On Jul 5, 10:47�am, Jerry Avins <j...@ieee.org> wrote:
> Dirk Bell wrote:
> > On Jul 4, 8:49 pm, "sxy6z" <icipiq...@yahoo.com.cn> wrote:
> >>> On Sat, 04 Jul 2009 10:40:44 -0500, sxy6z wrote:
> >>>> Hello Masters:
> >>>> � � � When I develop a Butterwoth IIR filter. there is a strange
> >>>> phnomenon. After filter a standard sine signal with it, noise signal
> >> has
> >>>> been removed. but the amplitudes/peaks of each �wave are not the same.
> >>>> for example, maybe one is 0.9994 and another is 0.977 etc.
> >>>> � � � In fact, Some FIR filters also have this phenomenon. It confused
> >>>> � � � me.
> >>>> � � � Any remarks are appreciated and thank you very much~~
> >>> Do you mean that different frequencies of sine waves have different
> >>> amplitudes? �If so, you're just seeing the fact that a Butterworth filter
> >>> has a DC gain of 1 and AC gains that are all strictly less than 1.
> >>> If you mean that you put _one_ sine wave through it, and that one sine
> >>> wave has peaks at different values, then you are either seeing the
> >>> transient response of the filter superimposed on it's continuous
> >>> response, or you are seeing the results of a bug in your code.
> >>> --
> >>>http://www.wescottdesign.com
> >> Signal is single frequency 0.5Hz. the peak of each wave is not the same.
> >> the added noise frequecy is random noise. After filter, different peaks
> >> have different value. Feel some difficult to discribe clearly. hehe ~~- Hide quoted text -
>
> >> - Show quoted text -
>
> > 1) You did add random wideband noise that may be mostly filtered out
> > but the remaining noise (i.e. noise not in stopband) still remains and
> > will be added to all samples including the peaks, so they wouldn't be
> > the same value. Steve Pope I believe was referring to this.
>
> > 2) Even if you did not add any noise, and if the sine wave is
> > essentially pure, and there is no quantization noise, and you let the
> > filter transient die out, you can get peaks of different amplitudes
> > depending on the relationship of the input frequency, sampling
> > frequency, and when you take the samples. �Is the 0.5Hz wave
> > mathematically generated and fed into your filter code? What is the
> > sample rate? Is there an integer number of samples per cycle of the
> > 0.5 Hz waveform?
>
> > If the answer to the last question in 2) is not TRUE, the
> > "peak" (largest sample in that half of the sinusoidal cycle) will not
> > be regularly sampled at the actual peak time of the waveform and the
> > "peak" amplitude may wander a bit.
>
> This just shows that, too observe the true nature of the filtered
> waveform, reconstruction filtering (or its equivalent for the purpose)
> needs to be applied.
>
> Jerry
> --
> Engineering is the art of making what you want from things you can get.
> �����������������������������������������������������������������������- Hide quoted text -
>
> - Show quoted text -
BTW, in the second case I mentioned, if the answer to the last
question is FALSE, while the "peak" value you get will be repetitive,
unless your timing is perfect, it will not be the true peak of the
sinusoidal waveform. So Jerry's comment is applicable to both cases.
Dirk Bell
Reply by Jerry Avins●July 5, 20092009-07-05
Dirk Bell wrote:
> On Jul 4, 8:49 pm, "sxy6z" <icipiq...@yahoo.com.cn> wrote:
>>> On Sat, 04 Jul 2009 10:40:44 -0500, sxy6z wrote:
>>>> Hello Masters:
>>>> When I develop a Butterwoth IIR filter. there is a strange
>>>> phnomenon. After filter a standard sine signal with it, noise signal
>> has
>>>> been removed. but the amplitudes/peaks of each wave are not the same.
>>>> for example, maybe one is 0.9994 and another is 0.977 etc.
>>>> In fact, Some FIR filters also have this phenomenon. It confused
>>>> me.
>>>> Any remarks are appreciated and thank you very much~~
>>> Do you mean that different frequencies of sine waves have different
>>> amplitudes? If so, you're just seeing the fact that a Butterworth filter
>>> has a DC gain of 1 and AC gains that are all strictly less than 1.
>>> If you mean that you put _one_ sine wave through it, and that one sine
>>> wave has peaks at different values, then you are either seeing the
>>> transient response of the filter superimposed on it's continuous
>>> response, or you are seeing the results of a bug in your code.
>>> --
>>> http://www.wescottdesign.com
>> Signal is single frequency 0.5Hz. the peak of each wave is not the same.
