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A simple audio-filter

Discussion in 'Electronic Design' started by Arild P., Jun 20, 2006.

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  1. Arild P.

    Arild P. Guest

    I need to find a way to build a simple audio filter.
    It's for an electronic talking toy which uses a piezo-speaker. I've
    removed the speaker and tried to make a line-out instead, but the sound
    is pretty "gritty" and "dirty". I believe I can make it better with
    some adjustments (filtering)
    This is what I've done so far:


    220K 10nF
    ----------------->---------------o---xxxx----o o--||----->
    | |
    50K x |
    audio Audio x<----| line
    out Taper x out
    |
    ----------------->-------------------------------o----------------->

    I've experimented with different capacitors. If I lower the value (say
    6 nF) the bass goes away (which includes some low frequency "rumble")
    and I get more treble. If I insert a capacitor with a higher value (say
    40 nF) the opposite happens.
    I would like to remove the annoying "rumble" and "dirty" part of the
    sound, but without removing all of the bass in the process. How do I do
    this in the simplest possible way?
     
  2. Rich Grise

    Rich Grise Guest

    The simplest possible way is to get a different transducer. Is it really
    a "piezo speaker", or is it merely a piezo sounder - those are usually
    used at their resonant frequency, and the frequency response is crap.

    If you're using a "real" piezo "speaker", then you have a tweeter and
    you're never going to get decent bass out of it anyway.

    Good Luck!
    Rich
     
  3. Arild P.

    Arild P. Guest

    It occured to me that explaining audio with words isn't the way to do
    it, so I've uploaded some MP3 examples:

    First, here's the type of sound I want to end up with (I'm using a
    similar toy here which does indeed have a headphone output):
    http://home.c2i.net/w-460389/audio_filtering/reference.mp3

    But here's the toy I'm working on, trying to add a suitable
    line-output, having removed the piezo-ceramic speaker from its audio
    output and added my line-out circuit, but without any capacitor:
    http://home.c2i.net/w-460389/audio_filtering/dirty.mp3
    Notice the very "harsh" and "dirty" sound compared to my reference.

    Here's the same toy with a 10nF capacitor which filters away a lot of
    the "harshness", but also removes more:
    http://home.c2i.net/w-460389/audio_filtering/cleaner_10nF.mp3


    To make it easier to compare, here are the two toys saying the same
    word.
    First the "reference" toy:
    http://home.c2i.net/w-460389/audio_filtering/reference_scratch.mp3

    Now the other toy:
    http://home.c2i.net/w-460389/audio_filtering/dirty_scratch.mp3


    My question is, how do I make the "dirty sounding" toy sound more or
    less like the "clean" one?
     
  4. Arild P.

    Arild P. Guest

    I'm not sure what it's called. Piezo something.
    I've googled for an image which I've found here, which looks similar to
    what my toy uses:
    http://www.mcld.co.uk/oddmu/tincanbass/piezotransducer.jpg

    The speaker/transducer/sounder or whatever is marked:

    Murata VSB41D25-07AR

    I couldn't find any specifications online. I'm sure it's long since
    been discontinued since the toy is from the 80s.

    The problem is that I seem to actually be getting too much bass out of
    it!
    I've just made and uploaded some MP3 files which will explain a lot
    more along with a new posting.
     
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