Band-reject filter approach

Discussion in 'General Electronics Chat' started by luigi.maroni, Feb 16, 2013.

  1. luigi.maroni

    Thread Starter New Member

    Feb 16, 2013
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    Hi there,
    I have constructed a project which has two audio sources and creates a final stereo mix-down from them.
    Along the way, one of the sources goes through a mid-frequency boost, and the other through a mid-frequency 'scoop'.

    This is how I've done it and I'd really appreciate feedback, because this is my own idea rather than anything I've read.

    The mid boost is a multiple feedback band pass centred at about 300 Hz. The output of that filter is then mixed with the 'dry' unfiltered signal via a virtual earth summing op amp. I'm happy that the phase-variances don't upset the sound.

    The mid scoop I do by passing the second channel dry signal through the SAME multiple feedback band pass circult but then SUBTRACTING that from the dry signal with an op amp. I do this because I want the mid-scoop to have the same shape as the mid-boost on the other channel.
    In a way I'm surprised I haven't seen this approach, as it seems 'obvious'. But most often things are done a certain way for very good reasons that just haven't occured to me.

    In short... any issues with doing my mid-scoop (band-reject) as I've described?

    Thanks in anticipation.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    It sounds like it should work. You likely haven't seen that approach because few designs need such a "scoop" response. What's the purpose of that in your project?
     
  3. luigi.maroni

    Thread Starter New Member

    Feb 16, 2013
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    1
    In actual fact, it's a part of a product prototype. This bit of it is to take in a mic'd up instrument input and boost it's main dominant frequencies. Then the other signal is ambient stage / crowd noice which I blend but with a matching mid-scoop. It creates a frequency separation to allow the musician a good, high clarity personal feedback. There's also user-configurable stereo feed widening and various other hifi processing parameters.

    So yeah the idea is to create a frequency gap in the ambients which cause the main instrument to cut through.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Sounds interesting. But I don't see why the musicians would want to hear amplified ambient stage/crowd noise of any type. :confused:
     
    Last edited: Feb 17, 2013
  5. luigi.maroni

    Thread Starter New Member

    Feb 16, 2013
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    Playing in a live setting it's critical to hear what your fellow band members are playing. Also, the full sound of an amplified instrument is coloured by the acoustics of the environment. Reverberation and other more subtle effects. This need to be heard to a certain extent.

    The desired approach is to alter then balance of these sounds in favour of allowing the player to hear him/herself much better, not to provide complete sound isolation.
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Yes, very interesting. Would this be used with an in-ear monitor system ?
     
  7. luigi.maroni

    Thread Starter New Member

    Feb 16, 2013
    7
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    Yes exactly. The prototype is finished now. I have:
    1. An instrument mic input socket
    2. A built-in electret mic
    3. A mono auxiliary input socket

    Each have their separate gain controls, and then 1 & 2 are mixed together using a virtual earth op amp configuration.
    Combined 1 & 2 then go through a controllable mid-scoop centered around 320 Hz
    The guitar goes through a controllable mid-boost centered at the same frequency.
    Both then go through independent treble cut controls which are identical 2nd order low passes with a cutoff of about 3KHz

    The two channels (the aux/ambient mix and the guitar) then go through a DSP stage which turns them into a stereo output with a controllable stereo field effect. At one extreme setting the guitar is very wide and the aux/ambient is less wide. At the other extreme the aux/ambient is very wide and the guitar less wide. In the middle there is no stereo field effect.

    Headphone amplifier volume control and that's it.

    Why all that effort?! To get tonal and spatial separation and a better mix of instrument vs. environmental accoustics (which often are awful). To make you sound awesome to yourself to make you play better.

    Seems to work pretty well.
     
    #12 likes this.
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Thank you so much for sharing the details with us.
    Very unique approach.
    It appears that this lets the musician have much more personal control of their instrument in the monitor mix than is normally available.
     
  9. #12

    Expert

    Nov 30, 2010
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    Thank you. This is the first novel approach to sound mixing I have heard about in years.
     
  10. luigi.maroni

    Thread Starter New Member

    Feb 16, 2013
    7
    1
    Spotted typo. 2 & 3 are mixed together. Not 1 & 2
     
  11. luigi.maroni

    Thread Starter New Member

    Feb 16, 2013
    7
    1
    Exactly so. And thank you.
     
  12. luigi.maroni

    Thread Starter New Member

    Feb 16, 2013
    7
    1
    Thanks for the kind words. Once I've done some more field trials of my hand-built prototype I will seek crowd-funding to take it to a small-scale production run.
     
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