Improving the SNR of a mic

I use a low end speaker connected to my mother board's AC97-type ADC to
input sounds. The problem is that I have a lot of noise.

How could I improve the SNR and get rid of a big part of the noise?
(I'm not talking here about post-processing but about getting the raw
signal from the ADC to a better SNR). Is my low end speaker the problem
or is it rather my ADC that's introducing the most noise? In other
words do I need a better ADC or a better speaker? Anything about the
two that I can do to make it better?

"Michel Rouzic" <Michel0528@yahoo.fr> wrote in message
news:1140737808.283548.99860@j33g2000cwa.googlegroups.com...
> I use a low end speaker connected to my mother board's AC97-type ADC to
> input sounds. The problem is that I have a lot of noise.
>
> How could I improve the SNR and get rid of a big part of the noise?
> (I'm not talking here about post-processing but about getting the raw
> signal from the ADC to a better SNR). Is my low end speaker the problem
> or is it rather my ADC that's introducing the most noise? In other
> words do I need a better ADC or a better speaker? Anything about the
> two that I can do to make it better?
>

You need to use the full dynamic range of the ADC without clipping of
course.

Tam

Michel Rouzic wrote:
> I use a low end speaker connected to my mother board's AC97-type ADC to
> input sounds. The problem is that I have a lot of noise.
> 
> How could I improve the SNR and get rid of a big part of the noise?
> (I'm not talking here about post-processing but about getting the raw
> signal from the ADC to a better SNR). Is my low end speaker the problem
> or is it rather my ADC that's introducing the most noise? In other
> words do I need a better ADC or a better speaker? Anything about the
> two that I can do to make it better?

What kind of noise? Hum? The fan in the computer? Low-end speaker or 
high, the impedance mismatch to the sound input is very high. Try a 
transformer. And shielding.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

Dougal McDougal of that Elk wrote:
> "Michel Rouzic" <Michel0528@yahoo.fr> wrote in message
> news:1140737808.283548.99860@j33g2000cwa.googlegroups.com...
> > I use a low end speaker connected to my mother board's AC97-type ADC to
> > input sounds. The problem is that I have a lot of noise.
> >
> > How could I improve the SNR and get rid of a big part of the noise?
> > (I'm not talking here about post-processing but about getting the raw
> > signal from the ADC to a better SNR). Is my low end speaker the problem
> > or is it rather my ADC that's introducing the most noise? In other
> > words do I need a better ADC or a better speaker? Anything about the
> > two that I can do to make it better?
> >
>
> You need to use the full dynamic range of the ADC without clipping of
> course.

umm yeah, good point, but without an amplifier I don't really have the
choice.

Jerry Avins wrote:
> Michel Rouzic wrote:
> > I use a low end speaker connected to my mother board's AC97-type ADC to
> > input sounds. The problem is that I have a lot of noise.
> >
> > How could I improve the SNR and get rid of a big part of the noise?
> > (I'm not talking here about post-processing but about getting the raw
> > signal from the ADC to a better SNR). Is my low end speaker the problem
> > or is it rather my ADC that's introducing the most noise? In other
> > words do I need a better ADC or a better speaker? Anything about the
> > two that I can do to make it better?
>
> What kind of noise? Hum? The fan in the computer? Low-end speaker or
> high, the impedance mismatch to the sound input is very high. Try a
> transformer. And shielding.

The noise is originating somewhere between the speaker and the ADC. It
seems to follow a pattern that is about 1.05 seconds wide. I'm quite
ignorant when it comes to electronics, what is a transformer (is it
those long wires rolled around metalic bars?) and shielding?

Michel Rouzic wrote:
> I use a low end speaker connected to my mother board's AC97-type ADC to
> input sounds. The problem is that I have a lot of noise.
> 
> How could I improve the SNR and get rid of a big part of the noise?
> (I'm not talking here about post-processing but about getting the raw
> signal from the ADC to a better SNR). Is my low end speaker the problem
> or is it rather my ADC that's introducing the most noise? In other
> words do I need a better ADC or a better speaker? Anything about the
> two that I can do to make it better?

Look at the Tascam US-122.  Amazing noise performance for the price 
(<$200).  I've measured Ein of about -127 dBu.


Bob
-- 

"Things should be described as simply as possible, but no simpler."

