Low Pass & High Pass Filter

Discussion in 'General Electronics Chat' started by kcneoh, Mar 2, 2010.

  1. kcneoh

    Thread Starter New Member

    Oct 10, 2008
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    Hi ,

    I just want to know in the REAL WORLD , we use low pass filter and high pass filter for what purpose ??

    As I understand the Low Pass Filter is used to reduce the noice level from input , in order to produce less noise output ==> Am I right at this point ??

    How about High Pass Filter ?? We use this for what purpose ??
     
  2. kkazem

    Active Member

    Jul 23, 2009
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    26
    Hi,
    You are partially correct. One use of Low Pass Filters (LPF) is to reduce high-freq noise on signals and this is probably it's most common use, but there are other uses. As far as High Pass Filters (HPF), one common use is to block DC (zero Hz) from a signal; another use is to set the lowest usable input signal for amplifiers, which is often 20 Hz for audio amps. The LPF is also used in the same audio amps to limit the maximum input freq, usually to 20 kHz. Although many audio amps use different max high and min low freq values in their filters, 20 Hz to 20 kHz is the range of audio frequencies that most humans can hear. Of course, as we grow older, say starting around 40 yrs old, most people start to lose the ability to hear high-frequencies. Therefore, most hearing aids boost the high-frequencies to restore a person's hearing back to "normal". This is done with a high-pass or a bandpass filter. A Bandpass filter is a combination of a low-pass and a high-pass filter such that there is a range of frequencies that are passed. There are also bandstop filters, one use of which is to reduce powerline (50 Hz or 60 Hz) humm in an audio amplifier that may be inadvertently picked-up by an electric guitar pickup. For this purpose, it would have a very narrow bandwidth and a very high attenuation at the center frequency. You may ask why one can't simply design the filter to operate only at 60 Hz and the answer is that this is simply impossible mathematically. It has to do with the fact that we don't have access to negative time. Therefore, all filters in the real world have a slope and bandwidth to them. Actually, one can have a "brick-wall" filter, but only on paper due to the limitation I mentioned above. There is another class of filters that you may not know about; called all-pass filters. These do not attenuate a signal's frequencies, but instead, introduce a delay which changes with frequency. I hope that answers your question. Feel free to ask other questions if you wish.

    Regards,
    Kamran Kazem
     
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  3. Audioguru

    New Member

    Dec 20, 2007
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    Telephones were invented when the transducers had horrible resonance and distortion above 3kHz so they designed the telephone system to cut all important high frequency speech consonants sounds (What did you say, what did you say? Sunny, money, funny and bunny all sound the same). They also cut all low speech frequencies to make the telephones sound "tinny".
     
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  4. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    On a very "Barney-style" level:
    Low-pass filters block AC signals, but pass DC levels.
    High-pass filters block DC levels, but pass AC signals.

    An example of a low-pass filter would be one in a DC power supply. The AC signal is mostly blocked out; and at the output you see a regulated supply.

    An example of a high-pass filter might be a cable TV box. Any DC level from the cable is blocked at the input of the box; only the RF signals get in.

    You can combine high-pass and low-pass filters to create band-pass filters; any AC input signal below a certain frequency is reduced in amplitude, and any AC input signal above another frequency is reduced in amplitude.

    LC filters are quite common. If you have a stereo system, it has capacitors and inductors that separate the low frequencies from the high frequencies, and route them to the appropriate speakers.

    Crystal filters are also quite common; they typically have a very narrow passband. The crystals are designed to resonate in a very narrow range of frequencies.
     
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  5. kcneoh

    Thread Starter New Member

    Oct 10, 2008
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    Hi ,
    Thanks for all reply and explanation.

    Additional question : For Low Pass Filter , how useful to learn to know the "cutoff frequency" in the REAL WORLD . In the other words , in the REAL WORLD , how we can apply this "cutoff frequency " .

    Thanks.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    The cutoff frequency of a filter is extremely important. Its rate of slope is also important.

    Remember vinyl records? They produced a sound effect called "wow". The amplifier amplified the low frequency wow and caused the speaker to vibrate wildly at a very low frequency. Then the other frequencies became messed up.

    You can hear down to 20Hz. Wow was at about 2Hz. So the cutoff frequency of the amplifier was selected to reduce wow frequencies and pass low audio frequencies.

    You can hear as high as 20kHz (if you are not deafened by guns, motorcycles and acid rock noise). If you are using a noisy old LM324 opamp as a preamp then it produces lots of hiss. If you filter out frequencies above 1kHz (using a cutoff frequency of 1kHz) then the hiss will be gone. But also most normal audio frequencies will be gone. Then you must decide on a compromise cutoff frequency.
     
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  7. kcneoh

    Thread Starter New Member

    Oct 10, 2008
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    AudioGuru,

    Can I summarize as below :-

    In most audio industry , Loss Pass Filter is used to limit the freq cannot go beyond 20HZ .

    But for "WoW" case , it limit the freq cannot go beyond 2Hz and in Telephone industry , it limit the freq cannot go beyond 3Hz because once above 3Hz will generate serious resonance and distortion

    And if the cutoff freq is set to 2HZ , that's means it block freq beyond 2HZ to pass through. So, "cutoff frequency" is a maximum limit for Low Pass Filter to allow frequency to pass through .

    Please correct me if I misunderstand on certain point .
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    No.
    A lowpass filter cuts high frequencies and passes low frequencies. LOWPASS.
    You want a highpass filter that cuts frequencies below 20Hz and passes frequencies above 20Hz. HIGHPASS.

    No.
    You want a highpass cutoff frequency of 20Hz (not 2Hz) so that the WOW at 2Hz is very much reduced and the audio frequencies above 20Hz are passed.

    No.
    Telephones have a 300Hz highpass filter, not 3Hz.

    No.
    A filter reduces frequencies beyond its cutoff frequency, it does not block them. A filter can have a gradual slope or a steep slope.
    At the cutoff frequency the signal voltage is reduced to 0.707 times which is half power.
     
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  9. kcneoh

    Thread Starter New Member

    Oct 10, 2008
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    AudioGuru,

    If the low pass filter's cutoff frequency is set to 2Hz , then this filter only can let 2Hz or below frequency to pass through this filter ??

    If the high pass filter's cutoff frequency is set to 300Hz , then , this filter only can let 300Hz or more to pass through this filter ??
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Yes.
    It is lowpass so it passes low frequencies.

    Yes.
    It is highpass so it passes only high frequencies.

    Filters do not block frequencies. They attenuate frequencies.
    A 3kHz 2nd-order Butterworth lowpass filter is -3dB at 3kHz, -6dB at 6kHz and -12dB at 24kHz. At 48kHz it still has an output at -18dB which is 1/8th the original amplitude.
     
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  11. Audioguru

    New Member

    Dec 20, 2007
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    At work I "souped-up" teleconferencing sound systems. Usually the high audio frequencies were muffled so I measured the 3kHz loss transmitting on one telephone line and receiving on another telephone line. The loss was 12dB!
    I complained to Bell Technical Support and they said it was normal and within their spec of -7.5db for each direction from the central office.
    From then on I added 3kHz pre-emphasis to the tele-conferencing systems that made speech sound crisp and clear.
     
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