"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 level" 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 Level" tape.

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.