                                              A. Einstein

"Michel Rouzic" <Michel0528@yahoo.fr> wrote in message
news:1140758561.557678.170880@v46g2000cwv.googlegroups.com...
>
> Dougal McDougal of that Elk wrote:
> > "Michel Rouzic" <Michel0528@yahoo.fr> wrote in message
> > news:1140737808.283548.99860@j33g2000cwa.googlegroups.com...
> > > I use a low end speaker connected to my mother board's AC97-type ADC
to
> > > input sounds. The problem is that I have a lot of noise.
> > >
> > > How could I improve the SNR and get rid of a big part of the noise?
> > > (I'm not talking here about post-processing but about getting the raw
> > > signal from the ADC to a better SNR). Is my low end speaker the
problem
> > > or is it rather my ADC that's introducing the most noise? In other
> > > words do I need a better ADC or a better speaker? Anything about the
> > > two that I can do to make it better?
> > >
> >
> > You need to use the full dynamic range of the ADC without clipping of
> > course.
>
> umm yeah, good point, but without an amplifier I don't really have the
> choice.

You always need a pre-amp!
>

Bob Cain wrote:
> Michel Rouzic wrote:
> > I use a low end speaker connected to my mother board's AC97-type ADC to
> > input sounds. The problem is that I have a lot of noise.
> >
> > How could I improve the SNR and get rid of a big part of the noise?
> > (I'm not talking here about post-processing but about getting the raw
> > signal from the ADC to a better SNR). Is my low end speaker the problem
> > or is it rather my ADC that's introducing the most noise? In other
> > words do I need a better ADC or a better speaker? Anything about the
> > two that I can do to make it better?
>
> Look at the Tascam US-122.  Amazing noise performance for the price
> (<$200).  I've measured Ein of about -127 dBu.

Maybe it's a slightly too big investement. Plus inputting a signal from
a extra-low end speaker with such a device seems to be pretty much like
putting the engines of a Ferrari in an old shitty Fiat. So far my plan
is to get the best of what I already own.

The noise waveform you describe (1.05uS pulse) sounds like digital
interference, which would probably be there regardless of choice of
transducer.

The last time I tried a (el-cheapo) speaker as a mike it was good for
about 20 millivolts p-p.  Your sound card ADC input probably wants
about 2 volts peak to peak.  The 100:1 voltage gain required (40 dB)
which you are presumably doing in software will bring up the level of
the noise by that amount.

Jerry Avins' comment about a transformer is the simplest way to fix
this.  Get a speaker transformer of reasonable quality (minimum size of
1 cubic inch - you'd be surprised at how many things are better when
they are heavier).

The primary winding needs to be rated as having a primary impedance of 5
Kohms or greater, and the secondary at 8 ohms or less.  Now connect the
speaker (mike) to the 8ohm secondary.  Connect the 5K winding to the
ADC input & signal ground.

The transformer is now running "back to front" and giving you a step-up
voltage multiplication of 25 to 1.  This is about 28 dB gain. which
should allow you to wind down your software gain by that amount.

Jim A.

Jim Adamthwaite wrote:
> The noise waveform you describe (1.05uS pulse) sounds like digital
> interference, which would probably be there regardless of choice of
> transducer.

I mesured on a sample of ten to be about 1.05 second long, but I
noticed on other parts of a recording that it can be shorter than that.

> The last time I tried a (el-cheapo) speaker as a mike it was good for
> about 20 millivolts p-p.  Your sound card ADC input probably wants
> about 2 volts peak to peak.  The 100:1 voltage gain required (40 dB)
> which you are presumably doing in software will bring up the level of
> the noise by that amount.
>
> Jerry Avins' comment about a transformer is the simplest way to fix
> this.  Get a speaker transformer of reasonable quality (minimum size of
> 1 cubic inch - you'd be surprised at how many things are better when
> they are heavier).
>
> The primary winding needs to be rated as having a primary impedance of 5
> Kohms or greater, and the secondary at 8 ohms or less.  Now connect the
> speaker (mike) to the 8ohm secondary.  Connect the 5K winding to the
> ADC input & signal ground.

ok thanks alot for the tip. i got alot of transformers hangin around so
i'll test so of them.

> The transformer is now running "back to front" and giving you a step-up
> voltage multiplication of 25 to 1.  This is about 28 dB gain. which
> should allow you to wind down your software gain by that amount.

Just wondering, how do you calculate the voltage multiplication out of
the impedance of each part? Anyways I guess a 230 to 12 volt (or less)
transformer will do it, right?