![]() ![]() This would needlessly complicate the project, which needs to be able to indicate average and peak power or signal levels, but for the purposes of this exercise does not need the absolute accuracy of the real thing. Nominal sensitivity for 0VU is 1.228V RMS, and the impedance is 3.9k.Īlthough these specifications are available as shown above, the meter movement itself will rarely behave itself well enough to meet the specs without a direct-coupled amplifier to control the meter's mechanical components. Since a VU meter is optimised for perceived loudness it is not a good indicator of peak It is driven from a full-wave averaging circuit defined to reach 99% full-scaleĭeflection in 300ms and overshoot not less than 1% and not more than 1.5%. VU: A VU meter is designed to have a relatively slow response. The decay rate of 1.5 seconds to a -20dB level (IEC specified) is met using a 650 ms time constant. These requirements are satisfied by anĪttack time constant of 1.7ms. That is 1dB down from a steady state reading for a 10ms tone burst, 2dB down for a 5ms burst, and 4dB down for a 3 ms burst. PPM: A standard PPM has a 5ms integration time, so that only peaks wide enough to be audible are displayed. The term 'ballistics' refers to the absolute movement of the meter's pointer, and for true VU and PPMs there are detailed specifications for the movement of the meter needle in response to a signal . The unit described here makes no pretence at being a real VU meter, and although it can also be used as a PPM, it does not meet the original BBC standards, which call for a linear meter and a logarithmic amplifier, with highly specified ballistics. Naturally, the two can be combined, but there is no PCB available for the circuit shown below. The later project relies on the meter's ballistics, but will be more than acceptable for most applications. See Project 128 for an alternative to the system shown here. A true PPM has a linear scale and uses a logarithmic amplifier to convert the linear change of input voltage into a log output to drive the meter. Some show both VU and Peak Programme on the same LED array, with one LED seeming to 'stick' at a higher level indicating the peak. PPMs are less common, although quite a few systems use LED arrays that are (more or less) PPMs. Oh yes, it will also say 'VU' on the meter face as well. Very few so-called VU meters come even close to the specification, and the little units on tape machines and sometimes provided on power amplifiers generally bear no resemblance to a real VU meter except that the meter dial is divided into the proper number of divisions, and has a red section from 0VU to +3VU. Ideally, a VU meter is supposed to take 300ms to stabilise, and should show only minor overshoot. The VU meter is average reading, and the ballistics are important if an accurate reading is to be obtained. The Peak Programme Meter (PPM) was originally developed by the BBC to overcome the shortcomings of the VU meter, which is notoriously bad at showing the peak signal level. Even in software, the most common level meter is made to look like an LED meter, although there are quite a few "analogue" software meters available as freeware. ![]() VU (Volume Unit) meters used to be the mainstay of audio metering systems, but they have been replaced by LED metering in a great many mixers and other applications. ![]()
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