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'[EE] Noise and spike removal in audio, and the dec'
2010\11\30@084833 by Olin Lathrop

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Tag changed to EE.

V G wrote:
> How would i clip off these loud spikes? In Audacity, they're clearly
> visible as tall spikes, compared to the rest of the recording.

The trick to reducing noise after the fact is to find a characteristic of
the noise not present in the real signal.  The first obvious criterion is
frequency, but you've probably already done that, and lots of the noise will
be in the same frequency range.  Everything else gets more tricky.

I remember when DSPs were first available and music was still on vinyl
records, someone made a box to eliminated the pops caused by little bits of
dirt in the record groove.  The algorithm looked for a sudden spike, then
cut out the spike and patched it over as if it never happened.  There was a
knob that adjusted how aggressive this was.  At one end you still got all
the pops.  At the other end it cut into the music noticably.  In the middle
it sometimes produced useful results.

> 3. I have a question about decibels. I understand that when measuring
> volume or sound intensity, 0 dB is the level at which sound is barely
> audible for the average human.

Not exactly.  DeciBels are really just a way to express a power ratio, so
0dB is just another way of saying a power ratio of 1.

There are some absolute scales based on dB, but these have specific names or
must somehow clearly state what their 0 reference is.  For example, small
radio transmitters are sometimes measured in dBm.  In that case 0dB is
specifically defined to be 1mW.

There is a sound pressure level scale based on deciBels (which is what
Alexander Graham Bell invented this logarithmic scale for in the first
place), but I don't know what its specific name is.  On that scale 0 is
indeed supposed to be the average leval at which humans can just barely hear
the sound.

> 3.2. My recorder has a sound intensity scale (I think) that is showing
> values from -48 dB all the way up to 0 dB on the high end. You can
> see the bar moving higher if you speak louder. I don't understand
> this scale.

Without a stated reference it is just a relative measure.  This sounds like
a scale often used for audio voltage signals and microphones, but I don't
remember the reference.  Some preamp "line" signals I worked with were
sometimes stated in dB with 0 reference being 1V RMS into 600 Ohms, but I'm
not sure how standard that was and you may have something quite different.

Back to your problem.  The best way to get good signal to noise ratio is to
eliminate as much noise as possible right up front.  Trying to detect and
remove the noise after it's mixed in with the real signal is difficult.

A directional microphone would help a lot, since direction is a clear
distinguishing characteristic of the signal versus the noise.  If you really
wanted to make a project out of it, you could use several small microphones,
record the stream from each with the same timebase, then post process them
to detect the professor's voice and eliminate the other sounds.  This is
basically making a phased array antenna for audio.  Think Aegis guided
missile radar.  However, that's a lot of work and I'm not seriously
suggesting it.

Try a plain directional microphone, or even ask the professor to wear a
wireless mic.


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(978) 742-9014.  Gold level PIC consultants since 2000

2010\11\30@090619 by Kerry Wentworth

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Is Aegis common knowledge these days?  Or do you have some specific experience with it?

Kerry


Olin Lathrop wrote:
{Quote hidden}

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Version: 7.0.289 / Virus Database: 267.11.13 - Release Date: 10/6/05

2010\11\30@093714 by Olin Lathrop

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Kerry Wentworth wrote:
> Is Aegis common knowledge these days?  Or do you have some specific
> experience with it?

The Aegis radar on the Arleigh Burke class destroyers has been publicly
known for a long time.  I'm sure a bunch of important details are
classified, but the general principal of a phased array radar is well known,
and didn't originate with the Aegis system anyway.  Besides, it wouldn't
take experts on the other side long to figure out what the funny looking
large hexagonal panels high on the sides of the ship are for.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2010\11\30@095931 by Kerry Wentworth

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I was just curious.  The precursor was the Tartar system, on Spruance class destroyers.  That was the system I worked on in the Navy.  The AN/SPS39 was a phased array height finding search radar, and the AN/SPG51 ('my' radar) was guidance.  The 51 was retained under Aegis, although the 39 was replaced by the AN/SPY1, which is the actual hexagonal antennas that people associate with Aegis.  I know it's not a secret, just didn't know how well it was known outside a small circle.

Kerry


Olin Lathrop wrote:
{Quote hidden}

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2010\11\30@105016 by alan.b.pearce

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> I was just curious.  The precursor was the Tartar system, on Spruance
> class destroyers.  That was the system I worked on in the Navy.  The
> AN/SPS39 was a phased array height finding search radar, and the
> AN/SPG51 ('my' radar) was guidance.  The 51 was retained under Aegis,
> although the 39 was replaced by the AN/SPY1, which is the actual
> hexagonal antennas that people associate with Aegis.  I know it's not
a
> secret, just didn't know how well it was known outside a small circle.
>
> Kerry
>
>
> Olin Lathrop wrote:
> > Kerry Wentworth wrote:
> >
> >> Is Aegis common knowledge these days?  Or do you have some specific
> >> experience with it?
> >>
> >
> > The Aegis radar on the Arleigh Burke class destroyers has been
publicly
> > known for a long time.  I'm sure a bunch of important details are
> > classified, but the general principal of a phased array radar is
well known,
> > and didn't originate with the Aegis system anyway.  Besides, it
wouldn't
> > take experts on the other side long to figure out what the funny
looking
> > large hexagonal panels high on the sides of the ship are for.

