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.

6 Responses to “Piezo’s VU Meters”

  1. Bill says:

    Have to say that I like the VU meters B.C. more than A.D. – seems to me that if you are designing a VU meter whose function makes more sense for analog recording (PPM is more suited for digital I think) you might as well go for a look of a classy upscale old VU meter inspired by a Nagra, or Ampex or Studer or whatever analog recorder

  2. Michael Dunn says:

    You *may* have misunderstood the “+4″ thing. +4dBu is still a commonly used physical interconnect – it’s what’s used on line-level XLR connectors.

    A level of +4dBu equals 0dBVU, but analog gear could often go much higher. It’s been a while – my numbers may be a bit off – but a mixing desk output might be able to hit, say, 15dBVU, or is it 15dBu. A tape deck would start to saturate at a few dBVU, but could be pushed quite a bit higher.

    I’m not aware if there’s a standard for “digital” meters. Maybe it is the “0dBVU = -4dBFS” that you’ve used – I dunno. In digital, it makes a lot more sense to me to simply use meters that make 0dB the max!

  3. Chris says:

    Thanks for the insight – there’s clearly been a lot of attention paid to the details. However, I have to say that I prefer the cleaner, more contrasty original version. The red on near-black-with-a-bit-of-shadow needle of the final version just doesn’t look crisp.

    Also, where in the original the tip of the needle follows the arc of the scale, in the final version it doesn’t, by quite a margin. At the two extremes, the needle falls below the scale and in the middle it’s in danger of ‘poking’ the numbers above the scale.

    To be brutally honest, I prefer the aesthetics of the original, in every aspect. The final version has raised features that don’t really deserve to be, and the one feature (the meter itself) that really warrants the illusion of depth below the front panel, only has it very marginally. To sum it up in a single phrase, I think I’d say the original is the more coherent design.

  4. Grant says:

    Bill: Its true a VU meter has less meaning in the digital world, but it was chosen as part of the style of Piezo. It conveys the flow of sound and people love the needles.

    Something made by Studer was amongst photos collected when we began this. Apart from inverted colors I thought it was reasonably similar – the photo might not be of the same equipment you remember though.

    Michael: No confusion with XLR interconnects, The 4dB offset is part of VU meters. You clearly know analog better than I do, but I’m aware that 0dBU is just a voltage level and analog signals can run hotter than that. Piezo is digital, but we chose a VU meter and it just doesn’t make sense without the offset (I’ve seen it).

    Chris: I’m happy you like the earlier artwork since I glued it together. But it was only ever meant as something temporary till it could be polished.

    As for the arc of the needle vs the scale, most real equipment was like that (including the Studer!), which undoubtedly was a compromise. Its not so obvious in use with just one needle in motion.

  5. John McLaughlin says:

    How did you solve the ballistics question? Did you play with animations until it looked correct or did you actually build a model/filter to give the right behavior?

    For what it’s worth I sort of prefer the first meter also — It seems more clearly done to me (but that’s just an opinion from someone who still get’s excited when apps support vi command keys)

  6. MVeeH says:

    The old meters are nice enough and they aren’t hideous but the new meters are beautiful! This isn’t a sophisticated audio analysis tool so going for accuracy over aesthetics would not make much sense. Grant, great work!!

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