Philipp,
Also check out Victor’s paper in the Journal of the Audio Engineering Society
on single side band FM.
J. Audio Eng. Soc., Vol. 56, No. 9, 2008 September,… it’s the one I spotted an AES typo. They enter this material by hand. Equation had a typo, page 686, in eq19 of the Taylor series. Should be cosx = 1 +… ,
not cosx = 1 = …
Victor had it correct, AES journal copied it incorrectly.
Love this paper on FM.
-Partev
Thanks Partev for the idea!
Sounds like a good solution!
Also, I saw mention of wanting a steeper roll-off rate. As I recall, many Csound
filters are second-order filters, would typically have 6dB/oct roll-off. Cascading
a filter into another adds an emphasis prior to cut-off, and a steeper roll-off rate.
I’ve cascaded filters that way and attained 18-24dB/oct roll-off rates. I was using
that when doing FM and wanted to filter select groups of side-bands. Also used
that method on a single-side FM from an AES paper Victor did,… which I loved.
Did straight as in the paper, then experimented on it with my 4th and 8th order filters.
Two audio paths, one was upper, the other lower side band, and used 4th and 8th
order cascaded filters on the different parts the the upper and lower side-bands.
Interesting two channel effects in the headphones.
If you find that useful, it could probably do something similar in AM.
-Partev
… you can then listen to the aSum or aDiff outputs, each containing the separate sideband
man. 11. nov. 2024 kl. 08:56 skrev Oeyvind Brandtsegg <obrandts@gmail.com>:
Hi Philipp
Maybe you’d like to try single sideband modulation? There, you can get the upper and lower sidebands as separate outputs.
Like this:
aCar oscili 1, icps ; replace this with your recording
; single sideband modulation
aSin, aCos hilbert aCar
aModSin oscili 1, imodFreq, giSine, 0.0
aModCos oscili 1, imodFreq, giSine, 0.25
aMod1 = aSin * aModCos
aMod2 = aCos * aModSin
aSum = aMod1 + aMod2
aDiff = aMod1 - aMod2
all best
Øyvind
man. 11. nov. 2024 kl. 08:21 skrev Philipp Neumann <0000119f78f3a4f9-dmarc-request@listserv.heanet.ie>:
Thank you, Victor, for your response!
I definitely meant LP; I’m not sure how that mistake slipped in.
I’m not certain if downsampling is actually necessary in my case, so perhaps it would help to explain my idea in more detail:
I’ve recorded some audio files containing ultrasonic frequencies. Now, I’d like to remove all audible frequencies with a very steep high-pass filter. After this, I plan to apply amplitude modulation to make the ultrasonic content audible, without changing the playback speed or pitch of the sound — only through the lower sideband generated by the AM.
Since AM also produces a higher sideband, I’d like to filter this out to prevent any aliasing.
So, I think I need both a very steep low-pass and high-pass filter.
Any thoughts?
First of all, I am not sure why you want a HP filter, since aliasing would happen above a certain frequency not
below. I assume you are talking about a LP filter instead.
Having said that, it only makes sense to use an LP filter if you are downsampling the signal at some point,
and for that you can try designing a linear phase FIR with the desired curve. GEN 53 can do this for you
https://csound.com/manual/GEN53.html
and implement it with ftconv.
If however you are not downsampling, LP filtering is not going to help you remove any aliasing.
Prof. Victor Lazzarini
Maynooth University
Ireland
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hello everybody!
I’m searching a way to avoid producing alias frequencies.
I’m working with ultrasonic sounds and do some AM on it. So i need a way to filter out the higher sidebands
I thought about using a high-pass filter with a very steep slope. but the filter i found are not that steep.
do someone has some recommendations for this?
Greetings,
Philipp
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