Audio filtering (was: Best equipment for stealth recording of voice (not music))

[R. L. Drake]

This is related to the other query I posted several days ago regarding
"electronic note taking" at meetings using a digital voice recorder and
Radio Shack microphone.

My main problem is that usually the audio quality is marginal at best, and
often large blocks of audio are indecipherable.

I received several helpful responses to that query, for which I thank you
and will try them.

I had another thought, though.

I wonder if I could put a bandpass filter on the audio output from the
recorder, and set the bandwidth from say around 500 Hz to 1500 Hz, or some
other range, to cut out as much of the unwanted frequency spectrum as I
can, and enhance that part of the audio range that I want.

There are audio DSP filters available such as the MFJ 784B
(http://www.mfjenterprises.com/products.php?prodid=MFJ-784B) that have
very sharp skirts, very flat tops, and will allow you to adjust the lower
and upper cutoff frequencies and the bandwidth. They are typically used
by amateur radio operators to help cut down on interference and help
their desired received signals come through. This particular one is
rather pricey and I'd like to know it will help make the audio more
intelligible before I spend the money.

Thanks...

[Richard Crowley]

"R. L. Drake" wrote...
> I wonder if I could put a bandpass filter on the audio output from the
> recorder, and set the bandwidth from say around 500 Hz to 1500 Hz, or
> some
> other range, to cut out as much of the unwanted frequency spectrum as
> I
> can, and enhance that part of the audio range that I want.
>
> There are audio DSP filters available such as the MFJ 784B
> (http://www.mfjenterprises.com/products.php?prodid=MFJ-784B) that have
> very sharp skirts, very flat tops, and will allow you to adjust the
> lower
> and upper cutoff frequencies and the bandwidth. They are typically
> used
> by amateur radio operators to help cut down on interference and help
> their desired received signals come through. This particular one is
> rather pricey and I'd like to know it will help make the audio more
> intelligible before I spend the money.

You can do it for free (or close to free) on your computer.
Try it and see how helpful it is. Don't set your expectations
too high.

[jwvm]

On Apr 7, 9:32 am, "R. L. Drake" > wrote:
> This is related to the other query I posted several days ago regarding
> "electronic note taking" at meetings using a digital voice recorder and
> Radio Shack microphone.
>
> My main problem is that usually the audio quality is marginal at best, and
> often large blocks of audio are indecipherable.
>
> I received several helpful responses to that query, for which I thank you
> and will try them.
>
> I had another thought, though.
>
> I wonder if I could put a bandpass filter on the audio output from the
> recorder, and set the bandwidth from say around 500 Hz to 1500 Hz, or some
> other range, to cut out as much of the unwanted frequency spectrum as I
> can, and enhance that part of the audio range that I want.
>
> There are audio DSP filters available such as the MFJ 784B
> (http://www.mfjenterprises.com/products.php?prodid=MFJ-784B) that have
> very sharp skirts, very flat tops, and will allow you to adjust the lower
> and upper cutoff frequencies and the bandwidth. They are typically used
> by amateur radio operators to help cut down on interference and help
> their desired received signals come through. This particular one is
> rather pricey and I'd like to know it will help make the audio more
> intelligible before I spend the money.
>
> Thanks...

You might want to try a restoration program like the one at the link
below.

http://www.acondigital.com/features/noise_reduction/index.html

[R. L. Drake]

On Sat, 07 Apr 2007 07:15:08 -0700, Richard Crowley wrote:

> You can do it for free (or close to free) on your computer.
> Try it and see how helpful it is. Don't set your expectations
> too high.

Good morning, Richard.

"jwvm" suggested something from Acon Digital Media, which looks like it
has possibilities. Is this what you were referring to? If not, can you
tell me what computer application you had in mind? My expectations aren't
high at all, or at least I don't think they are... anything that will give
me decipherable output most of the time is fine for me, doesn't need to be
hi-fidelity or anything close to it.

[Boris Lau]

R. L. Drake wrote:
> "jwvm" suggested something from Acon Digital Media, which looks like it
> has possibilities.
> Is this what you were referring to?
that's not free. I'd rather start with a simple equalizer to cut the
frequencies you don't want.

> If not, can you
> tell me what computer application you had in mind?
I'm not him, but you could try for example Audacity:
http://audacity.sourceforge.net

It's a free program for editing audio files, in the effect menu you find
an equalizer function. Of course you have to get your recording into
your computer first. But if you use of those little digital recorders,
it might have USB so you can simply transfer the file.

Boris



--
http://www.borislau.de - computer science, music, photos

[hank alrich]

R. L. Drake wrote:

> Richard Crowley wrote:
>
> > You can do it for free (or close to free) on your computer.
> > Try it and see how helpful it is. Don't set your expectations
> > too high.
>
> Good morning, Richard.
>
> "jwvm" suggested something from Acon Digital Media, which looks like it
> has possibilities. Is this what you were referring to? If not, can you
> tell me what computer application you had in mind? My expectations aren't
> high at all, or at least I don't think they are... anything that will give
> me decipherable output most of the time is fine for me, doesn't need to be
> hi-fidelity or anything close to it.

