Tape operating level aka ‘reference fluxivity’: what’s it all about (and why should you care)?

So, I’ve been investigating the subject of tape operating levels. Or, to use the more technical-sounding term, reference fluxivity.

I actually started looking into it a few months ago but it’s taken me this long to get my thoughts into enough of a semblance of order to write something vaguely coherent! Why? Well, it turns out that reference fluxivity is a veritable curate’s egg, a can of worms… a seemingly simple concept and yet the more you research it, the more variables, differing terms, different units of measurement and contrasting yet strongly held beliefs you find. And, as I eventually worked out, some of these beliefs are simply wrong. Others, meanwhile, are just different opinions and preferences. Some people consider tape operating levels to be all-important while others take little notice of them. Of those in the former camp, some support a view of ‘the higher the level, the better’ whereas others stick resolutely to decades-old standards. As for me, my starting point was when Ed Pong of UltraAnalogue Records brought it to my attention, and so off I went exploring (as ramblers tend to).

Dave, dazed and confused

I’ll admit that this hasn’t been the easiest of explorations. I think I now understand the rudiments and the key implications of reference fluxivity, but whether I can then explain it in a remotely useful way remains to be seen! In the spirit of reel-to-reel rambling, let’s give it a shot… but first I must re-emphasize that I’m not claiming to be any kind of expert here, just a journeyman sharing his tales from the road, as it were. So if there’s something here that strikes you as being off the mark, please do drop me a line and steer me back towards the righteous way.

What the devil is/are ‘reference fluxivity’ and/or ‘tape operating levels’?

Before we dive into the detail (and there’s lots of it) let’s start with a quick précis of what we’re talking about here. To the recording engineer, the studio and the record label, the choice of reference fluxivity, or tape operating level, is made to minimize tape hiss and maximize dynamic range and resolution, ultimately to achieve the highest level of realism in the recording. In practice it’s something that a studio would have decided upon and then stuck with, perhaps changing once or twice over the course of a few decades (as tape formulations evolved and audio electronics became more advanced).

So, what does the beginner need to know? Tape operating level, or reference fluxivity, is a term used to describe the strength at which the record head ‘prints’ the signal on to the tape. However, this does not directly relate to the level of the signal that your tape machine outputs to your amplifier, nor does it directly relate to the recording level…

“0VU = 320nWb/m”… erm, sorry??

Say, what? (Good question). Usually I’m a fan of straight-talking and plain English, but in this case I find the more complex term ‘reference fluxivity’ to be slightly more useful to my understanding than the simpler ‘tape operating level’, which is why I’m switching between the two here. ‘Level’ is a broad term that could potentially mean all manner of different things and so it’s easy to confuse it with other parameters. ‘Reference fluxivity’, on the other hand, is quite specific so let’s kick off with that as our starting point.

So, ‘flux’… magnetic flux is a measure of the strength of a magnetic field at a particular location, so in the case of R2R tape, we’re talking about the strength (or level) of the signal that the recording head ‘prints’ onto the tape. The tape is said to be recorded at a certain level, which is measured in Nano-Webers per meter (nWb/m or 10-9 Wb/m) – hence the term, ‘tape operating level’.

Reference fluxivity, then, is a given magnitude of magnetic flux (for example 250nWb/m, typically measured at 1kHz) that is used to calibrate the input and output circuits of a tape machine such that the signal level meters (VU or PPM) indicate 0VU (or PPM5), when a reference level signal is recorded or played back. These settings are applied to your tape recorder using laboratory-recorded reference tapes specifically made to calibrate tape deck recording and playback levels. A recorder calibrated to, say, a higher fluxivity will record at a higher level of magnetisation on the tape, but will play back at a lower level, thus producing the same output level. So, for example, a machine calibrated to 320nWb/m will record at approximately 2dB higher level on the tape compared with a machine calibrated to 250nWb/m but will play back at -2dB level and so produce the same output level. Whatever your deck is calibrated to, the key point is that a level of 0VU in will give a level of 0VU out, whether recording or playing back.

