This is largely due to the use of slow responding VU meters in almost dbu analog professional audio equipment which, by their design, and by specification responded to an average level, not peak levels. F68 digital alignment levels the EBU Rec. Different broadcasters may handle the above requirements differently. This article possibly contains original research.
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Red and green needles are used for left and right. This colour convention comes from port and starboard navigation lights, in accordance with an ancient BBC in-joke about Broadcasting House resembling a battleship. White and yellow needles are used for sum and difference M and S. A more recent variation is to use a black needle with a dayglo orange tip for S instead of yellow.
As a result, signals of identical amplitude and phase in the left and right channels make the M meter show exactly the same deflection as for the individual L and R meters. The M6 standard means that dual mono sources e. This M3 format is designed to give a more accurate indication of the level of the summed mono signal when working with conventional stereo material. The premise is that in summing two signals of similar level but carrying non-phase-coherent sounds i.
The M3 standard means that true stereo material can be peaked to 6 in both channels, with the M meter also showing 6. However, dual-mono sources can only be peaked to 5. Commercial broadcasting in the UK initially used M3  but had switched to M6 by It was felt that this was too small to justify the much greater expense. It was also found that VU meters gave more consistent readings than PPMs when comparing programme levels at the sending and receiving end of long lines subject to group delay , which altered the waveform.
IEC Digital PPM[ edit ] IEC is a partial standard for a PPM designed for use with digital audio in both professional and consumer use, using "incremental dot or bar type displays or numerical displays". Such a display shows level relative to 0 dBFS. The return time has the same value as a Type I meter: 1. Types IIa and IIb differ only in the scale marks. Parameters for the VU meter and Nagra modulometer are included in the table below for comparison.
EBU R68 PDF
Peak programme meter