On Fri, 28 Oct 2011 18:34:49 +0000 (UTC), glen herrmannsfeldt
<gah@ugcs.caltech.edu> wrote:
>robert bristow-johnson <rbj@audioimagination.com> wrote:
>
>(snip)
>> i guess not. i understand the multipath problem, i was just assuming
>> that the direct path (from the single signal source to the two
>> microphones) is shorter and louder than the reflected paths.
>
>I was just remembering, how we learn in 3rd grade science class
>(or maybe 4th grade) that light goes in straight lines, and
>sound doesn't. They can't teach diffraction at that point, though.
>
>You can easily be in a place where the direct path involves lots
>of diffraction and absorption, where another path reflects off
>a large object, such as a building. That could be true for audio
>and, for people in big cities, for radio signals.
>
>-- glen
In EM propagation we call that the "urban canyon" environment. It's
one of the most aggressive multipath cases, especially when you're
mobile.
Eric Jacobsen
Anchor Hill Communications
www.anchorhill.com
Reply by dbd●October 28, 20112011-10-28
On Oct 28, 10:52�am, robert bristow-johnson
<r...@audioimagination.com> wrote:
> On Oct 28, 1:06�pm, dbd <d...@ieee.org> wrote:
> > So Hardy is right that cross correlation gives you the delay (lots of
> > them, from many effects) and Maurice is right that further processing
> > is required to produce useful results.
>
> well, i didn't see what maury was saying about further processing.
I took "further processing" than cross correlation to be additional
processing not necessarily just later processing. Use of the start of
the envelope is one I have used. It requires that you have some method
of identifying signal present as different from background level so
that you can select the "front end" of the envelope to avoid multipath
and reverberation. Towards this end, I've used time domain processing
to identify wideband signals in parallel with frequency domain
processing to identify narrowband signals. There can be a lot of
"further processing" so the system is far more than a simple cross
correlator. Even the conversion of time delay to direction of arrival
can involve further processing to avoid needing to measure propagation
velocity directly.
Dale B. Dalrymple
Reply by maury●October 28, 20112011-10-28
On Oct 28, 1:17�pm, robert bristow-johnson <r...@audioimagination.com>
wrote:
> On Oct 28, 1:16�pm, maury <maury...@core.com> wrote:
>
> > On Oct 28, 10:43�am, robert bristow-johnson
>
> > > i don't see how AMDF will work at all. �AMDF is more related to *auto*-
> > > correlation (operating on a single input signal), not the cross-
> > > correlation between two inputs.
>
> > Robert, it does work for estimating delay between a speech signal and
> > its delayed form, not two independent signals.
>
> so what if the delayed signal has a significant amount of attenuation
> besides the delay? �how well does AMDF work for that?
>
> r b-j
It worked fine. You still get a minimum at the point of (envelope)
correlation.
The AMDF isn't usually used this way, but when you're trying to "cram"
a bunch of stuff in a 44-pin quad ASIC, the reduced complexity of the
AMDF was needed.
Reply by glen herrmannsfeldt●October 28, 20112011-10-28
(snip, someone wrote)
>> and their correlation function is a mess. That happens due
>> to multipath and reverberations,
Reminds me of our library book sale, which is in a really large
building (previously owned by the Navy, but now the city).
They have a PA system and sometimes make announcements, talking
really slow. There are a few speakers around, but also large brick
walls which make good reflections, and you really can't hear well
at all.
I was wondering at the last sale, if you have the appropriate DSP
equipment, and some number of amplifiers and speakers, what would
be the best way to make a system that is most audible in the room?
I was wondering about enough speakers and delay modules to try
to cancel out some of the reflections. It might be that sound
absorbing pads are cheaper, but maybe not.
-- glen
Reply by glen herrmannsfeldt●October 28, 20112011-10-28
robert bristow-johnson <rbj@audioimagination.com> wrote:
(snip)
> i guess not. i understand the multipath problem, i was just assuming
> that the direct path (from the single signal source to the two
> microphones) is shorter and louder than the reflected paths.
I was just remembering, how we learn in 3rd grade science class
(or maybe 4th grade) that light goes in straight lines, and
sound doesn't. They can't teach diffraction at that point, though.
You can easily be in a place where the direct path involves lots
of diffraction and absorption, where another path reflects off
a large object, such as a building. That could be true for audio
and, for people in big cities, for radio signals.
-- glen
Reply by glen herrmannsfeldt●October 28, 20112011-10-28
Vladimir Vassilevsky <nospam@nowhere.com> wrote:
> robert bristow-johnson wrote:
(snip)
>> so you'll get noisy peaks in the auto-correlation (and there will also
>> be other sources from other angles that will have other path-length
>> differences - so then when you see a peak, that will be a legitimate
>> candidate for the time delay, but of a different source). but
>> (alternatively to what Vlad is saying) i would expect that, for a
>> source that is significantly louder than the competing sources
> The reality is that if you hear the same audio source from two different
> positions, those are going to be two very different signals and their
> correlation function is a mess. That happens due to multipath and
> reverberations, as well as because the same audio source behaves quite
> differently at different view angles. It is a naive idea to expect any
> accurate result from trivial AMDF or correlation approach. DOA is no
> simple problem; tons of books are written about it. IIRC doctor Rune was
> specializing at that; perhaps he could clarify.
Is cross-correlation of the absolute value or square better?
It seems that might reduce some phase effects that could confuse
the correlation. Otherwise, you hope that the amplitude of multipath
and reverberation is smaller that the main signal.
