VuMeter


 * The Audacity VU meters could be thought of as a hybrid between a VU and a PPM meter. Should we be using a standards-based VU or PPM meter instead?
 * Conform to EBU standard R128.
 * Measures Programme Loudness (normalised to a Target Level of -23.0 LUFS), Loudness Range and Maximum True Peak Level (with a permitted maximum of -1 dB True Peak).
 * Measures with a meter compliant with both ITU-R BS.1770 and EBU Tech Doc 3341.
 * Includes a gating method as specified in ITU-R BS.1770-2. The relative gate ensures that audio below a given threshold is left out from Loudness measurement, because human perception of loudness is chiefly determined by the loudest passages or "foreground sound".
 * It's not an ANSI C16.5 VU meter, it's a digital peak meter.
 * EBU Technical Publication 3205-E: Section 2.4: Reversibility error < 0.5 dB
 * IEC 60268-17 ANSI C16.5: Reversibility error < 0.2VU, generally 0.2 dB
 * positioned very near the record level control
 * phase-error indicator (if absolute averages of (L-R) > (L+R) then your inputs are probably out of phase)
 * What if you were recording with a true stereo microphone, I think the sounds on the Left would show an out of phase condition when both channels are compared. likewise the sounds on the Right side. (phase-error indicators are mainly to show if you have your inputs wired incorrectly).
 * Equation strictly is |L-R| > |L+R|, i.e. ignore the signs on the results. Hence for a blumein or similar crossed pair, this will only occur if the sound is outside the V of the pair of microphones at the side. This is equivalent to side on sound in a Mid / Side microphone setup. Either way the key thing is that mono listeners to the result will get little or no sound, which is very confusing. Any recording produced like this is likely to get a poor reception if played in mono.
 * An alternative implementation of this is based on an X-Y display of the inputs turned through 45 degrees, which will show horizontal or vertical depending on the phase of the inputs.
 * Given the above, maybe it should be possible to send the mix to the meters as well as the inputs to the meters. Very easy to cause by crossing leads inside balanced audio connectors.
 * As well as a peak meter, it would be very useful to have an RMS meter (as seen in Wavelab).
 * From the radio point of view a PPM meter is much more useful than the much slower VU type. This would be very useful. Currently use a, but would be much easier/better to have one built into Audacity.