"Dolby Level" Calibration Tapes
The Dolby Labs noise reduction systems are "companders" (short for
compressor-expanders). In recording, they raise the recording level of
the quiet passages; then in reproduction, the lower the level of these
passages back to where it originally was. In the process, the recorded
signal is restored to its original sound, but the noise from the
recording system is reduced by something like 10...15 dB, depending on
the noise reduction system.
For these systems to work correctly, it is absolutely necessary that
the frequency response of the recorder/reproducer be flat; and it is
necessary that the level of the signal going into the Dolby expander be
the same as the original signal that went into the Dolby compressor.
So when a tape recording system is going to be used with a Dolby Noise Reduction
System of any type -- A, B, C, or SR -- the frequency response of the
reproducer must first be standardized with a calibration tape, such as
the MRL Multifrequency Tape. Then if the machine will be used to record, the frequency response of
the recorder must be standardized. Then, finally, the reproducing and recording
reference fluxivities ("recording and reproducing levels") must be set by setting the reproducing and
recording gain controls.
When the Dolby systems were first sold, the "standard recording
was the so-called "Ampex Operating Level", which is a reference
fluxivity of 185 nanowebers per meter (nWb/m) at 700 Hz.
MRL now makes and sells a "Dolby Level Calibration Tape", with a
recording of 185 nWb/m at 400 Hz, with a 10 % upward frequency
modulation for 15 milliseconds every 500 milliseconds. This makes a
characteristic sound that identifies the "Dolby Level". We can
make such an open-reel tape with this signal at whatever tape width and
speed that you need -- please contact us for a part number and price.
Alternatively, if you use a MRL Multifrequency Tape at 185 nWb/m to
calibrate your tape reproducer, you do not need a separate "Dolby
The early blank recording tape was 3M Type 111, or something similar.
In the mid-1960s, other blank tapes were introduced that could be used
at higher recording levels and reference fluxivities. Some studios
started using higher reference fluxivities with their Dolby
systems. Dolby Labs eventually published recommendations for
these alternate recordings levels.