We said that the meters have been created to facilitate the reading of the voltage circulating in audio equipment so that you know when the signal level is low, high, right, so as not to create distortions, maintain proper dynamic and correctly interface different devices audio.
As it regards to the structure of modern meters these can be of various sizes, from simple led bars on hardware devices such as outboard and audio mixer (fig. 1) or even as today they are little used in the form of rack as external outboard (fig. 2) which allow to have a wider and more precise graphic scale or even in the form of software (fig. 3) that always allows you to have a complete and precise graphic scale. They can use various display technologies, from bar Graf to Liquid Crystal LCD Monitor, all in replacement of old electromechanical systems including how we’ll see the VU Meter used around the 60 ‘- 70’ – 80 ‘ professional audio environment.
The types of meter are:
VU Meter, RMS Meter, Peak Program Meter, Digital Meter.
VU Meter (fig. 4)
The VU meter (Volume Unit – 1939) was the first and only electromechanical tool used to analyze the level of the analog audio signal voltage, developed in collaboration by Bell Labs and the broadcasters NBC and CBS, it is a real voltmeter and their only that instead of representing the voltage level graphically shows the level in dB.
The main problem of these systems is the correct dynamic representation as they are constructed with a mechanical mass composed of a needle that indicates to the technician the present value of dB inserted into a moving coil that depending on the working voltage has the task of moving the indicator needle.
This whole mass has its weight and inertia to the displacement, which fall performance during the aging process as well as to the occurrence of phenomena such as the oxidation of the components. None of this makes fast and effective system for use in the display of signal peaks that have the characteristic of being very fast (a few milliseconds), so the VU Meter was used most often to display the average value of the signal, facts were often also called RMS Meter.
In audio environment the RMS (Root Mean Square) (fig. 5) is a value does not exist in nature but created by man to identify psychoacoustic perception of the average values heard to determine the loudness or the perceived loudness of a sound, more other parameters that we will see in other arguments, and identify electroacoustic specifications of various instruments, such as power amplifiers (electric power) and speakers (sound pressure and power).
The RMS value is only present in the analog environment and is calculated on the sinusoidal signal.
As can be seen from Figure 5 the dashed line represents the RMS value sometimes also called effective value.
The RMS, indicatively is found by multiplying the value of the peak of the half-wave of frequency for 0.707. For example, if the frequency has a peak 10 to the RMS value is: 10 * 0.707 = 7.07.
We will see this value, its calculation and its applications in more detail in other arguments. It should however be said that with this value does not consider the possible use of dynamic processors.
Returning to the VU meter, the instrument was originally built to represent as well as the value of the voltage (very often realized for the RMS signal detection) also the sensitivity of the ear with the aim of moving the needle indicator such and as perceptual response of the ear, according to data by scientist psychoacoustic known at the time. The reason for this is related to the simple fact that once did not exist today such graphics RTA and Impulse and VU Meter was the only solution to be able to see the dynamics of the signal thus being able to better calibrate outboard processors such as compressors, expander and gate.
The main problem was able to create a circuit that would be suitable and sensitive to properly convey the dynamic response of the musical instrument signal. Given the impossibility to create different instruments for each musical instrument to analyze it was decided to create a circuit that would fit as an average of all this and it’s used the ear response to sound with short duration pulses.
n.b. As today, through RTA and Peak Meter graphics systems are easily able to represent the dynamics of any musical instrument in order to work properly with the various signal processors.
The specifications of the VU Meter require a circuit with a 3,900 Ohm impedance, and the former were constricted with a scale at its maximum value posed 0 dB (the reference value). As very often can be seen even today, viewing Figure 4 is known as the scale in the vicinity of 0 dB is finer with steps of 1 dB up to – 7 dB, while lower down is seen instead as it is more approximate to – 10 dB to – 20 dB with a step of 10 dB, this is because the closer one gets to the maximum tolerable value before the distortion and will need to be more careful and precise so as to send the most correct possible signal. The reaction time of the needle for the ascent from the minimum to 0 dB was about 300 ms (calibrated in accordance with the theory of the human ear response time).
As I said it was impossible to represent the dynamics of a musical instrument in a precise way, aware of this fact, however, it was possible to know what were the margins of error, for example considering percussive instruments like drums and toms could be a representative error from 10 dB to 20 dB, if the needle of the VU Meter indicated – 20 dB for a signal of a percussion instrument, it was very likely that in reality its peak value was 0 dB (as it is the peak that has high voltage values while the RMS is seen as only one of its media, and it is the peak that sends distortion and damage the equipment).
To voice the error was generally of 6-9 dB.
Knowing this subsequent VU Meter were constructed with a scale that went even beyond to 0 dB, sometimes up to + 6 dB + 9 dB and over dependent on the quality and cost of the VU Meter.
The VU meter were more than anything else present in audio equipment such as mixers and processors to detect the level of the input and output signals, generally (the most modern) were accompanied by a red light next to 0 dB which is illuminated when the signal exceeded the or 0 dB. Above the 0 dB numbering notches are generally red while below 0 dB are black.
