On Tuesday, March 5, 2019 at 9:51:48 PM UTC-5, Steve Pope wrote:
> dbd  <d.dalrymple@sbcglobal.net> wrote:
> 
> >On Tuesday, March 5, 2019 at 7:51:47 AM UTC-8, mako...@yahoo.com wrote:
> >
> >> I agree with your statement, but
> >> 
> >>  I think the beam created by this "time domain method" will still vary
> >as a function of frequency?
> >> 
> >> Because delay lines  create a phase shift that varies with frequency.
> >> 
> >> Because it is phase shift (not time delay)  that controls the beam
> >> 
> >> Am I wrong?
> >
> >Yes.
> >
> >> 
> >> m
> >
> >In the broadband time delay beamformer, the delays set the beam pointing
> >angles, which are constant in frequency. The beam widths will vary with
> >frequency as the effective size of the array, in wavelengths, changes
> >with frequency.
> 
> I am thinking that a yagi antenna is a narrowband beamformer whereas
> a log periodic antenna is broadband, and their must be equivalents
> in DSP.
> 
> Steve

OK I get it now.
The geometry of the array creates signals that are offset in TIME, not offset in PHASE as I was thinking.  Therefore to get the signals to add constructively over a wide range of frequencies,  "all you need to do"  is use time delays to  time align the multiple signals and they will remain time aligned over a range of frequencies.  I was so accustomed to thinking in terms of phase that  I was incorrectly thinking that the array created signals that are shifted by a given amount of phase.  This is false.  They are shifted by a given amount of time, so all it takes is time delays to get the signals back  in alignment.       

Thank you all for the discussion.

Mark

dbd  <d.dalrymple@sbcglobal.net> wrote:

>On Tuesday, March 5, 2019 at 7:51:47 AM UTC-8, mako...@yahoo.com wrote:
>
>> I agree with your statement, but
>> 
>>  I think the beam created by this "time domain method" will still vary
>as a function of frequency?
>> 
>> Because delay lines  create a phase shift that varies with frequency.
>> 
>> Because it is phase shift (not time delay)  that controls the beam
>> 
>> Am I wrong?
>
>Yes.
>
>> 
>> m
>
>In the broadband time delay beamformer, the delays set the beam pointing
>angles, which are constant in frequency. The beam widths will vary with
>frequency as the effective size of the array, in wavelengths, changes
>with frequency.

I am thinking that a yagi antenna is a narrowband beamformer whereas
a log periodic antenna is broadband, and their must be equivalents
in DSP.

Steve

On Tuesday, March 5, 2019 at 7:51:47 AM UTC-8, mako...@yahoo.com wrote:

> I agree with your statement, but
> 
>  I think the beam created by this "time domain method" will still vary as a function of frequency?
> 
> Because delay lines  create a phase shift that varies with frequency.
> 
> Because it is phase shift (not time delay)  that controls the beam
> 
> Am I wrong?

Yes.

> 
> m

In the broadband time delay beamformer, the delays set the beam pointing angles, which are constant in frequency. The beam widths will vary with frequency as the effective size of the array, in wavelengths, changes with frequency.

Dale B. Dalrymple

> > 
> >>
> >> Wideband beamforming is achieved using time delays.
> >>
> >>
> > 
> > I was hoping someone would chime in here.
> > 
> > A time delay that is constant over frequency will STILL yield a phase shift varies over frequency and therefore not be suitable for a wideband beamformer?
> > 
> > 
> > Perhaps you meant to say DSP can be used to create wideband beamforming by providing a time delay that varies over frequency such that the PHASE shift is constant over frequency, example Hilbert xform?
> > 
> > Mark
> > 
> > 
> >   
> > 
> It looks like you are assuming that beamforming must be done in the 
> frequency domain.  If your receivers feed delay lines with a relatively 
> fine delay resolution, you can beamform entirely in the time domain.
> 
>

I agree with your statement, but

 I think the beam created by this "time domain method" will still vary as a function of frequency?

Because delay lines  create a phase shift that varies with frequency.

