Under The Microscope

Piezo’s VU Meters

Piezo’s VU meters are one of the first things people notice when they try it out, and thanks to our designer Christa, the meters are quite pleasing to the eye. I’m the guy who had to make them work though, so I get to tell the story of their creation. To begin with I imagined they’d be simple enough to build, the decibel (dB) values were already provided by Rogue Amoeba’s comprehensive audio code, so I just had to make some needles move? Easy!

Are They Calibrated?

Long before Piezo was done, Quentin asked if the meters would be calibrated, with the warning that this would be the first question users asked when we shipped. I didn’t have a good answer. In fact, I had no idea, so I set out to do some research before I looked too stupid.

What Is a VU Meter Anyway?

Well, it’s not quite what I expected.

Wikipedia says:

Volume Unit (VU) defined: The reading of the volume indicator shall be 0 VU when it is connected to an AC voltage equal to 1.23 Volts RMS (equal to +4 dBu) at 1000 cycles per second.

That’s all true, but it doesn’t explain much, so let’s try some additional information:

Physical VU meters were cheap to manufacture, with a simple mechanism that gave an average of the signal. They were created in 1939, and standards soon followed for many aspects: voltage levels and sine wave frequencies for calibration, the scale, which is in units of dBu (but offset 4dB from what you’d expect), as well as ‘ballistics’ describing the rise and fall time of the needles.

Ballistics are interesting. They dictate that needles should take 300 milliseconds, almost a third of a second, to rise from the lower stop to the zero dB marking. The effect of the ballistics is to make consistent sounds read louder, whilst sharp, percussive sounds will not display as loud as they truly are – similar to our own perception of loud noises.

VU meters were useful for many purposes and widely used in all manner of audio equipment. That said, many meters in consumer equipment had little in common with the true standard, despite having ‘VU’ clearly written on them.

Worth noting, VU meters were not the same as ‘Peak’ meters. ‘Peak Program Meters’ (PPMs) attempted to more accurately reflect fast voltage peaks and had many standards of their own. Analog PPMs were more expensive to manufacture and calibrate, so they were never as common when meters were physical objects. Now that our audio is digital, a computer can provide a true peak meter or an RMS power meter easily, and that’s what you normally see displayed in software. For example, Rogue Amoeba’s own Audio Hijack Pro provides meters in a few places that are switchable between Peak and RMS display.

Piezo’s VU Meters:

Now that I knew what ‘calibration’ meant for a VU meter, I could get down to business. That meant:

* Adding a table of dB values to angles describing the meter’s layout
* Using some math to interpolate between table values and sweep a log scale
* Writing test code to confirm motion and reference marks
* Adjusting sampling frequency and animation speeds
* Offsetting the dB values by that tricky 4dB in the standard
* And verifying against other meters I could find in audio software.

Yep, just as easy as “making some needles move”.  All this took some time, but if we were going to call them VU meters, it was all necessary.

Once the meters were correct, we still had to decide upon labels for the meter and their placement, so that final artwork could be made to suit. Experimentation and a nod to aesthetics led to our final result. Here’s how it looked with my placeholder artwork early on, as test code drew a needle for each possible marking:

Piezo's VU Meter Needles Test
VU Meters B.C. (Before Christa)

Zero dB on the digital scale reads as +4 on the meter, but because the meter values and the needles don’t respond instantly, it doesn’t reach the red zone very often. The ballistics of Piezo’s needles don’t attempt to follow the standard exactly, but they aren’t too far from it. Our testing showed that running the needles too fast made them appear frenzied and tiring to watch. On the other hand, running them too slow made them lag behind the music. Finding the right balance was hard, and required plenty of experimenting. In the end we made the fall time half the speed of the rise time – keeping some of the smooth liquid motion whilst matching peaks in your music.

Piezo's VU Meter Needles Final Version
VU Meters A.D. (After Designer)

Since shipping we’ve had users tell us they keep Piezo’s meters on screen all day because they enjoy them so much.  I think we did alright.

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