I have been in places where sound reflecting off a building was
the most direct source from an audio standpoint. (More direct
from the diffraction around other objects.)
-- glen
Reply by robert bristow-johnson●October 28, 20112011-10-28
On Oct 28, 1:16�pm, maury <maury...@core.com> wrote:
> On Oct 28, 10:43�am, robert bristow-johnson
>
>
> > i don't see how AMDF will work at all. �AMDF is more related to *auto*-
> > correlation (operating on a single input signal), not the cross-
> > correlation between two inputs.
>
>
> Robert, it does work for estimating delay between a speech signal and
> its delayed form, not two independent signals.
so what if the delayed signal has a significant amount of attenuation
besides the delay? how well does AMDF work for that?
r b-j
Reply by Vladimir Vassilevsky●October 28, 20112011-10-28
robert bristow-johnson wrote:
> On Oct 28, 11:24 am, Vladimir Vassilevsky <nos...@nowhere.com> wrote:
>
>>
>>The reality is that if you hear the same audio source from two different
>>positions, those are going to be two very different signals
>
> do they lose their property of correlation?
The envelope of correlation function would be a hump of several tens of
millisecond wide. The underlying fine structure inside the hump is
pretty much random. The naive method won't make for any accurate estimate.
>>and their
>>correlation function is a mess. That happens due to multipath and
>>reverberations,
>
> yeah, that will result in correlations at different lags. at *longer*
> lags.
Different results for different signals as well.
>>as well as because the same audio source behaves quite
>>differently at different view angles. It is a naive idea to expect any
>>accurate result from trivial AMDF or correlation approach.
>
> i don't see how AMDF will work at all.
For AMDF, use 1-bit quantization (used to be common with radars), or use
some sort of AGC to normalize amplitudes, or find a gain which makes for
the best AMDF match.
> AMDF is more related to *auto*-
> correlation (operating on a single input signal), not the cross-
> correlation between two inputs.
Why not.
>
>>DOA is no simple problem
>
> i guess not. i understand the multipath problem, i was just assuming
> that the direct path (from the single signal source to the two
> microphones) is shorter and louder than the reflected paths.
Heh. Been there. I've burned with that once. Won't fall into this
mistake again.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Reply by robert bristow-johnson●October 28, 20112011-10-28
On Oct 28, 1:06�pm, dbd <d...@ieee.org> wrote:
>
> So Hardy is right that cross correlation gives you the delay (lots of
> them, from many effects) and Maurice is right that further processing
> is required to produce useful results.
>
well, i didn't see what maury was saying about further processing.
what i *didn't* understand from maury was this thing about the AMDF.
i see (and wrote) how AMDF can be related to *auto*-correlation (AMDF
should minimize at lags where auto-correlation maximizes). but auto-
correlation is a single input signal operation (it's the cross-
correlation between a signal and itself). cross-correlation is a two
input operation and i do not see how AMDF could be adapted to perform
a similar operation unless somehow the two signals were of the same
energy (but i don't expect that to be the case for time delay
difference, the delayed signal is likely to be lower in energy).
but i fully understand and agree with the need for post-processing of
the output of AMDF, ASDF, auto-correlation, cross-correlation, the
output of an FFT, whatever. it's one thing to get this intermediate
data, it's another thing to tease outa this intermediate data a
salient parameter of interest. usually that requires some sorta
"expert systems" programming, not the sorta DSProgramming we normally
do.
r b-j
Reply by maury●October 28, 20112011-10-28
On Oct 28, 10:43�am, robert bristow-johnson
<r...@audioimagination.com> wrote:
> On Oct 28, 11:24�am, Vladimir Vassilevsky <nos...@nowhere.com> wrote:
>
>
>
>
>
> > robert bristow-johnson wrote:
> > > On Oct 27, 12:15 am, "steveu" <steveu@n_o_s_p_a_m.coppice.org> wrote:
>
> > >>Isn't that only going to work well for clean signals from the source? The
> > >>OP said these are signals from two mics, so they are going to have a lot of
> > >>reverb mixed in, and the reverb will be quite different at each mic. As
> > >>Vlad said, the cross correlation might look near to random.
>
> > > so you'll get noisy peaks in the auto-correlation (and there will also
> > > be other sources from other angles that will have other path-length
> > > differences - so then when you see a peak, that will be a legitimate
> > > candidate for the time delay, but of a different source). �but
> > > (alternatively to what Vlad is saying) i would expect that, for a
> > > source that is significantly louder than the competing sources
>
> > The reality is that if you hear the same audio source from two different
> > positions, those are going to be two very different signals
>
> do they lose their property of correlation?
>
> > and their
> > correlation function is a mess. That happens due to multipath and
> > reverberations,
>
> yeah, that will result in correlations at different lags. �at *longer*
> lags.
>
> > as well as because the same audio source behaves quite
> > differently at different view angles. It is a naive idea to expect any
> > accurate result from trivial AMDF or correlation approach.
>
> i don't see how AMDF will work at all. �AMDF is more related to *auto*-
> correlation (operating on a single input signal), not the cross-
> correlation between two inputs.
>
> > DOA is no simple problem
>
> i guess not. �i understand the multipath problem, i was just assuming
> that the direct path (from the single signal source to the two
> microphones) is shorter and louder than the reflected paths.
>
> r b-j- Hide quoted text -
>
> - Show quoted text -
Robert, it does work for estimating delay between a speech signal and
its delayed form, not two independent signals. The original post
implied this situation.