Dorrough 40-A Specifications download pdf (Page 18)

Gate Level (-a)

Range: -8 to -20 dB (1 dB steps).

It is the high level parts of a program which are often of most interest for assessing compliance

with broadcast criteria. To prevent an extended length of low level program content reducing

the averaged loudness value to a degree that affects the measured value, the meter has an

optional gating function. Once the signal drops below the gate threshold, the meter will continue

to hold, display and use for averaging purposes, the previous value that was above the

threshold. Setting it to -20 causes the gate to remain open and the unit displays the actual

loudness for the vast majority of the time.

ITU BS1770 recommends a -10dB relative threshold for the loudness gate.

Gate Level (-b)

Range: Floating or Absolute.

The gate level can be an absolute value or a floating one. Absolute values offer a degree of

simplicity but at the risk of a high setting causing gating of quieter sections which might

sometime comprise a significant part of material such as a feature film soundtrack. Adopting a

low absolute value can cause background noise that is at a relatively high level (for example a

documentary by a waterfall) not to be gated out, even though it is not a significant contributor

to the perceived loudness. Making the gate level relative may then offer results that more closely

reflect perceived program loudness.

Most international loudness recommendations specify a relative gating scheme.

Peak Mode

Range: Digital or True Peak.

Selecting Digital will base the reading on individual sample values.

True Peak uses oversampling to create the maximum intersample peak possible taken from the

maximum value of the interpolated wave shape.

ITU BS1770 specifies ‘True peak’ as the correct peak mode.

Reference

This is the frequency chosen for the calibration alignment and is either 400 or 1000 Hz.

Capture

The Loudness capture functionality may be changed between two different modes; Start-Pause

and Auto-Start.

Loudness measurement or Capture is essentially always done over a timespan - hence, it is

necessary to define when the Loudness capture starts and stops.

As explained in the Loudness & Terminology section, several different Loudness measures are

relevant i.e. Momentary Loudness, Short-term Loudness, Integrated Loudness and Loudness

Range – all time dependent.

The Start-Pause Mode

Is in effect a manual mode that allows you to decide when the Loudness capture starts, pauses,

and restarts or is reset.

The button expresses the Loudness capture state the meter is in, e.g. ‘Start’ indicates that the

Loudness Captured has been started.

Loudness Capture is dependent on the incoming signal being higher than -70dBFS.

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Dorrough 40-A Specifications Page 18