Because it is phase shift (not time delay)  that controls the beam

Am I wrong?

m




 

On 3/4/2019 12:13, makolber@yahoo.com wrote:
> 
>>
>> Wideband beamforming is achieved using time delays.
>>
>>
> 
> I was hoping someone would chime in here.
> 
> A time delay that is constant over frequency will STILL yield a phase shift varies over frequency and therefore not be suitable for a wideband beamformer?
> 
> 
> Perhaps you meant to say DSP can be used to create wideband beamforming by providing a time delay that varies over frequency such that the PHASE shift is constant over frequency, example Hilbert xform?
> 
> Mark
> 
> 
>   
> 
It looks like you are assuming that beamforming must be done in the 
frequency domain.  If your receivers feed delay lines with a relatively 
fine delay resolution, you can beamform entirely in the time domain.

-- 
Best wishes,
--Phil
pomartel At Comcast(ignore_this) dot net

> 
> Wideband beamforming is achieved using time delays. 
> 
>

I was hoping someone would chime in here.

A time delay that is constant over frequency will STILL yield a phase shift varies over frequency and therefore not be suitable for a wideband beamformer?


Perhaps you meant to say DSP can be used to create wideband beamforming by providing a time delay that varies over frequency such that the PHASE shift is constant over frequency, example Hilbert xform?

Mark


 

On Tuesday, February 19, 2019 at 10:13:37 AM UTC-5, mako...@yahoo.com wrote:

> 
> maybe the point you miss is that beam forming is NOT a wideband process but is wavelength (frequency) dependent.

Beamforming can be either narrowband or wideband. Narrowband beamforming applies phase shifts to the signals - this equates to a time delay at a particular frequency. For narrowband systems you're assuming that the phase delay value doesn't change much over the frequency range of interest.

Wideband beamforming is achieved using time delays. Time delays can be achieved by upsampling the signal to achieve higher time resolution needed for beamforming - usually some form of interpolation is also used. It can also be achieved by applying a tapped delay line filter to each element of the array. The filter is trying to delay the signal by the appropriate amount. 

Most systems are assuming a constant speed of propagation, but in some environments the speed of propagation may vary.

David

On Saturday, February 16, 2019 at 6:53:36 PM UTC-5, stch...@gmail.com wrote:
> Hi,
> A typical beamforming method widely used in ultrasound technology is to apply delays between channels and then summing the signals of all the channels. However, when properly calculated delays are applied, it could happen that the phase of the arriving signal is 180 deegres (half the ultrasound wavelength) shifted between channels and summed amplitude is decreased. This results in decreasing SNR and dynamic range. Does anybody here have an idea how to solve this problem?

but that means that the delays were NOT "properly calculated"

if the delays were properly calculated ___for the given wavelength and angle of arrival___  , the signal will be enhanced, not degraded.

maybe i don't understand the question?

maybe the point you miss is that beam forming is NOT a wideband process but is wavelength (frequency) dependent.


mark

<stchebel@gmail.com> wrote:

>A typical beamforming method widely used in ultrasound technology is to
>apply delays between channels and then summing the signals of all the
>channels. However, when properly calculated delays are applied, it could
>happen that the phase of the arriving signal is 180 deegres (half the
>ultrasound wavelength) shifted between channels and summed amplitude is
>decreased. This results in decreasing SNR and dynamic range. Does
>anybody here have an idea how to solve this problem? 

If you are trying to obtain maximum ratio combining, applying the
appropriate phase shift (perhaps with a Hilbert transformer) to
each channel should work.

How to obtain the needed phase shifts would depend on application,
but a training signal should work.

Steve

On Saturday, February 16, 2019 at 3:53:36 PM UTC-8, stch...@gmail.com wrote:
> Hi,
> A typical beamforming method widely used in ultrasound technology is to apply delays between channels and then summing the signals of all the channels. However, when properly calculated delays are applied, it could happen that the phase of the arriving signal is 180 deegres (half the ultrasound wavelength) shifted between channels and summed amplitude is decreased. This results in decreasing SNR and dynamic range. Does anybody here have an idea how to solve this problem?

When we sample a single time domain channel, assuming the baseband (DC to maximum desired frequency) case, we must satisfy the Nyquist criteria which requires that we sample for a rate of more than 2 samples per cycle in time of the highest desired frequency. A beamformer takes in samples from a set of locations in space and calculates samples for a signal from a selected bearing. There is a Nyquist criteria for the location of the sample positions in space. Consider a linear array of equally spaced sampling sites. The Nyquist criteria is that there must be more than 2 sample locations Array sensors_ for every wavelength (for the signal propagating in the media you work in) of the highest desired frequency to avoid spacial aliasing. If you satisfy that, you cannot have an unexpected 180 degree error.

Dale B. Dalrymple