Electronics > Beginners
Triangle wave and low-pass filter (help needed with experiment!!)
13hm13:
I currently don't have a lab or equipment to conduct an experiment, so if any of you EEVbloggers can help, muchos gracias in advance!!
What's needed:
21khz (fundamental freq.) triangle wave
-->
filter (LPF brickwall, that begins steep rolloff at 22khz)
-->
spectrum analyzer (screen capture spectrum, post in this thread)
Put the spectrum analyzer probe on the output of the filter, and post the screen capture.
(And if you have an o'scope convenient, please screen capture that as well. I want to see what's left of that triangle wave!)
Highly recommend (but not required):
The triangle wave and filter be in the analog domain (old school).
What's this experiment for:
Determining behavior of digital audio signals. If I get some help, I'll post more detail.
Thanks again!
ogden:
Do your "experiment" yourself at 1KHz - using PC audio output/input and Audacity or similar software (with triangle wave generation and FFT functions).
T3sl4co1l:
Are you expecting something other than a sine wave?
Keep in mind that a "brick wall" filter is nonphysical, or equivalently, takes ~the age of the universe to start up. Practical filters have nonzero transmission in the cutoff band*, but can be made sharp enough that the first nonzero harmonic of the triangle wave (the 3rd harmonic, as it happens... as if that wording wasn't confusing :P ) is below detection thresholds.
*(except at finitely many points)
If this is in response to an observation --
Keep in mind that, say, sound cards aren't brick wall filtered, partly for the above reason but partly also because of cost. It's cheaper to, say, oversample the audio from 44.1kHz, up to say 192kHz or more, interpolating between samples according to a digitally synthesized filter profile. The subsequent analog filter then can be quite modest. All the harmonics and aliasing in the 22.05 to 96kHz range can be reduced arbitrarily in digital, so that the analog filter only needs to go from passing at 22.05kHz to stopping at (192 - 22.05)kHz -- a far easier problem to solve.
Or maybe they don't filter exactly like that, because of various practical and laziness reasons. In short, the waveform may look goofy as hell, but it doesn't matter to them because no one can hear much over 20kHz and there is no requirement for phase correctness. Typically you'll see a lot of ringing on sharp edges/corners, more exaggerated on square waves (of course) but still apparent on triangle waves, too. Strictly speaking, this is not distortion, but still a linear process; the catch is, "linear" applies to the amplitudes, not the wave shape, so what you see on the oscilloscope may be quite odd.
Tim
RoGeorge:
https://www.google.com/search?q=triangle+wave+spectrum
ogden:
--- Quote from: RoGeorge on October 30, 2018, 04:03:23 pm ---https://www.google.com/search?q=triangle+wave+spectrum
--- End quote ---
Spectrum of 21KHz triangle wave filtered with 22KHz lowpass filter will not look like "triangle+wave+spectrum".
Actually LTSpice software is very good free(!) tool to do said experiment
Navigation
[0] Message Index
[#] Next page
Go to full version