Technical Bulletin #2, 8/82

METERS AND MEASUREMENTS
HOW TO SET RECORDING LEVELS
Terence D. O’KeIIy

One of the most important quality measurements of a tapeis its dynamic range or signal-to-noise ratio. This
specification describes in decibels the difference between
the two limits which confine all tape—the maximum
acceptable signal (the maximum output level or MOL) andthe inherent noise level of the tape (Fig. 1). Whenrecording, one must set the levels high enough so that theprogram masks the tape hiss but not so high that thesignals overload the tape and create excessive distortion.
If the recording level is too low, a steady hiss will be
audible throughout the program; if the recording level istoo great, the sound will be harsh and grating with“spitting” sounds on sudden musical changes and a loss ofhigh frequencies. The proper recording level is determinedby reading the meters on the recorder. Since different
types of meters measure in different ways, each type must
be read in its proper way to interpret the information
correctly.

decibels above the zero reference point, so keeping theaverage reading at 0 allows 6 to 8 decibels to be used as“headroom” for peak signals occurring 6 to 8 dB higher
than the average level.

All meters on tape recorders measure the amount of signal
being delivered to the tape so that the recording level canbe set as high as possible to mask noise but low enoughto avoid excessive distortion. (The maximum “tolerable”
distortion in tape recording is generally agreed to be 3%
third harmonic distortion of a low frequency—315 Hz forcassette recorders, for example—or 3% intermodulation
distortion of two high frequencies.) The meters have ascale in decibels above and below a reference levelmarked by a zero. The “positive” area above the zero isusually marked in red to indicate the danger of increasing
distortion.

Most open-reel decks use the VU-type meter. Recordinglevels are set so that the meter barely goes into the red,
indicating that the average signal is at 0-VU with peaks at
6 to 8 dB above. Peaks can occur so quickly thatsometimes VU-type meters will appear completely
motionless even when the sound is easily audible. At highaverage levels around 0-VU, the inability of the meters toshow peaks is no problem for an open-reel deck because
its fast speed provides a lot of tape for peaks; and themeters are designed around this capability. Cassette tape,
however, moves at one half the slowest open- reel speed
commonly used. With much less tape running past therecord head, the 6 to 8 dB of headroom is cut down
considerably. VU-type meters should not be adjusted toregister 0-VU on a cassette deck because the peak signals
would be severely distorted if the average level were at the0-mark. Many cassette decks use a VU-type meter in
combination with a light emitting diode (LED) that flashes
instantly when a sudden peak reaches the danger point.
The meter might be registering -12 on the scale or evennothing at all, but the fast-acting LED will still show a very
brief peak. Recording levels on these meters should be set
so that the LED rarely and only faintly flashes. VU-type
meters without LED’S should be set conservatively until
experience and practice familiarize the recordist withinterpreting musical dynamics through averaging meters.

VU Meters

Peak-reading Meters

The most common meter with a needle-type pointer is theVU (volume unit) meter. A true professional VU meter
conforms to very rigid specifications and is quite
expensive. The VU-type meters found on consumer
recorders are not as accurate, but they work well enoughfor home use. The purpose of the VU meter is to measure
average or over-all signal levels. The scale typically runs
from -20 to +3 with the 0 to +3 section marked in red. The
point of 3% third harmonic distortion is usually 6 to 8

Peak-reading meters, especially the LED strings orfluorescent light types have become commonplace on
cassette decks. These meters concentrate on the peaks
rather than average readings, although some areswitchable between the two modes. Their scales can run
from -40 to +10; and they need the expanded scalebecause the meter or display is almost always moving, incontrast to the slower, averaging meter. The peak meter isgenerally easier to use for cassette applications becauseslow tape speeds reduce tapes’ headroom capabilities,
and one should be more concerned about peak levels thanaverage levels. The only drawback to some of the LED or
fluorescent-type peak meters is that the accuracy ofdefinition depends on the number of light elements. A lightmeter with a scale of -30 to +10 over only six LEDs makes
it quite difficult to distinguish -4 through +4 dB when thesame LED is lit for the entire range. The greater thenumber of light elements, the greater the definition of themeter.
The actual recording level set on the meters depends not
only on the tape, but also on the content of the program.
Premium Type I normal bias tapes generally haveexcellent headroom at low frequencies, that is, theirmaximum output is so good that they do not reach 3%
distortion until a +6 or more on peak meters. Type II tapes
generally do not do as well, but their much lower noise
floor more than makes up for any MOL difference, andthey often provide better high frequency SOL (“SaturatedOutput Level”) than Type I tapes. Type IV metal tapes
have the best MOL values at both high and low
frequencies at least theoretically, but much depends on
the quality of the recorder. In many cases a Type I tapecan outperform a much more expensive Type IV at low
frequency MOL if the record head cannot handle the extra
bias current required for Type IV tape, and a true chrome
Type II tape is much quieter than any metal formulation.