Of course, if you are playing back a tape that has been recorded with a higher fluxivity than your machine has been calibrated with, the output signal will be higher. For example, if your machine is calibrated to 250nWb/m and you are playing back a tape recorded at 320nWb/m, the resulting output signal will be 2dB higher, and show +2VU on your level meters.

The name of the game: signal vs. noise

In very crude layman’s terms, the reference fluxivity, or the tape operating level (to use the simpler term from here forwards), of a R2R tape is a measure of ‘how magnetized’ the tape is. And so, as in almost all things audio, there’s a push-pull between ideals. ‘More’ magnetization has the potential to offer more musical desirables – information, detail, dynamic range, shifting the recorded musical signal further away from the inherent noise floor of the tape itself. But then as you move up the scale, the arch enemy of distortion creeps in until, ultimately, you reach a tape’s saturation point – the point at which it’s so magnetized it’s kind of ‘whited-out’.

What we’re talking about, then, is that all-important ratio of signal-to-noise. Or in this case perhaps we could say, finding the optimum sweet spot in a scale of noise–to-signal-to-saturation.

Sounds simple enough – so where’s the rub?

So far, so graspable. But the complications that then arise are due to the fact that (as with all things analogue) standard or common practice in the world of tape has evolved with time, experience and technology. Therefore, tapes from different eras were recorded at different levels. What’s more, different decks intended for studio or domestic use were calibrated for different tape operating levels. And even now, recording engineers have varying preferences and practices and so modern master tape copies also differ in their levels.

To help make sense of all these variations, let’s take a very brief tour from the good old days to modern practice.

Tape variations

First off, the physical quality of magnetic tape has improved over time. Older tapes had lower saturation levels than many modern tape formulations. By recording at a higher level, any tape hiss will relatively be lessened. So different tape recordings may have different operating levels depending on when they were recorded, reflecting the quality of magnetic tape being manufactured at the time.

Studio/recording preferences & deck variations

Typical vintage consumer tape machines, which were obviously designed for home rather than studio use, were either calibrated to 160nWb/m, which was known as the NAB Broadcast Cartridge Standard (not to be confused with NAB equalisation), or to 185nWb/m (@ 700Hz, it’s actually 180nWb/m @1kHz), which was known as the Ampex Operating Level. Either of these was considered to be a sensible tape recording and playback level at the time (from around the 1950s). The Ampex Operating Level became the de-facto standard reference point for recording studios and so from here forward we’ll refer to it as the 185nWb/m reference, or ‘0db’.

Over time, as tape manufacturing improved, many studios changed to using a higher recording level. As a result, by the 1960s and 1970s many professional studios had calibrated their decks at 250nWb/m (it’s a logarithmic scale, so that’s nearly 3dB ‘louder’ than 185nWb/m). So a steady test tone recorded at 0dB on such a studio machine would replay as a steady +3dB tone, well into the red, on your vintage consumer machine.

As tape manufacturing continued to improve, studios began to raise their operating levels accordingly. In the United States, 250nWb/m remained the norm for most of the analogue era, but in much of Europe it was increased to 320nWb/m. 320nWb/m is nearly 5dB over the old 185nWb/m standard, and 2dB higher than 250nWb/m.

These days, some tape formulations, such as Recording The Masters SM900, can comfortably be recorded at +9dB above that 185nWb/m baseline, therefore some users (studios and consumers alike) choose to calibrate their machines even higher in order to achieve even lower tape noise and greater dynamic range. It’s certainly not uncommon to find machines calibrated to 516nWb/m (+9dB relative to 185dB).

What does this mean for me as a listener / occasional recorder?

As a listener and occasional recorder (say, making back-up copies of your tapes), your concern is essentially twofold. First, when having your tape deck calibrated during initial set-up and subsequent servicing, to what reference fluxivity should you calibrate your machine? And second, there’s the tape-by-tape basis to consider. If you’re lucky enough to have a deck with adjustable operating levels, it’s a simple question of checking the tape’s cover information to identify the level, or playing the test tone if there is one, and adjusting the output level so that reads at the 0VU mark. But if your deck doesn’t offer that option, then you’re stuck with the level to which your deck’s been calibrated, and so you might want to have a quick check of the fluxivity of a tape before buying it.