>> the added noise frequecy is random noise. After filter, different peaks
>> have different value. Feel some difficult to discribe clearly. hehe ~~- Hide quoted text -
>>
>> - Show quoted text -
>
> 1) You did add random wideband noise that may be mostly filtered out
> but the remaining noise (i.e. noise not in stopband) still remains and
> will be added to all samples including the peaks, so they wouldn't be
> the same value. Steve Pope I believe was referring to this.
>
> 2) Even if you did not add any noise, and if the sine wave is
> essentially pure, and there is no quantization noise, and you let the
> filter transient die out, you can get peaks of different amplitudes
> depending on the relationship of the input frequency, sampling
> frequency, and when you take the samples. Is the 0.5Hz wave
> mathematically generated and fed into your filter code? What is the
> sample rate? Is there an integer number of samples per cycle of the
> 0.5 Hz waveform?
>
> If the answer to the last question in 2) is not TRUE, the
> "peak" (largest sample in that half of the sinusoidal cycle) will not
> be regularly sampled at the actual peak time of the waveform and the
> "peak" amplitude may wander a bit.
This just shows that, too observe the true nature of the filtered
waveform, reconstruction filtering (or its equivalent for the purpose)
needs to be applied.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Dirk Bell●July 5, 20092009-07-05
On Jul 4, 8:49�pm, "sxy6z" <icipiq...@yahoo.com.cn> wrote:
> >On Sat, 04 Jul 2009 10:40:44 -0500, sxy6z wrote:
>
> >> Hello Masters:
>
> >> � � � When I develop a Butterwoth IIR filter. there is a strange
> >> phnomenon. After filter a standard sine signal with it, noise signal
> has
> >> been removed. but the amplitudes/peaks of each �wave are not the same.
> >> for example, maybe one is 0.9994 and another is 0.977 etc.
> >> � � � In fact, Some FIR filters also have this phenomenon. It confused
> >> � � � me.
>
> >> � � � Any remarks are appreciated and thank you very much~~
>
> >Do you mean that different frequencies of sine waves have different
> >amplitudes? �If so, you're just seeing the fact that a Butterworth filter
> >has a DC gain of 1 and AC gains that are all strictly less than 1.
>
> >If you mean that you put _one_ sine wave through it, and that one sine
> >wave has peaks at different values, then you are either seeing the
> >transient response of the filter superimposed on it's continuous
> >response, or you are seeing the results of a bug in your code.
>
> >--
> >http://www.wescottdesign.com
>
> Signal is single frequency 0.5Hz. the peak of each wave is not the same.
> the added noise frequecy is random noise. After filter, different peaks
> have different value. Feel some difficult to discribe clearly. hehe ~~- Hide quoted text -
>
> - Show quoted text -
1) You did add random wideband noise that may be mostly filtered out
but the remaining noise (i.e. noise not in stopband) still remains and
will be added to all samples including the peaks, so they wouldn't be
the same value. Steve Pope I believe was referring to this.
2) Even if you did not add any noise, and if the sine wave is
essentially pure, and there is no quantization noise, and you let the
filter transient die out, you can get peaks of different amplitudes
depending on the relationship of the input frequency, sampling
frequency, and when you take the samples. Is the 0.5Hz wave
mathematically generated and fed into your filter code? What is the
sample rate? Is there an integer number of samples per cycle of the
0.5 Hz waveform?
If the answer to the last question in 2) is not TRUE, the
"peak" (largest sample in that half of the sinusoidal cycle) will not
be regularly sampled at the actual peak time of the waveform and the
"peak" amplitude may wander a bit.
Dirk Bell
Reply by Tim Wescott●July 4, 20092009-07-04
On Sat, 04 Jul 2009 19:49:04 -0500, sxy6z wrote:
>>On Sat, 04 Jul 2009 10:40:44 -0500, sxy6z wrote:
>>
>>> Hello Masters:
>>>
>>> When I develop a Butterwoth IIR filter. there is a strange
>>> phnomenon. After filter a standard sine signal with it, noise signal
> has
>>> been removed. but the amplitudes/peaks of each wave are not the same.
>>> for example, maybe one is 0.9994 and another is 0.977 etc.
>>> In fact, Some FIR filters also have this phenomenon. It confused
>>> me.
>>>
>>> Any remarks are appreciated and thank you very much~~
>>
>>Do you mean that different frequencies of sine waves have different
>>amplitudes? If so, you're just seeing the fact that a Butterworth
>>filter
>
>>has a DC gain of 1 and AC gains that are all strictly less than 1.
>>
>>If you mean that you put _one_ sine wave through it, and that one sine
>>wave has peaks at different values, then you are either seeing the
>>transient response of the filter superimposed on it's continuous
>>response, or you are seeing the results of a bug in your code.
