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2001\09\24@224039 by

Hello All,

I have a project that I want to do in a pic but
want to make sure it hasn't been done before.
I really hate re-inventing the wheel.

Anyway,

What the circuit will do is have a speaker connected
in a test setup and pass a range of frequencies to the
speaker testing the impedance at each frequency. I would
like to have the pic connected to a computer running
a basic program for graphing the results. The results will
later be used to calculate the enclosure size for the speaker.

Questions

I know I can use a pic with an A/D for measuring a voltage
but how would you have a pic generate a frequency?

The frequencies needed are in the audio range from 1hz to about
20khz.

If it hasn't been done already I have a schematic of the manual
test setup using a freq generator and dvm to do the test manually.
I could scan and send the schematic if anyone is interested.

Any help would be great,
Steve
//kcerhome.com

--

> I know I can use a pic with an A/D for measuring a voltage
> but how would you have a pic generate a frequency?

Use a timer interrupt. There is code at http://www.piclist.com for
setting the interrupt period. If you can generate a triangle wave
with a D/A, this circuit

http://home.clear.net.nz/pages/joecolquitt/tri2sine.html

will convert it to a pretty good sine wave. You can send this to
the audio amp to drive the speaker

I hoped to follow up on using a PIC pin a voltage source, which
would have been handy for the +/-12 and -5 needed (in this and
other analogue circuits), but perhaps you can derive them from

> The frequencies needed are in the audio range from 1hz to about
> 20khz.

20kHz is OK, that's 250 instructions/hertz (at 20MHz) to approximate
a triangle wave

> If it hasn't been done already I have a schematic of the manual
> test setup using a freq generator and dvm to do the test manually.
> I could scan and send the schematic if anyone is interested

You could host it on your own page at PIClist, one of the admin

--

I'm no PIC expert (yet), but wanted to point out that
the "audio" range is typically specified as 20 Hz to
20 kHz.  No need to generate quite as low a frequency
as you mentioned (I know, not much difference, but it
could save you some time/programming).

I've done my share of speaker testing on Audio
Precision and LMS.  It could be interesting to see how
a PIC-based measurement system turns-out.   :)

Good luck,

ATM

--- Kasm Consulting And Electronic Repairs
<kcerHOME.COM> wrote:
{Quote hidden}

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Your best bet would be the MAX038 sine wave generator chip.
It has a bandwidth of 0.1Hz to 20MHz.  Its frequency can be
changed by changing a voltage.  You can easily interface
a DAC to digitally control the frequency.

Go to http://www.maxim.com and check out the spec sheet.

Ben

{Original Message removed}
Steve,
No comment on the pic part of your project, but are you sure you want to
calculate box size from the freq/impedance data? Are you aware that what you
need to measure are things like Qc, Qt, Fs etc which is done at specific
frequency points. No offense meant!

Jan Didden

{Original Message removed}
>I know I can use a pic with an A/D for measuring a voltage
>but how would you have a pic generate a frequency?

Steve,

http://www.fairchildsemi.com/pf/ML/ML2036.html

More than once, I've thought about using this chip for another type of
audio measuring instrument (non-acoustic). I would be pleased to know how
you get along with it, if you decide to give it a shot...

Mike Hardwick

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Steve, take a look at Fairchild's (formerly Micro Linear's) ML2036
Programmable Sinewave Generator. I've been looking into this as part of my
PIC-based DAQ system. I'm currently testing the ML2036, MAX038, and looking
at Analog Devices' DDS family.

For your application, the ML2036 seems ideal. All you need is a crystal
(literally, no capacitors). It requires +/- 5V and has a 3-wire SPI-style
interface. You clock-in 16 Bits, LSB first, and set a Latch Enable line to
set the frequency from 1 to 50KHz (I tested it to 65,535KHz). By using a
8.388608MHz crystal (Digi-Key PN: SE3415 / Epson CA-301 family), you get a
direct one-to-one program data to frequency setting with a 1Hz resolution.
The chip provides Osc/2 and Osc/8 outputs which may be of use if you want
to clock a PIC at 4.194304MHz.

There is a voltage reference input. Tying this to +5V gives a maximum
output swing of +/- 2.5Vp-p. You can reduce this via an external reference.
Harmonic distortion is -45db max. The device comes in a 14-pin DIP or 16-pin

http://www.fairchildsemi.com/pf/ML/ML2036.html

- Tom

At 07:18 25-09-01 -0500, Steve wrote:
{Quote hidden}

------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

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Jan,

No offense taken at all.
I'am aware of the Qc,Qt,Fs figures.
The optimum way to get these figures for a given
speaker pair (For Matching) would be to take an
actual impedance reading over a range of freqs and then
calculate from the graph produced. There is quite a bit
of variance amoung speakers, even if the same model.

Steve.

{Original Message removed}
On 26 Sep 2001 21:28:33 -0400, Kasm Consulting wrote:
> Jan,
>
> No offense taken at all.
> I'am aware of the Qc,Qt,Fs figures.
> The optimum way to get these figures for a given
> speaker pair (For Matching) would be to take an
> actual impedance reading over a range of freqs and then
> calculate from the graph produced. There is quite a bit
> of variance amoung speakers, even if the same model.
>

Not that this particularly my field (he hides behind his CS academia!)
but the way a colleague and I have done this in the past is to use an
impulse noise spike (a maximum length sequence) and then capture this
(with a calibrated mic) and do the FFT...

We have an old bit of kit called the "IMP" which plugs into a PC
parallel port which does all the noise/capture and then you can do your
own thing and get all the Q's and impedance curves and response curves
too...

Just my \$0.02
/Kevin
--
Dept. of Computer Science   Ph : +61 8 8303 5845
University of Adelaide      Fax: +61 8 8303 4366
SOUTH AUSTRALIA

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Hello,

> There are several software packages around that makes up an audio testing
> center around a sound card.

I've been using it for some time. The programs I use are "Sonic
Foundry's Sound Forge" and "Sound Technology's SpectraLAB".

I've heard ElectroVoice have a good audio testing software. Have you
tried it? Any other suggestion?

Thanks,

Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2138
Blumenau  -  SC  -  Brazil
http://www.citronics.com.br
-----------------------------------

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