level meters "jump" before waveform rises

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Gord
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level meters "jump" before waveform rises

Post by Gord »

Curiosity question:

When I move the finish marker of the selection I see that the level meters change to indicate (what I assume to be) the volume level at that point. I also notice that when I move the finish marker rightward from a silent area into an area with significant volume, such as a song with an abrupt beginning, the level meters "jump" about 50 ms before the waveform rises.

Is this because the level meters are showing the volume averaged over a certain period of time, as opposed to an instantaneous level that would jump all over the place from one sample to the next?
DewDude420
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Post by DewDude420 »

Basically, yes. Here's what wiki has to say about VU meters:
It is intentionally a "slow" measurement, averaging out peaks and troughs of short duration to reflect the perceived loudness of the material. It was originally developed in 1939 by the combined effort of Bell Labs and broadcasters CBS and NBC for measuring and standardizing the levels of telephone lines. The instrument used to measure VU is called the volume indicator (VI) instrument. Most users ignore this and call it a VU meter.
It's an average indicator, not a peak. it doesn't actually follow the waveform but rather the RMS of the waveform.

You need to remember that a waveform is an AC signal...it's varying from positive to netagive constantly where as a VU meter only shows volume...if the meter was set to move with the waveform, you'd need a meter that would move both ways giving you an inaccurate way of knowing what the volume is, but it'd also move much much much faster than you could ever possibly see. I also think (but won't swear to it, we'll have to wait till Chris responds) that some of the control visuals are based on FFT processing..I know spectrogram and Spectrum are...but I'm not sure just how many of them use FFT. If the VU meter uses FFT, then another explanation would be FFT window overlap.
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Re: level meters "jump" before waveform rises

Post by GoldWave Inc. »

Visuals receive about 4000 samples at the marker's position. So the peak level of the meters is determined from the highest amplitude of all samples near the marker (not just the sample it is currently over). If you select the Waveform visual, that may give you a better idea of how it works.

If you zoom in 1:1, the numerical amplitude of the sample at the marker is shown in the status bar.

Chris
Gord
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Post by Gord »

Okay, thanks guys. The numbers seem to add up, too - if the 4000-sample window is centered around the marker point then there would be 2000 samples on each side and

(2000 samples) / (44100 samples/sec) = 0.045 seconds
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