Product Family - Tech Service (other)
Deciding what operating level to use, let alone understanding the different options of operating level, can be confusing and frustrating. This bulletin should take some of the confusion out of operating level terms and out of choosing what level to use.
|Measurement Units||Before you can get very far into this subject, you must become familiar with the different terms used. Operating levels are discussed using 3 different units; dB, W. and nano Webers per meter. These are different units all measuring the same thing. dB stands for decibel, W for volume units. Nano Webers per meter (nWb/m) is the measurement unit of recorded flux on-the tape. dB and W are the same except that a W is measured using a standard Volume Indicator or W meter. dB can be measured on many different meters with different metering ballistics. The W meter has standardized, defined characteristics intended to make all W meters react the same way to dynamic levels of sound.|
|Flux Level||A number of years ago,
it became standardized that the recorded flux level be indicated for playback level rather
than W where the zero calibration can be changed. The fluxivity measurement is expressed
in units of flux per unit of track width, so that the number is independent of what
recording format is being used. The standard numbers express as nano Webers per meter of
This measurement is difficult and isn't commonly done in the field. From a practical standpoint the user purchases a playback alignment tape which is defined as having a certain fluxivity level recording. He then calibrates his machine to the alignment tape playback. There are 2 different measurement techniques used in measuring recorded tape flux. One technique was used by a German alignment tape manufacturer and another by the U.S. manufacturers. These different techniques result in about a 10% difference in value. Thus if you use fluxivity values, you should know whether you are using the U.S. short circuit measurement number or the European open circuit flux measurement. The following table lists the commonly used values for each:
ANSI stands for the American National Standards
Institute, the U.S. standards body.
|Fluxivity vs. 0 VU||The zero on a tape
recorder can be recalibrated to a wide range of recording levels. Back in the 50's and
60's, the W meter was calibrated so that the zero conformed with the broadcast standard
operating level, i.e. the NAB (National Association of Broadcasters) level. This was done
so that recordings exchanged would all play back at the same level throughout the
broadcast industry and that these recordings could be interspliced with each other.
In the early 70's 3M introduced a new recording tape for the original recording market which may have started the confusion of different operating levels. That tape #206 was so successful that it became an industry standard. A specific lot of #206 is defined as the standard reference tape in the specifications of the IEC (International Electrotechnical Commission). #206 and 207 -tapes were sold to the recording studio and hi fi enthusiasts because they could be recorded at a higher level (+3 dB) than the NAB level without increasing the amount of distortion. The user was given the choice of keeping the same operating level or increasing it up to 3 dB to get his recorded program 3 dB higher above the tape noise.
Competitive products soon followed #206's higher MOL (maximum output level) lead, thus new tapes with yet even higher MOLs were introduced over the years. First was 3M #250, later was 3M #226 & 227, and most recently 3M #996. This great variety of choices has offered the recording studio many options and also possibly created some more confusion. The chart on the adjoining page illustrates where today's 3M open reel audio tapes fall in this spectrum of different maximum operating levels.
|Headroom vs. Operating Level||The maximum
recommended operating level shown in the chart on page 3 reverts back to the basic
practice of where to set the operating level. That level is 8 dB below the recording level
which yields 3% third order harmonic distortion in the playback of a mid range sine wave
(pure tone) recording. The tone is usually 1 kHz. As the recording level of the tone is
increased, eventually a point is reached where significant harmonics can be heard and
measured in the playback of this recording. It is generally accepted that when the
playback of a recorded tone has third order harmonic information which is 3% in amplitude
of the fundamental, that that recording level is the maximum level for the tape. It is
felt that this recording level will produce harmonic distortion products in program
material that can be just barely heard.
One practical problem with audio recording is that the information you wish to record is rarely steady and constant in level. It is very dynamic in level. In order for good recordings to be made, there needs to be some cushion between the peak levels the W meter reads and where distortion actually occurs. Sound contains many level transients and if the W meter were to accurately follow these levels, it would be unreadable. The sound would occur and the meter would respond before you would see it. The W meter deliberately responds more slowly to sound level dynamics, so that it can be ready more easily.
In order to compensate for the "slowness" of the W meter, the zero level is set at a significantly lower level than the level where distortion occurs. This is where the 8 dB between operating level and MOL comes in. This 8 dB is the headroom that has been commonly used over the years. Is 8 dB of headroom enough for your recording applications? That is the basic question. It is also where the professional technique of a skilled recording engineer comes in to play.
|How Much Headroom Do You Need?||With the wide variety
of tapes having different MOLs, the options for recording become much greater. Not only do
the tape's "sonics" come into the equation, but also what operating level should
be used. The classic approach of 8 dB of headroom has been questioned by many people now
that the higher MOL tapes are available. Listening tests of different tapes recorded at
different operating levels have been done by many engineers. The results haven't always
been the same, since ears and the recorded program material both vary.
The recording engineer must choose an operating level based on the compromise of how peak levels and transients sound and how much tape noise will be heard underneath the program material. The higher the recording level, the less tape noise will be under the signal, but the greater the risk of having transient sounds lose their "punch" and realism.
Today's electronic music devices have also given us the ability to create sounds which don't occur in nature and can have significantly greater peak levels during these transients. Someone recording this type of program material is likely to determine a need for greater headroom than the conventional 8 dB. Thus an engineer would calibrate the VU meter's zero point for a fluxivity level which is greater than 8 dB below the tape's MOL.
When recording live performances, it is likely that the recording engineer will want greater headroom. Live performers almost always perform more loudly in the actual concert than they did in any rehearsal. The adrenaline of the live performance seems to take over and they play or sing louder. Here is another place where the recording engineer might wish to use 11 dB of headroom rather than 8.
One time only, unique sound effects might be another situation where the engineer wants more headroom. There is no rehearsal in these situations and thus the one recording has to be good and free of excessive distortion.
The engineer must also consider how to use the meters, how carefully to maintain recording levels at or near 0 VU. Maybe added headroom is needed as insurance because you can't always be monitoring the levels carefully.
Some engineers like to hit the tape hard with high recording levels in order to use the tape as sort of a "soft" limiter. If this is the recording technique preferred then switching tapes to one with a different MOL means the 0 VU meter should be recalibrated so the same headroom is employed. This means the engineer will get approximately the same compression with the new tape as what would be expected from the old one.
How much headroom to use, i.e. what operating level should one use with a given tape type? That's up to the recording engineer. It's part of what makes the engineer a professional at this job. The headroom may be 8 dB for some projects, 11dB for others, and maybe even 14 dB for a few. Which ever level selected, 3M has a product that can be used. The highest output products allow the most flexibility of choice. The lower output products offer good value performance at lower costs.
84-9811-5450-7 *123.11) R1 ©3M 1992
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