>>
>>--
>>http://www.wescottdesign.com
>>
>>
> Signal is single frequency 0.5Hz. the peak of each wave is not the same.
> the added noise frequecy is random noise. After filter, different peaks
> have different value. Feel some difficult to discribe clearly. hehe ~~
>Signal is single frequency 0.5Hz. the peak of each wave is not the same.
>the added noise frequecy is random noise. After filter, different peaks
>have different value. Feel some difficult to discribe clearly. hehe ~~
This could be additive, low-frequency noise that is not
being filtered out.
Steve
Reply by sxy6z●July 4, 20092009-07-04
>On Sat, 04 Jul 2009 10:40:44 -0500, sxy6z wrote:
>
>> Hello Masters:
>>
>> When I develop a Butterwoth IIR filter. there is a strange
>> phnomenon. After filter a standard sine signal with it, noise signal
has
>> been removed. but the amplitudes/peaks of each wave are not the same.
>> for example, maybe one is 0.9994 and another is 0.977 etc.
>> In fact, Some FIR filters also have this phenomenon. It confused
>> me.
>>
>> Any remarks are appreciated and thank you very much~~
>
>Do you mean that different frequencies of sine waves have different
>amplitudes? If so, you're just seeing the fact that a Butterworth filter
>has a DC gain of 1 and AC gains that are all strictly less than 1.
>
>If you mean that you put _one_ sine wave through it, and that one sine
>wave has peaks at different values, then you are either seeing the
>transient response of the filter superimposed on it's continuous
>response, or you are seeing the results of a bug in your code.
>
>--
>http://www.wescottdesign.com
>
Signal is single frequency 0.5Hz. the peak of each wave is not the same.
the added noise frequecy is random noise. After filter, different peaks
have different value. Feel some difficult to discribe clearly. hehe ~~
Reply by Tim Wescott●July 4, 20092009-07-04
On Sat, 04 Jul 2009 10:40:44 -0500, sxy6z wrote:
> Hello Masters:
>
> When I develop a Butterwoth IIR filter. there is a strange
> phnomenon. After filter a standard sine signal with it, noise signal has
> been removed. but the amplitudes/peaks of each wave are not the same.
> for example, maybe one is 0.9994 and another is 0.977 etc.
> In fact, Some FIR filters also have this phenomenon. It confused
> me.
>
> Any remarks are appreciated and thank you very much~~
Do you mean that different frequencies of sine waves have different
amplitudes? If so, you're just seeing the fact that a Butterworth filter
has a DC gain of 1 and AC gains that are all strictly less than 1.
If you mean that you put _one_ sine wave through it, and that one sine
wave has peaks at different values, then you are either seeing the
transient response of the filter superimposed on it's continuous
response, or you are seeing the results of a bug in your code.
--
http://www.wescottdesign.com
Reply by HardySpicer●July 4, 20092009-07-04
On Jul 4, 8:40�am, "sxy6z" <icipiq...@yahoo.com.cn> wrote:
> Hello Masters:
>
> � � � When I develop a Butterwoth IIR filter. there is a strange
> phnomenon. After filter a standard sine signal with it, noise signal has
> been removed. but the amplitudes/peaks of each �wave are not the same. for
> example, maybe one is 0.9994 and another is 0.977 etc.
> � � � In fact, Some FIR filters also have this phenomenon. It confused me.
>
> � � � Any remarks are appreciated and thank you very much~~
Look at the frequency response and calculate what the magnitude should
be.
It will drop depending on the passband attenuation. How many dB
attenuation in the passband is there?
3dB?
Hardy
Reply by Rune Allnor●July 4, 20092009-07-04
On 4 Jul, 20:57, spop...@speedymail.org (Steve Pope) wrote:
> sxy6z <icipiq...@yahoo.com.cn> wrote:
> > Hello Masters:
> > When I develop a Butterwoth IIR filter. there is a strange
> > phnomenon. After filter a standard sine signal with it, noise
> > signal has been removed. but the amplitudes/peaks of each �wave
> > are not the same. for example, maybe one is 0.9994 and another
> > is 0.977 etc.
>
> Not normal. �You have a bug somewhere.
Seems slike an overflow in the pole vault.
Maybe in the Bubka coefficient? Shouldn't
be higher than about 6.14-6.15.
Rune
Reply by Steve Pope●July 4, 20092009-07-04
sxy6z <icipiq_ka@yahoo.com.cn> wrote:
> Hello Masters:
> When I develop a Butterwoth IIR filter. there is a strange
> phnomenon. After filter a standard sine signal with it, noise
> signal has been removed. but the amplitudes/peaks of each wave
> are not the same. for example, maybe one is 0.9994 and another
> is 0.977 etc.