Not that getting it wrong, or paying little or no attention to it, is likely to blow up your deck or anything! In fact you could happily ignore the whole issue if you wanted to. But if you’re an audiophile, you’re unlikely to want to, since it’s all about optimizing the signal-noise ratio of your deck and tapes, and hence your musical pleasure. Let’s face it, if you’re paying several hundred pounds to hear the ultimate copy master of an album, then you’ll probably want that ‘ultimate’ to be, well, ultimate!

Deck calibration and adjustment of output levels

As if all this wasn’t complicated enough, R2R decks can be fitted with either volume unit (VU) meters, peak programme meters (PPM), or no meter at all.

Studer A807 VU Meters, UNCAL select buttons and REP/SYNC LEVEL adjustment controls

A VU meter is a simple galvanometer and uses the physical inertia of the movement to average out the reading. A PPM is more sophisticated and has a very fast rise time and slow decay. PPM is favoured by the BBC as it shows peaks that exceed a certain duration that could overmodulate a tape (or even blow a radio transmitter!). These peaks are not shown by VU meters. However, VU meters have simpler circuits and are easy to use for most domestic applications.

Murraypro PICO Peak Program Meter (image credit: ES Broadcast)

When the deck is calibrated, which is done at the point of manufacture and then in subsequent servicing, the reference point of the VU or PPM meter is set at a given level. This is the deck’s calibrated operating level.

So when you’re listening to a tape on an optimally calibrated deck, the baseline of a recording – say, the backing track average or the main vocal – will be at or slightly below this 0VU point (i.e. the position on your VU meter where it changes from black to red, with peaks perhaps overshooting very slightly into the red).

Studer A812 VU meters, showing UNCAL mode and levels adjusted to 0VU
MRL calibration tapes

So how do you know what reference fluxivity / tape operating level your deck’s been calibrated to? And what are the implications of having it calibrated at any one of the various possible levels? You can find out by playing a calibration tape to see where the reference level is set. Undoubtedly the most reliable source of reference calibration tapes is Magnetic Reference Laboratory Inc (abbreviated to MRL, and based in San Jose, California).

MRL calibration tapes are created under strict laboratory conditions

It doesn’t matter if you don’t have a calibration tape recorded at your required fluxivity as you can easily convert from one to the other. For example, if you have a calibration tape for 250mWb/m and you want to set up your machine for 320nWb/m, simply set the repro levels at -2VU (-2dB from your reference 0VU). Then when you set up the record levels, you record a tone at 0VU in and get 0VU out, but the tone will be recorded 2dB higher on the tape than one recorded at 250nWb/m – simple!

MRL calibration tapes are specifically made for calibrating tape machines. My personal preference is to calibrate my Studers, which were both pro machines, at 355nWb/m. That’s +6dB over the old standard of 185 nWb/m, and +3dB above the later studio standard of 250nWb/m. My Technics, on the hand, which was a consumer machine, is calibrated to 185nWb/m. Note: While I would always have my decks calibrated by a professional, you can find a useful guide and video tutorial on deck calibration at www.recordingthemasters.com/machine-adjustment

Technics RS1500 output level control showing reference dot at 8

Now, in addition to your engineer-set calibration level, you should also be able to manually adjust the output level – assuming that your particular deck has output level controls. On most (but not all) decks, you’ll have an output knob so that you can adjust that level above or below the calibrated setting. The one on my Technics has a small dot adjacent to the ‘8’ position to indicate that’s where the calibrated level is (see image left). So at position 8 it’s playing exactly to its 185nWb/m calibration. But I can adjust above and below that, giving a little room to accommodate replay of higher or lower level recordings.

On a studio/pro deck these controls might not be so obvious – it’s just as likely to be labelled ‘repro’ than ‘output’, and on Studers you select these controls by pressing the ‘UNCAL’ button.