Oh, you mean panels like this land based one ...
http://www.bbc.co.uk/northyorkshire/uncovered/fylingdales/index.shtml

-- Scanned by iCritical.

2010\11\30@150738 by Carey Fisher

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On Tue, Nov 30, 2010 at 9:06 AM, Kerry Wentworth <
spam_OUTkwentworthTakeThisOuTspamskunkworksnh.com> wrote:

> Is Aegis common knowledge these days?  Or do you have some specific
> experience with it?
>
> Kerry
>
> I was the manufacturing manager for the phased array elements on early
Aegis builds. (Also Phalanx, MILSTAR satellite system, IR seekers for the
Hellfire missile...)

Back then phase control elements were made from ferrite (iron oxide with
other stuff in it) inside a piece of waveguide that you would magnetize very
precisely to various states to achieve a desired amount of phase delay at a
specific microwave frequency.  Put a whole bunch of phase control elements
together, combine the signals of different phases and you can have multiple
radar beams, null out jamming signals and all sorts of neat stuff.

I don't think they use ferrite much any more except for maybe very high
power applications.

Care


'[EE] Noise and spike removal in audio, and the dec'
2010\12\01@035426 by Sean Breheny
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That's a PAVE PAWS radar. We have one of them here in Cape Cod,
Massachusetts to cover the eastern US - its main task is fast alert of
submarine launched missiles, and its secondary task is tracking space
junk so that manned missions and other high priority satellites can be
placed in safe orbits. I once got a tour of the one here as part of an
amateur radio goodwill visit. I was not allowed to bring a camera and
I was asked not to put down any significant details about what I saw
in any written form. So I won't :) However, I can say that the one
here has more up-to-date equipment than is shown in the photos on this
BBC web site. I am somewhat surprised that they allowed these photos
to be taken and published. Does the RAF operate this radar by itself
or are there USAF personnel, too? The one here is a joint USAF/RCAF
operation.

Sean


On Tue, Nov 30, 2010 at 10:50 AM,  <.....alan.b.pearceKILLspamspam@spam@stfc.ac.uk> wrote:
> Oh, you mean panels like this land based one ...
> http://www.bbc.co.uk/northyorkshire/uncovered/fylingdales/index.shtml
>
> --
> Scanned by iCritical.
>
>

2010\12\01@040931 by Sean Breheny

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I should have looked at Wikipedia before I responded! It looks like
the radar at Fylingdales is similar to PAVE PAWS but not exactly the
same, which would explain some of the differences I see in the photos.
Fylingdales is still considered part of the BMEWS (Ballistic Missile
Early Warning System).

Sean


On Wed, Dec 1, 2010 at 3:54 AM, Sean Breheny <shb7spamKILLspamcornell.edu> wrote:
{Quote hidden}

>> -

2010\12\01@055139 by alan.b.pearce

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> I should have looked at Wikipedia before I responded! It looks like
> the radar at Fylingdales is similar to PAVE PAWS but not exactly the
> same, which would explain some of the differences I see in the photos.
> Fylingdales is still considered part of the BMEWS (Ballistic Missile
> Early Warning System).

Also look at the date on the link - the article is a reasonable number
of years old. I have a feeling there has been a major upgrade of
Fylingdales since then, and that it is still used to detect the bombers
that they periodically intercept coming in over the Arctic Ocean to the
north of Scotland, attempting to probe UK airspace.
-- Scanned by iCritical.

2010\12\01@080408 by Olin Lathrop

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Sean Breheny wrote:
> That's a PAVE PAWS radar. We have one of them here in Cape Cod,
> Massachusetts to cover the eastern US

In Truro, if I remember right.  I didn't realize it was a phased array, I
never got that good a look.

Where are you located if Cape Cod is local to you?


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Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2010\12\02@031809 by Sean Breheny

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I think Truro is correct. Strangely, even though it is not open to the
public, there are several signs on the roads around it that say "PAVE
PAWS --->".

I live in Lexington and work in Woburn.

Sean


On Wed, Dec 1, 2010 at 8:04 AM, Olin Lathrop <EraseMEolin_piclistspam_OUTspamTakeThisOuTembedinc.com> wrote:
{Quote hidden}

>

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