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#1
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Skewed waveforms
What does it mean when the waveform of a sound is essentially symmetrical,
except that the positive excursions of the wave are bigger than the negative excursions, so that it looks a bit lopsided? And what does it mean when a wave form looks as though it has been raised or lowered as a whole, so that what would have been the zero crossings are now above or below zero? I've seen both occasionally but I haven't been able to determine what circumstances cause them. Someone here said something about trombones not producing symmetrical waveforms, which reminded me of these anomalies. How or why would a particular instrument produce a waveform that isn't symmetrical around zero? |
#2
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Skewed waveforms
On Tue, 24 Jan 2012 17:33:29 +0100, Mxsmanic
wrote: What does it mean when the waveform of a sound is essentially symmetrical, except that the positive excursions of the wave are bigger than the negative excursions, so that it looks a bit lopsided? And what does it mean when a wave form looks as though it has been raised or lowered as a whole, so that what would have been the zero crossings are now above or below zero? I've seen both occasionally but I haven't been able to determine what circumstances cause them. Someone here said something about trombones not producing symmetrical waveforms, which reminded me of these anomalies. How or why would a particular instrument produce a waveform that isn't symmetrical around zero? That's just how waveforms are. You can, if you fancy whiling away a few hours, think about maybe the way reeds work, or the embouchure on a mouthpiece but none of that will really get you very far. It's just how real life is. d |
#3
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Skewed waveforms
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#4
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Skewed waveforms
On Tue, 24 Jan 2012 17:33:29 +0100, Mxsmanic
wrote: What does it mean when the waveform of a sound is essentially symmetrical, except that the positive excursions of the wave are bigger than the negative excursions, so that it looks a bit lopsided? And what does it mean when a wave form looks as though it has been raised or lowered as a whole, so that what would have been the zero crossings are now above or below zero? I've seen both occasionally but I haven't been able to determine what circumstances cause them. Someone here said something about trombones not producing symmetrical waveforms, which reminded me of these anomalies. How or why would a particular instrument produce a waveform that isn't symmetrical around zero? -- For the first case, assimetrical waveforms of certain instruments and human voice frequencies, I consider this as being a good thing. It shows to me that the recording has an adequate content of harmonics in the case of loud and clear tones. Please see http://www.audiomisc.co.uk/asymmetry/asym.html . -The second case is not good at all. It shows significant DC offset which only steals a lot of bandwidth, ie. dynamics, among other things. Luckily, the remedy can be done easily and is straightforward. I wonder why is this being so often overlooked at production of CDs. To eliminate DC offset, activate the DC offset removal tool in your audio editor or do a high pass filtering at very low frequerncies such as 16 Hz. Edi Zubovic, Crikvenica, Croatia |
#5
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Skewed waveforms
Mxsmanic wrote:
What does it mean when the waveform of a sound is essentially symmetrical, except that the positive excursions of the wave are bigger than the negative excursions, so that it looks a bit lopsided? It means it's asymmetric. Real waveforms are that way. And what does it mean when a wave form looks as though it has been raised or lowered as a whole, so that what would have been the zero crossings are now above or below zero? That could be DC offset, but it could also just be an asymmetric waveform. I've seen both occasionally but I haven't been able to determine what circumstances cause them. People blow into a trumpet... and the positive going peaks are much higher pressure than the negative-going peaks because that's how a trumpet works. The human voice is that way too, but to a lesser extent. Back in the old AM broadcast days before fancy phase rotator systems, and because the AM system is inherently asymmetric and can tolerate occasional positive peaks over the line but not occasional negative ones, the station engineers would often audition announcers and determine which polarity their microphones should be set to for optimum performance. Someone here said something about trombones not producing symmetrical waveforms, which reminded me of these anomalies. How or why would a particular instrument produce a waveform that isn't symmetrical around zero? Because air is moving faster coming out than it is going in. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#6
