Before recording to tape it is important to record a calibration tone to make sure that all monitoring/recording meters are the balanced equally. It also means that volume controls can be set correctly when playing back the tape.

There are 3 types of meters :

A VU voltmeter showing relative loudness of various audio signals. The recording level should be 0 VU but should read -6 to -8 VU for percussive instruments (drums, percussion, piano) to allow for slow meter movement.

LED bargraph.

A column of lights that shows peak recording level .For all instruments set the levels to peak at 0 to +6 dB according to manufacturers recommendations.

LED peak indicator.

Flashes when peak recording levels are excessive. Set the level as high as possible before it flashes (sometimes red)

Calibration Tones.

If the master tape is open reel, calibration tones should be recorded. Record the following tones:
1 kHz at 0 VU. Optionally, 15 kHz for azimuth alignment at 0 VU for 15 or 30 ips (the speed reel tapes run) to prevent tape saturation (-10 VU for 7.5 ips).

Any tone can be used providing it is a constant and clear pitch. If you have a synthesizer try a flute voice on note C6 and make sure that the reverb, note decay and other effects are turned off. It sometimes helps to weight the key down so that both hands are free to set the faders on the mixer or recorder.