[sdiy] removing dc offset

[Scott Gravenhorst]

That sounds like the offset is not constant, that it rises to some level at
the start of a sound and falls back to another level when the sound is
finished.  It may be the result of a very simple/inexpensive VCA and the
associated thump that goes with it.  A VCA like a single ended FET VCA might
do this.

"gregory zifcak" <zifcak at hotmail.com> wrote:
>i see. so maybe what i'm feeling in the speaker cone is just the edges of 
>the transition from zero to the dc offset? i get a thump at the start of a 
>sound and a thump at the end. now i'm guessing that the capacitor passes the 
>initial rising edge, then brings it back to zero, then passes the falling 
>edge, and back to zero. the resulting pulses are filtered out substantially 
>when i raise the filter frequency to about 300 hz.
>hmm,
>greg
>
>>
>>What happens if you take the cutoff below 300 Hz?
>>
>>If the DC offset is constant, it should be eliminated by _any_ size
>>capacitor.  It's just that smaller ones will have a higher cutoff frequency
>>and will attenuate bass more as the capacitor is decreased in size.  I 
>>don't
>>believe the problem is the filter slope.
>>
>>
>>"gregory zifcak" <zifcak at hotmail.com> wrote:
>> >this is what i was doing, but i have to use a cutoff frequency of about 
>>300
>> >hz to substantially reduce the dc. i'm assuming this is because the 6db
>> >slope is just not steep enough.
>> >i guess that because the toy originally used a small internal speaker, 
>>they
>> >didn't need to worry about the dc. i've added an output and i'm worried
>> >about damaging speakers with the thump.
>> >>
>> >>Are you using a capacitor in series with the output as a DC blocking
>> >>capacitor?  It needs to be larger for lower frequencies to pass through
>> >>it.  It's really a simple highpass filter, so the larger the cap, the
>> >>lower the cutoff.
>> >>
>> >> >--- gregory zifcak <zifcak at hotmail.com> wrote:
>> >> >
>> >> >> hi,
>> >> >> i am modifying a toy that has very rich bass, but due to the fact
>> >> >> that it is
>> >> >> run from a single battery supply, there is a dc offset. i've tried
>> >> >> filtering
>> >> >> this with caps of various sizes, but i can't seem to get rid of the
>> >> >> offset
>> >> >> without getting rid of the bass. is there a simple way to do this?
>> >> >> thanks for any suggestions,
>> >> >> greg
>> >> >>
>> >> >>
>> >> >>
>> >> >
>> >> >
>> >> >
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>> >>---------------------------------------------------------
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>> >>
>> >>-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
>> >>-- Linux Rex         | RedWebMail by RedStarWare
>> >>-- FatMan: home1.gte.net/res0658s/fatman/
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>> >>-- Autodidactic Master of Arcane and Hidden Knowledge.
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>> >
>> >
>>
>>---------------------------------------------------------
>>- Where merit is not rewarded, excellence fades.
>>- Hydrogen is pointless without solar.
>>- What good are laws that only lawyers understand?
>>- The media's credibility should always be questioned.
>>- The only good terrorist is a dead terrorist.
>>- Governments do nothing well, save collect taxes.
>>
>>-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
>>-- Linux Rex         | RedWebMail by RedStarWare
>>-- FatMan: home1.gte.net/res0658s/fatman/
>>-- NonFatMan: home1.gte.net/res0658s/electronics/
>>-- Autodidactic Master of Arcane and Hidden Knowledge.
>>
>
>

---------------------------------------------------------
- Where merit is not rewarded, excellence fades.
- Hydrogen is pointless without solar.
- What good are laws that only lawyers understand?
- The media's credibility should always be questioned.
- The only good terrorist is a dead terrorist.
- Governments do nothing well, save collect taxes.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- Autodidactic Master of Arcane and Hidden Knowledge.