Dieter wrote:
[ could you post in plain text instead of quoted-printable please? ]
We=20can=20just=20use=20a=20wide=20BPF,=20or=20even=20use=20the=
=20aforementioned=20brick=20wall=20filter
our=20ears=20have=20(if=20the=20frequency=20is=20high=20enough,=
=20which=20it=20should=20be=20with=20192Khz
audio).
You don't want to use up amplifier power and tweeter power handling
on 192 KHz stuff that the tweeter can't reproduce and we can't hear
anyway.
Yes, class D amps have a 2nd order output filter to reduce EMI and to
protect the speakers.
I haven't read the rationale (if there is one) behind 192 KHz but
I assume the idea is to raise the frequency enough that the
side effects from the LPF are above the audible range.
Actually, if you use a Bessel filter alignment for the output filter
with F0 the proper multiple of 20KHz -- SQRT(3) IIRC, there are minimal
effects although pre-emphasis with a shelf filter centered at 20K is a
good idea. The issue with the filter is in the cutoff. The cutoff
attenuation should reach -138.5 dB before it reaches 1/2 the carrier
frequency of the amp -- at least in theory. I read this in various
papers, but what does it matter since we can't hear it!
IIUC, a lot of amps use a higher carrier frequency than 44.1KHz or
48KHz. I don't really understand this. It reduces the efficiency of
the amp and increases the distortion caused by the finite rise and fall
times of the output transistors.
There is no way to digitally drive an output stage with the much
information (24bits @ 192Ks/s). So, the only advantage is that you have
more information to use in a digital front end (volume, tone, equalizer,
loudness, etc.), a digital filter, and predistortion. And, there I do
see an advantage, 44.1KHz needs to be oversampled to use the digital
equivalent of a control preamp.
--
JRT
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