It was still a difficult instrument and required skills for its use.
PPM (Peak Program Meter)
The evolution of the VU Meter is the Peak Program Meter (always remain analog domain), first popularized in Europe especially in the broadcast and then around the world in video and audio environment.
Initially it was always used graphs of VU Meter only constructed with different circuits, the most modern and suitable for a more precise resolution for display of the peaks and of the dynamics of the instruments. These PPM had a rise time of about 10 ms and a fall time of 2 s. They also had major scales up to – 60 dB. Instead of moving coils was using amplifiers (faster and more reliable).
They were not representative of a true peak value but they were close and for they were often called QPPM (quasi-peak program meter).
Standards and References
At the time of diffusion of VU Meter and QPPM does not exist real standards (on the construction methods representative) we say that the most widely used at the professional level for VU Meter was explained in Figure 4 while many were QPPM.
The first manufacturer to introduce the QPPM was AM radio broadcasting that outline two types of quasi-PPM, the first with a graphical equal to that of VU Meter, while the second with graphic like the one in figure 6.
In figure 6, quasi-PPM with 1 to 7 scale where 0 dB is 6.
The quasi-PPM have different characteristics as well as also graphic as technical specifications based on those determined by the manufacturer.
N.B. Some manufacturers in the past proposed systems composed of VU Meter to analyze the signal RMS and Quasi-Peak Program Meter to analyze the peak signal. To date, the VU Meter and and QPPM are used only in vintage instruments.
The QPPM as well as the VU Meter are used to represent primarily the mono signal, developments enabled to create some QPPM they could see the stereo signal like the one in figure 7.
Others might measure the level of stereo combination, as sum L + R or MS.
Others still accompanied the value in dB of with percentages where 100% was the value at 0 dB (Fig. 8).
Most modern QPPM instead showed graphic displays such as those of True Peak Meter we’re going to see.
True Peak Meter or PLM (Peak Level Meter)
The True Peak Meter are essentially constructed from integrated circuits which have the task of sending the voltage corresponding to that passes along the circuit to light bulbs or today to the LEDs that make up the display of which is composed of the meter, illuminating all the necessary led up to one that identifies the value of the working voltage for which the circuit has been calibrated.
The most modern True Peak Meter analog have times of attack and release times of a few milliseconds ( almost transparent and only dependent on the input signal or output without introducing additional delays value) and hence are ideal to analyze in real time or close to the dynamics and the peak signal values of musical instruments.
The True Peak Meter today can be found on any type of mixer, outboard and analog amplifier to detect the value of the input and output signal.
With the advent of Peak Meter display (fig. 6) also born the first meter of rack (fig. 7), then not only implemented in audio equipment as seen for VU Meter or QPPM mixer and outboard.
Those Rack saw the space and with different purposes of use allow you to make custom measurements how to show the peak values or RMS values of the signal to determine the speed of attack and release of the lights, see numerical level of dB value ( with precision to the point) or the circulating voltage, to establish an average weight (less weighted, and the faster the display in real time), set the blocking parameter to the peaks (peak hold) (useful as the audio technician when mixes, records, edits, must follow different signals and different devices for which detracts from the attention to the level of the circulating signal in that particular meter, if we set a n ms block this helps by placing in a fixed lighting the led with the maximum peak reached for the set period of time, so that when the attention of the technician look back on that particular signal can see the maximum level at which it arrived and possibly if the signal has gone into distortion without requiring a technician noticing it attenuate).
n.b. Today all of these operations are mostly performed by Digital Peak Meter present on digital hardware and software devices and have replaced almost all of Analog Meter Rack.
In figure 7 a representation of a Peak Meter with Peak Hold set, is known as the level remains illuminated previously reached distortion.
The Peak Meter generally have much larger scale compared to VU Meter (being much more precise) at times with a minimum of – 60 dB and a maximum of + 6 dB, +9 dB, +12 dB, +15 dB.
Their scale is divided into three colors, green generally up to a few dB below 0 (generally 60% – 65% of full scale), then orange and red up to 0 dB over 0 dB.
Even in Peak Meter the scale is generally more precise in the proximity of 0 dB and less for lower level signals.
n.b. Indicatively through the true peak meter or as we will see the digital meter is also possible to detect the RMS value of the signal, in practice, all the LEDs that remain permanently switched to passage of the signal are the average value, while those LEDs that illuminate and constantly off are the peak values.
PPM and QPPM are regulated by Directive IEC 60268-10.
The IEC (International Electrotechnical Commission) is represented by a Committee on the definition and organization of standards electricity, electronics and the related.
More on Decibel and Meter:
Decibel and Meter – I ( Decibel and Standard Types )
Decibel and Meter – III ( Digital Meters and Software )
Decibel and Meter – IV ( Normalization and LUFS Meter )
Decibel and Meter – V ( Meters in Audio Equipment )
Decibel and Meter – VI ( Loudness Manager, Loudness Engineer )