Analog or digital lowpass filter???

Discussion in 'The Projects Forum' started by mr_computer, Mar 20, 2009.

  1. mr_computer

    Thread Starter New Member

    Feb 12, 2009
    7
    0
    Hello
    I'am working on a speech recognition project to be implemented on FPGA (spartan 3E starter kit).

    Now, I'am trying to record a 8KHz speech signal using the kit's ADC (it contains AMP and ADC) via an electret microphone.

    My question is about the band limiting of the speech signal (LPF with fc=8KHz) implementation !!!

    What is the best implementation of that filter ??? in analog domain? befor amplification (but i'am worry about noise because the Mic. output is very low since i'am limited by the kit's Vcc (3.3v) for powering the Mic.) or in digital domain? to capture the the frequencies(22KHz) then apply digital filtering with decimation to extract the 8KHz signal since i have a sufficient area in FPGA chip.

    So, what is your suggestions ?
    Thanks.
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Tell us about the amplifier that comes with the kit. Specification? Circuit?

    It's probably good practice to amplify and band limit the microphone signal before ADC is done.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
    2,654
    632
    Try it without band limiting. You might get a little aliasing, but maybe the sound quality will be fine. You never know; the microphone itself might limit your bandwidth sufficiently.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    It depends on why you want to band limit the signal. If it is to avoid aliasing, it must be an analog filter, preceding the A/D converter. Once aliasing has occurred, no amount of filtering, analog or digital, can remove the resulting unwanted spectral components.
     
  5. mr_computer

    Thread Starter New Member

    Feb 12, 2009
    7
    0
    thanks for your replying

    well,The Spartan​
    ®-3E FPGA Starter Kit board includes a two-channel analog capture circuit, consisting of a LTC6912 Dual Programmable Gain Amplifiers with Serial Digital Interface,below its data sheet link :
    http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1121,P7596,D5359

    and an analog-to-digital converter LTC1407A-1 Serial 14-bit Simultaneous Sampling ADCs with Shutdown, below its data sheet link :​
    http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1001,C1158,P2420,D1295

    Thanks.

     
  6. mr_computer

    Thread Starter New Member

    Feb 12, 2009
    7
    0
    Dear Ron H,
    thanks for replying

    the purpose of band limiting is to avoide alising and also to reduce the amount of memory required to store the signal.
    Thanks
     
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