energy calculated using Parseval's equation in the complex
> plane.

didn't know Parseval's theorem (this is that the energy in the frequency
domain is equal to the energy in the time domain with a scaling factor
tossed in depending on how the DFT definition is scaled) had much to do
with the complex plane.

> Does it use real&  imaginary and their phase counterparts for
> calculation also?

it's magnitude-squared which is the same as real-squared plus
imaginary-squared.

> 5) what is the purpose of windowing and whether this is done in time or
> frequency domain?

the purpose is to taper the edges of the ends of the time-domain data
set so that discontinuities are minimized and so the spurious
frequency-domian components that are a result of this discontinuity are
minimized.

--

r b-j                  r...@audioimagination.com

"Imagination is more important than knowledge."

```
______________________________

# Re: DFT & other basic questions - Randy Yates - 2012-10-14 11:00:00

```robert bristow-johnson <r...@audioimagination.com> writes:

> i have no idea what "-ve" or "+ve" is.  never read those terms before
> regarding the DFT.

negative/positive
--
Randy Yates
Digital Signal Labs
http://www.digitalsignallabs.com
```
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# Re: DFT & other basic questions - 2012-10-15 23:12:00

```On Sunday, October 14, 2012 12:22:22 AM UTC-4, robert bristow-johnson wrote:
> On 10/13/12 10:47 PM, manishp wrote:
>
> > Sirs,
>
> >
>
> > Few basic questions on DFT:
>
> >
>
> > 1) what is the signifiance of -ve amplitude in frequency domain
>
> > 2) what is the significance when cosine component has -ve and sine has +ve
>
> > amplitudes
>
>
>
> boy, my ignorance again slaps me in my face.
>
>
>
> i have no idea what "-ve" or "+ve" is.  never read those terms before
>
> regarding the DFT.
>
>
>
> > 3) In some literatures, the frequency domain is shown with magnitude and
>
> > phase. Shouldn't this be magnitude and phase for real and imaginary parts
>
> > (so totally 4 components)?
>
>
>
> the phase of the imaginary part is always +90 degrees.  the phase of the
>
> real part is 0.
>
>
>
> > 4) How is the energy calculated using Parseval's equation in the complex
>
> > plane.
>
>
>
> didn't know Parseval's theorem (this is that the energy in the frequency
>
> domain is equal to the energy in the time domain with a scaling factor
>
> tossed in depending on how the DFT definition is scaled) had much to do
>
> with the complex plane.
>
>
>
> > Does it use real&  imaginary and their phase counterparts for
>
> > calculation also?
>
>
>
> it's magnitude-squared which is the same as real-squared plus
>
> imaginary-squared.
>
>
>
> > 5) what is the purpose of windowing and whether this is done in time or
>
> > frequency domain?
>
>
>
> the purpose is to taper the edges of the ends of the time-domain data
>
> set so that discontinuities are minimized and so the spurious
>
> frequency-domian components that are a result of this discontinuity are
>
> minimized.
>
>
>
> --
>
>
>
> r b-j
>
>
>
> "Imagination is more important than knowledge."

Robert,

Parseval's is much more general than generally taught in DSP.

It says if you have two functions g(t) and h(t) and their fourier transforms G(w) and H(w) that
the integral of conj(g(t)) * h(t) dt = scalefactor* integral of conj(G(w))*H(w) dw. What most
call Parseval's is really a special case where g(t) and h(t) are the same function. This is
called Bessel's identity.

Clay

```
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# Re: DFT & other basic questions - Rick Lyons - 2012-10-21 11:30:00

```On Sun, 14 Oct 2012 11:00:49 -0400, Randy Yates
<y...@digitalsignallabs.com> wrote:

>robert bristow-johnson <r...@audioimagination.com> writes:
>
>> i have no idea what "-ve" or "+ve" is.  never read those terms
before
>> regarding the DFT.
>
>negative/positive
Hi Randy,
This a great example of laziness on the part of the
original poster causing confusion for those trying
to offer help.

[-Rick-]

```
______________________________

# Re: DFT & other basic questions - Randy Yates - 2012-10-21 15:22:00

```Rick Lyons <R...@_BOGUS_ieee.org> writes:

> On Sun, 14 Oct 2012 11:00:49 -0400, Randy Yates
> <y...@digitalsignallabs.com> wrote:
>
>>robert bristow-johnson <r...@audioimagination.com> writes:
>>
>>> i have no idea what "-ve" or "+ve" is.  never read those terms
before
>>> regarding the DFT.
>>
>>negative/positive
> Hi Randy,
>    This a great example of laziness on the part of the
> original poster causing confusion for those trying
> to offer help.