However, some studio machines didn’t have them at all. The machines were calibrated internally by a technician and then left at the calibrated level. So, if that’s your scenario (or if an engineer has bypassed your output controls in the interests of improved hi-fidelity), it pays to think carefully about your calibration level, according to the types of tape you want to buy and play – since you’ll need to ensure that you’re buying tapes that are at least close to the level of your machine. But to be honest, your best bet is to make sure you buy a deck that does have adjustable output.

Buying and playing tapes

If you’re buying vintage consumer tapes, they’re likely to have been recorded at 185nWb/m or lower. Vintage master tapes, meanwhile, are most likely to be 185nWb/m or 250nWb/m.

What’s more, a good number of contemporary master tape copies are also recorded at this level. The Tape Project, for example, records at 250nWb/m giving them plenty of headroom so they’re never going to overload the tape. But there are others who favour higher levels. Open Reel Records records onto SM900 tape at 320nWb/m, which being a ‘+9dB tape’ still leaves bags of headroom but lowers the noise floor to 3dB less than it would be at 250nWb/m. UltraAnalogue Recordings uses the highest level I’ve come across in contemporary recordings, at 396nWb/m, further lowering the noise floor and increasing the dynamic range. Sensibly, recording at this level, they include a 1kHz test tone (also, very sensibly separated – before the leader tape, so that you can check and set your repro level accordingly). For example, if your machine is calibrated to 250nWb/m the 1kHz tone on an UltraAnalogue Recording tape would read +4 on your VU meter and so you should turn down the output level accordingly. Can’t adjust your output levels? Don’t worry, on request UltraAnalogue will make 250nWb/m copies for those that can’t handle these levels. Still, my advice (again) would always be to invest in a machine with an easily adjustable output level.

Pros and cons of high / low levels

At this point you might be thinking, if recording at a higher level means a lower noise floor and a greater dynamic range, then why not always record at the highest possible level?

Because you have to balance this against saturation, the point where the tape is ‘full’ and begins to distort.

Secondly, well before you get to the saturation point, increasing the tape operating level increases the potential for print-through: the recorded signal imprinting a copy of itself on top of the adjacent layer of tape (kind of like a magnetic brass rubbing). The stronger the imprint, the greater the likelihood of this transferral of information between successive layers. So, I’ve come to the conclusion that there’s a sweet spot.

Summing up

Well, several pages of rambling later, does this explain or simplify anything? Or are you more confused than ever?!

The basic rule of thumb is this:

  • Know what operating level (reference fluxivity) your tape machine is calibrated to.
  • Know the operating level of your recordings on tape and adjust your repro level to make sure the VU meter reads ‘0’ at the reference test tone. If there are no tones (not unusual) then find out what operating level the tape is and adjust the output accordingly, or adjust so that the VU needle just flicks into the red on the loudest sections of the music.

Of course, in the business of replay only, none of this is that critical. If a tape sounds really loud, lower the repro level. If it seems quiet, raise it.

It’s only when recording that you really need to get het up with all this stuff. But that’s a whole other ramble!

Here’s a quick reference guide for getting your head around the logarithmic nature of the numbers, or how increases in nWb/m relate to loudness in decibels:

Reference fluxivity nWb/m @1kHz

* 185 nWb/m @700Hz

Relative level (dB) referenced to 185nWb/m (to nearest 1dB)
185* 0
250 +3
320 +5
355 +6
396 +7
520 +9

Note that the oft-used reference of 185nWb/m is actually measured at 700Hz which equates to 180nWb/m if measured at 1kHz. Throughout this article, to save confusion we’ve referred to 185nWb/m. Similarly the other figures are approximate.

Right, now I’m off for a lie down (you may feel the same way after reading all of this!). But not before I offer my sincere thanks to Ed Pong of UltraAnalogue Recordings and hi-fi journalist Neville Roberts for allowing me to pick their brains and therefore helping me to make sense (I hope) of this mind-boggling subject.