Audio Level Metering with Sound Devices Field Mixers

This article moves beyond the basics to specifically discuss metering with Sound Devices 442, MixPre, and MP-2 portable products. A good general understanding of audio level metering is helpful before understanding the nuances of Sound Devices meters. Every manufacturer has a unique perspective on how audio level information should be communicated to the user. No matter what type of equipment used, there is a learning curve associated with translating the metered level to real-world applications. In our opinion, peak-responding metering is more appropriate for signal acquisition and recording than VU metering. VU meters are excellent for judging signal loudness — more important in sound reinforcement and mix-down applications. With today's modern inputs, knowing where a signal peaks is essential to the recordist. Sound Devices products present peak information in a consistent, quickly understood format.

Physical Type

There are three commonly used meter types used on portable audio equipment, LED, LCD, and mechanical. Sound Devices has chosen LED-based meters for several important reasons.
  • LED meters are not affected by ambient temperature conditions, whereas LCD meters often darken in high humidity/heat conditions and slow down in cold climates.
  • LED meters are not affected by shock or mechanical vibration like mechanical meters.
  • LED meters can be driven with varying voltage to change their intensity.
  • LED meters can be viewed easily from a distance.
  • LED meters free the mechanical design and allow smaller product size.
Sound Devices uses an LED technology called Gallium Nitride (GaN) for output meters on the MixPre and MP-2 as well as for all LED's on the 442. These high-performance LED's are excellent for viewing in all light conditions, including direct sunlight. GaN LEDs are extremely efficient and draw very little current even at high brightness. GaN LED's incorporate a lens in their design; the light output of GaN LED's is reduced when viewed off-axis. The output meters on the MixPre and MP-2, when set to the highest brightness setting, can cause eye irritation or damage with prolonged exposure when viewed directly on-axis. The super-bright setting is intended only for direct sunlight conditions or with off-axis viewing angles. Set the meter brightness to the lowest acceptable level for viewing. This will also maximize battery life.

302 and 442 Meter Ballistics

The output meter ballistics (speed of response) on both the 302 and 442 can be selected among three settings:
  1. VU - appropriate for music recording,
  2. Peak - with true PPM decay times,
  3. Peak/VU combination- the most informative setting.
The Peak/VU setting is unique among portable gear. With this setting, the 302 and 442 users knows the absolute maximum output level -- important when interconnecting with digital recorders -- plus the VU level below.The more dynamic the program content, the greater the difference between VU and peak levels. For instance, typical speech has a peak-to-average ratio of around 14 dB, while theatrical speech can have peak to average ratios near 25 dB. These differences are easily seen with the combination meter setting.
  • Percussive sound sources, 50 dB + peak to average levels,
  • Speech, 14 - 25 dB peak to average levels,
  • Jam band, 3 - 8 dB peak to average levels .
Knowing both the peak and average levels greatly aids in optimizing the dynamic range and signal-to-noise of the signal sent to the recorder.

MixPre and MP-2

LEDs on the MixPre and MP-2 have an attack time constant of 0.1 ms and a decay time constant of 100 ms. This is defined as a peak reading meter (our decay time is shorter than the BBC specification for peak meters, a specification defined in an analog-only world). Peak monitoring allows for the best understanding of maximum signal strength, which is the most important measurement when interconnecting with digital-based products. By knowing peak signal levels, the operator can optimize gain structure to not exceed O dBfs. Again, properly set on-board limiters on the MP-2 and MixPre are the best assurance of keeping signals in a safe range with digital inputs.

Meter Scale

The MP-2, MixPre, and 442 meters are calibrated to a dBu scale. This scale, combined with peak responding ballistics, provides a very accurate indication of signal level. Remember, the LED's labeled "0" refer to 0 dBu, not 0 VU! When viewing the 442 in VU mode, the 442 meter shows in VU increments.0 VU often refers to either a +4 dBu or a +8 dBu signal. The 1 kHz tone oscillator on the MixPre is calibrated to output a 0 dBu signal, corresponding to many line level input stages as 0 VU. With a 0 dBu indication on the meters, output levels have at least 20 dB of headroom before clipping. The 442 0 VU reference can be set via a User Setup between a 0 dBu, +4 dBu, or +8 dBu reference. The LED's above each input illuminate very near clipping (illuminating amber when limiting). When you see red, you are quickly running out of headroom. With almost all sound sources, Sound Devices recommends activating the on-board limiters; these will prevent the inputs from clipping. The LED above the headphones indicates that only the headphone circuit is clipping, not your master outputs! The LED's are the best indication of signal strength activity.