I agree Rick. I try to avoid cutesie abbreviations in email and usenet
for this reason.
--
Randy Yates
Digital Signal Labs
http://www.digitalsignallabs.com
```
______________________________

# Re: DFT & other basic questions - Jerry Avins - 2012-10-21 18:07:00

```On 10/21/2012 11:30 AM, Rick Lyons wrote:
> On Sun, 14 Oct 2012 11:00:49 -0400, Randy Yates
> <y...@digitalsignallabs.com> wrote:
>
>> robert bristow-johnson <r...@audioimagination.com> writes:
>>
>>> i have no idea what "-ve" or "+ve" is.  never read those terms
before
>>> regarding the DFT.
>>
>> negative/positive
> Hi Randy,
>     This a great example of laziness on the part of the
> original poster causing confusion for those trying
> to offer help.
>
> [-Rick-]

We could put a stop to this by refusing to acknowledge posts that
contain unreasonable abbreviations. We could wast the time by instead
discussing at length which abbreviations were "reasonable" and which
were not.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
```
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# Re: DFT & other basic questions - Allan Herriman - 2012-10-22 06:08:00

```On Sun, 21 Oct 2012 08:30:08 -0700, Rick Lyons wrote:

> On Sun, 14 Oct 2012 11:00:49 -0400, Randy Yates
> <y...@digitalsignallabs.com> wrote:
>
>>robert bristow-johnson <r...@audioimagination.com> writes:
>>
>>> i have no idea what "-ve" or "+ve" is.  never read those terms
before
>>> regarding the DFT.
>>
>>negative/positive
> Hi Randy,
>    This a great example of laziness on the part of the
> original poster causing confusion for those trying to offer help.

It must be an American thing; +ve and -ve are commonly used around here.

Regards,
Allan
```
______________________________

# Re: DFT & other basic questions - Rob Gaddi - 2012-10-23 13:00:00

```On 22 Oct 2012 10:08:21 GMT
Allan Herriman <a...@hotmail.com> wrote:

> On Sun, 21 Oct 2012 08:30:08 -0700, Rick Lyons wrote:
>
> > On Sun, 14 Oct 2012 11:00:49 -0400, Randy Yates
> > <y...@digitalsignallabs.com> wrote:
> >
> >>robert bristow-johnson <r...@audioimagination.com> writes:
> >>
> >>> i have no idea what "-ve" or "+ve" is.  never read those
terms before
> >>> regarding the DFT.
> >>
> >>negative/positive
> > Hi Randy,
> >    This a great example of laziness on the part of the
> > original poster causing confusion for those trying to offer help.
>
>
> It must be an American thing; +ve and -ve are commonly used around here.
>
> Regards,
> Allan

As a dirty colonial, I've never seen 'em before, and was wondering what
the original poster's emitter voltages had to do with anything.

--
Rob Gaddi, Highland Technology -- www.highlandtechnology.com
Email address domain is currently out of order.  See above to fix.
```
______________________________

# Re: DFT & other basic questions - Rick Lyons - 2012-10-23 14:03:00

```On 22 Oct 2012 10:08:21 GMT, Allan Herriman
<a...@hotmail.com> wrote:

>On Sun, 21 Oct 2012 08:30:08 -0700, Rick Lyons wrote:
>
>> On Sun, 14 Oct 2012 11:00:49 -0400, Randy Yates
>> <y...@digitalsignallabs.com> wrote:
>>
>>>robert bristow-johnson <r...@audioimagination.com> writes:
>>>
>>>> i have no idea what "-ve" or "+ve" is.  never read those
terms before
>>>> regarding the DFT.
>>>
>>>negative/positive
>> Hi Randy,
>>    This a great example of laziness on the part of the
>> original poster causing confusion for those trying to offer help.
>
>
>It must be an American thing; +ve and -ve are commonly used around here.
>
>Regards,
>Allan

Hi Allan,
Ah ha.  That explains things.  I don't recall
ever seeing "+ve" and "-ve" before.

Does "around here" mean the place where people
can "open the boot" of their cars?

[-Rick-]
```
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