From: Jens Groh ()
Date: May 8, 2008
Subject:
Re: FAQ is BAQ! - frequency shifting.
One thing about Weaver's trick that could at least be called elegant
is that it works with IIR, not only FIR filters.
They are pretty efficient. (Well, probably not too too much better
compared to FFT-based FIR in terms of "decibels signal-to-alias ratio
per multiply-accumulate operation"...)
They have low latency compared to FIR filters (unless you want to pay
for that low latency convolution patent...).
The phase respone is arbitrary in this case, and the passband ripple
is uncritical, in contrast to the Hilbert transformer.
Apart from that, I would say the Hilbert transformer method is
actually the simpler one. It is much easier to understand.
Still I'm happy that I seem to be the first one who translated that
old tricky ham radio geek circuit to the DSP audio world and that a
couple of people seem to like it.
In fact I had only remembered how it works and the name Weaver and it
was very hard to find the reference at that time. Today it's easy --
here you are: http://www.h4.dion.ne.jp/~ja5fp/weaver.pdf. (Isn't that
tube circuit cool? :-) )
Jens Groh (author of that Csound Mag article)
Am 08.05.2008 um 19:10 schrieb robert bristow-johnson:
>
>> ----- 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
>>
>>
>>
>> Great Andy, thanks! It looks so much better.
>>
>>
>> I've also cleaned up the structure a bit so that the dsp FAQs are
>> on the main
>> page and the admin FAQs are just one of the sections.
>>
>> http://music.columbia.edu/cmc/music-dsp/musicdspFAQ.html
>>
>>
>> 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 regarding "frequency shifting". in FAQ #19 (the numbering
> doesn't quite correspond with the table of contents at the top, but
> that's really small potatoes) "How do I do pitch shifting?", it says
> regarding "frequency shifting":
>
> "Frequency shifting is actually an older technology from the analog
> days, and 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 process (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]
> efficient, 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 real-time Hilbert transformers) involves a single
> "filter" (the Hilbert transformer, i won't count the corresponding
> delay line as another filter) and two sine/cosine multiply
> operations. a quick Google search finds this link to 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.
>
> --
> [EMAIL REMOVED] >
> "Imagination is more important than knowledge."
>
> --
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