A professional recording engineer knows not only how
much headroom his mastering tape has and at which
frequencies it varies, he also knows the approximate
energy levels across the frequency spectrum of theprogram about to be recorded. Recording a piano
concerto, for example, would require slightly lower levels
than recording a song with a heavy drum beat because thetransient peaks of the piano are more likely to create tapedistortion at a given level than the low frequency energy ofa drum. Cassette tape has far greater restrictions thanopen-reel tape, especially at high frequencies; so evenmore caution is in order. Unless the person recording canread a 1-3 decibel difference in a rapidly moving meter andknows what kind of energy—high or low frequency—ismoving the meter or flashing the LEDs at any particular
moment, in addition to knowing how much headroom
exists on the tape at that frequency, it is best to beconservative, It is important to preview the program
material or conduct a microphone test to determine theloudest passages the tape will have to handle and to set
the meters accordingly. One should practice with different
formulations, intentionally over- and under-recording togain familiarity with the limits of distortion and noise andthe levels that best fit each tape. It is quite difficult to utilize
a tape’s full maximum output level to the last dB.

Unfortunately, many “improved” tapes have boosted MOL
at a sacrifice in noise so that the dynamic range, thedifference between MOL and noise, is exactly the same.
These “improvements” demand that the user increase
recording level by each 1 dB gain in MOL or lose that 1 dB
in increased noise. BASE’S true chromium dioxide PRO II
cassette, on the other hand, has the lowest noise levelpossible today and improves MOL without sacrificing
noise. Since PRO II cassettes have the least noise of allcassette tapes, there is less need to record at levels soextreme that they are always on the brink of excessive
distortion. Every improvement in dynamic range is a true
and practical benefit for the user.
In setting recording levels one must keep in mind several
points: 1.) the type of meter in use, its reference level (seebelow), and how to read it; 2.) the type of tape in use andits headroom characteristics in the recorder; and 3.) thedynamics of the program to be recorded, that is, whether
the peak energy is in the low or high frequency range. The

Meter Type
markCassette VU-type withchoice of levels based on these considerations should
result in the best sounding recordings possible.
The graph of music energy in Figure 1 shows particularly
strong peak signals which would not appear on a VU-typeaveraging meter but which would be resolved by the fasterpeak display. The point marked by the arrow above reads-4dB on the VU-type meter, but the LED peak shows its
instantaneous value as + 3dB. The steady-state nature of
the test tone will produce exactly the same reading on bothmeters because the peak and average values are thesame.
The numbers marked on meter scales express the numberof decibels above or below a 0-reference point. The 0point can represent various levels of magnetic flux(strength) depending on the design of the recorder. The
is a good guide in relating one
Dolby calibration markcassette meter to another because it represents a defined
flux level of 200 nanowebers per meter (nWb/m) ofmagnetic energy.

Flux level at 0 dB point
140 nWb/m-3.10 dB

200 nWb/m

0.00 dB

Consumer open-reel deck with VU-type

185 nWb/m-0.68 dB*

Professional open-reel deck with VU-type

250 nWb/m+2.62 dB

Cassette peak meter withat +3 dB point

Difference fromat O-dB point*A consumer open-reel deck with Dolby NR encoding and decoding would use its own 0-dB point at 185 nWb/m as its Dolby
reference level rather than the 200 nWb/m level used in cassette decks.