Richard is appropriately suggesting that you can load the files into
your computer and experiment with EQ to your heart's delight without
incurring much expense.

--
ha
Iraq is Arabic for Vietnam

[Richard Crowley]

"R. L. Drake" wrote ...
> Richard Crowley wrote:
>> You can do it for free (or close to free) on your computer.
>> Try it and see how helpful it is. Don't set your expectations
>> too high.
>
> Good morning, Richard.
>
> "jwvm" suggested something from Acon Digital Media,
> which looks like it has possibilities. Is this what you
> were referring to?

No, but it is a likely candidate. You problem won't be
trying to find something, but to wade through all the
many possibilities to find what you want.

> If not, can you tell me what computer application you
> had in mind?

I was thinking of (free) Audacity
http://sourceforge.net/projects/audacity

> My expectations aren't high at all, or at least I don't think
> they are... anything that will give me decipherable output
> most of the time is fine for me, doesn't need to be
> hi-fidelity or anything close to it.

You may not realize that what you are asking to do is
actually significantly more difficult that it sounds. But
perhaps in your case, it is less demanding because you
need only "transcription-quality". But still, removing
broadband noise (street traffic, clothing rubbing, etc.)
is difficult without severely impacting the desired signal
also. And you may have already noticed that microphones
don't "hear" like your ears do. Especially in reverberant
spaces, you need to get the mic a lot closer to get a
decent recording than you ears would have to be, etc.

[R. L. Drake]

On Sat, 07 Apr 2007 17:32:52 +0200, Boris Lau wrote:

> I'm not him, but you could try for example Audacity:
> http://audacity.sourceforge.net
>
> It's a free program for editing audio files, in the effect menu you find
> an equalizer function. Of course you have to get your recording into
> your computer first. But if you use of those little digital recorders,
> it might have USB so you can simply transfer the file.

Good afternoon, Boris.

Yes, my recorder has a USB port and I can easily download audio into the
computer as WAV files.

Thanks to you and everyone who suggested Audacity. I'll give it a try.

[jwvm]

On Apr 7, 11:17 am, "R. L. Drake" > wrote:

<snip>
> "jwvm" suggested something from Acon Digital Media, which looks like it
> has possibilities. Is this what you were referring to? If not, can you
> tell me what computer application you had in mind? My expectations aren't
> high at all, or at least I don't think they are... anything that will give
> me decipherable output most of the time is fine for me, doesn't need to be
> hi-fidelity or anything close to it.

The advantage of programs like Acoustica is that you can select a
portion of the waveform where there is only noise and let the program
figure out how to filter the data. It can greatly reduce background
noise although audible artifacts become apparent as noise suppression
increases. Audacity apparently has similar capabilities so given that
it is free, you might try that first. You can also download a trial
version of Acoustica if you want another option.

[[email protected]]

>R. L. Drake wrote:
>> "jwvm" suggested something from Acon Digital Media, which looks
>>like it has possibilities.
>> Is this what you were referring to?
>that's not free. I'd rather start with a simple equalizer to cut the
>frequencies you don't want.
<snippage>
>I'm not him, but you could try for example Audacity:
>It's a free program for editing audio files, in the effect menu you
>find an equalizer function. Of course you have to get your
>recording into your computer first. But if you use of those little
>digital recorders, it might have USB so you can simply transfer the
>file.
I'd say you want to process *after* you record anyway. My
problem with any kind of filtration, noise gate etc. on
recorded tracks while recording is that I can't undo them.

i was mastering a gospel album for some clients at a friend
of mine's studio a few years ago, and one of the album
tracks was a song that began just with high hat cymbal and
electric piano before the bass and full kit plus other
layers came in. tHe afternoon we'd done that track that
keyboard he was using had a ground loop problem and was
delivering me a little 60 hz, but he wouldn't retake the
damned thing. I wanted to retake anyway with the drums
mic'd somewhat differently as we'd been doing in later
sessions, or at least get him to retake the electric piano.
NO soap, he was satisfied.

My host for the evening's mastering session is always
wanting to show me what his magic plug-ins will do for
mastering, and 90% of the time i turn up my nose. HE can't
interest me in the old use the eq curve of the client's
favorite album trip.

TO make a long one short he grabbed cedar and hit the intro
pulling it off when the arrangement was full and nobody
would be likely to notice the 60 hz hummmmm.
WE were able to dial in just enough to wipe the 60 hz
without really hurting the electric piano sound too badly at
all.

STill for live recording combined with sr I like to have
some good parametric eq available.




Richard webb,
Electric Spider Productions
Replace anything before the @ symbol with elspider for real
email address.



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