From: robert bristow-johnson ()
Date: May 8, 2008
Subject:
Re: FAQ is BAQ! - frequency shifting.
> ----- Original Message -----
> From: "douglas repetto"
> To: "A discussion list for music-related DSP"
> Subject: Re: [music-dsp] FAQ is BAQ!
> Date: Thu, 08 May 2008 12:32:00 -0400
>=20
>=20
>=20
> Great Andy, thanks! It looks so much better.
>=20
>=20
> I've also cleaned up the structure a bit so that the dsp FAQs are on the =
main=20
> page and the admin FAQs are just one of the sections.
>=20
> http://music.columbia.edu/cmc/music-dsp/musicdspFAQ.html
>=20
>=20
> music-dsp members, please feel free to send in update/comments/additions.
i have to take a little exception to a wee part of the technical content re=
garding "frequency shifting". in FAQ #19 (the numbering doesn't quite corr=
espond with the table of contents at the top, but that's really small potat=
oes) "How do I do pitch shifting?", it says regarding "frequency shifting":
"Frequency shifting is actually an older technology from the analog days, a=
nd is also somewhat simpler than pitch shifting. Nonetheless there are some=
things to watch out for. When done with ring modulation as part of the pro=
cess (most methods do this), you must filter carefully to prevent aliasing."
so far this is fine. then,
"The best (simple efficient clean) current method is Weaver modulation; see=
this link: http://www.csounds.com/ezine/summer2000/processing/"
there is a technical value judgement here ("best, simple[st], [most] effici=
ent, clean[est]") that might not be universally held. another method that =
is current (even more current because in Weaver's day, they didn't have rea=
l-time Hilbert transformers) involves a single "filter" (the Hilbert transf=
ormer, i won't count the corresponding delay line as another filter) and tw=
o sine/cosine multiply operations. a quick Google search finds this link t=
o illustrate:
http://www56.homepage.villanova.edu/scott.sawyer/fpga/II_frequency_shifting=
.htm
i s'pose, if you want, i could code up a simple C-code snippet, but someone=
else would have to build something around it to test it.
just a couple of $0.01 tossed in for your consideration.
--
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"Imagination is more important than knowledge."
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