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Skewed waveforms
Edi Zubovic edi.zubovic[rem writes:
snips -The second case is not good at all. It shows significant DC offset which only steals a lot of bandwidth, ie. dynamics, among other things. Luckily, the remedy can be done easily and is straightforward. I wonder why is this being so often overlooked at production of CDs. To eliminate DC offset, activate the DC offset removal tool in your audio editor or do a high pass filtering at very low frequerncies such as 16 Hz. Edi is absolutely right. A trick a multi-Grammy winning engineer taught me was to HPF every mix input channel with a 24 dB/octave (or steeper) slope. (He uses the Oxford EQs and has 36 dB/oct available; I'm using the regular PT EQ and only have 24 dB available.) There's usually a subtle but noticeable increase in overall clarity to a mix when you do this. Not only DC, but simply subsonic and LF noise can chew up overall dynamics. This wasn't a problem in the olden days when tape and transformers passed little below 20 Hz. But with digital going to nearly DC, junk down there is a consideration. But beware DC removal tools, at least the one in Sound Forge 8. A while back I was on this topic and discovered the following: - the PT high pass actually is removing DC (I had my doubts for a while), but LF noise appears to re-introduced in the mix bus -- internal dither, perhaps? Depending on what I'm doing, I'll sometimes put a final HPF as the very last plugin on the master fader strip. - Sound forge 8 DC removal appears to /add/ junk at 1, 2, and 3 hz that WASN'T there before running tool -- but not always. Odd. On the other hand, I've seen great-sounding CDs from the Mastering lab show the same big hunks of energy at 1, 2, and 3 hz. Are they using Sound Forge??? Is there some common DC removal algorithm that does this for some obscure reason? I'm more concerned about noise between 10 and 20 hz, as this might have a chance of getting through in a better home stereo. We can probably ignore noise below 10 hz (but not DC). OTHO, remember the LP days, when a bad warp on a good system caused your LF drivers to jump around A LOT at something way less than 10 Hz? That can't be good for IM distortion performance.... Regarding asymmetries: Some instruments and voices indeed produce more air compression and less rarefaction as they make noise -- thus the differences in waveforms... 'Tis nature. Frank Mobile Audio -- |
#7
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Skewed waveforms
On 1/24/2012 11:33 AM, Mxsmanic wrote:
What does it mean when the waveform of a sound is essentially symmetrical, except that the positive excursions of the wave are bigger than the negative excursions, so that it looks a bit lopsided? That's a sign of non-linearity somewhere, but it's also natural for a lot of things (including the human voice) to not be symmetrical around zero. And what does it mean when a wave form looks as though it has been raised or lowered as a whole, so that what would have been the zero crossings are now above or below zero? That's a DC offset. Most converters have one, and it can be calibrated out, or simply "fixed" by running the signal through a high pass filter somewhere below 10 Hz (which is usually what a "remove DC offset" function does). -- "Today's production equipment is IT based and cannot be operated without a passing knowledge of computing, although it seems that it can be operated without a passing knowledge of audio." - John Watkinson http://mikeriversaudio.wordpress.com - useful and interesting audio stuff |
#8
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Skewed waveforms
On Tue, 24 Jan 2012 17:33:29 +0100, Mxsmanic wrote:
Someone here said something about trombones not producing symmetrical waveforms, which reminded me of these anomalies. How or why would a particular instrument produce a waveform that isn't symmetrical around zero? To add to the good answers already given, it might be worth pointing out that a wave form that is asymmetric doesn't necessarily have a DC offset. A high-pass filtered regular wave of short positive going pulses will have equal area under the short but tall positive pulses and the longer but lower amplitude negative-going part of the waveform. So the *average* is zero (no DC offset), but the positive *peaks* are bigger in amplitude than the negative peaks. As for the mechamism of production: brass intrument tone is produced by the player's lips opening and closing rapidly and interrupting a flow of air. The waveform is asymmetic because the closed and open times are very different (I'm not sure which way round). -- Anahata --/-- http://www.treewind.co.uk +44 (0)1638 720444 |
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