Level meter
A modulation meter (also: level meter) is a measuring device for checking the modulation of audio signals . VU are the appropriate level or adjustment of the level of audio signals for sound recordings , radio - transmissions needed or live concerts, so get into the radio technology of, recording studio technology and event equipment used. In the past, they were part of the standard equipment of analog hi-fi cassette decks and tape recorders , which have now been pushed out of the market by digital technology .
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The display of the measured value by mechanical pointer , light pointer , light emitting diodes or liquid crystal displays . The physical properties of these displays influence the display characteristics of the level meter. Light-emitting diodes and liquid crystals react without inertia, mechanical pointers and light pointers, on the other hand, have mechanical inertia and tend to form a mean value typical of the design . The measured values of different level meters cannot therefore be compared directly with one another; depending on the type of signal, different levels of dB are obtained for the level .
Most level meters, for example in hi-fi devices and software products, have an undocumented display behavior. Its exact properties are often only known to the manufacturer if it has been tested at all. There are also standardized level meters or electrical level meters with precisely defined display behavior, which are used in the recording studio , radio , television technology and measurement technology . These include the QPPM ( Quasi Peak Program Meter ), the SPPM ( Sample Peak Program Meter ), the True Peak Meter (both peak level meters ), the vu meter and the volume meter ( ITU Rec. BS.1770 and BS .1771).
Technical characteristics
Ballistic properties
The ballistic properties of an analog level meter include:
- Inertia ( integration time ), differentiated according to
- Settling time
- Overshoot
- Return time
- Polarity reversal deviation (the same signal, phase rotated, must result in the same display value)
The display behavior of an analog level meter is not only determined by the physical design of its display instrument. The level indicator is preceded by a rectifier , which in practice does not work entirely linearly. Moving-coil measuring mechanisms are also not ideally linear in practice.
Level meters with digital displays without inertia, such as light-emitting diodes, liquid crystal or fluorescent displays , simulate such ballistic properties in their measuring mechanism. This has proven to be useful in practice because the eye is considerably slower than the hearing. The display appears much quieter and is therefore easier to read. The digital level meters realized purely in software are also given an artificial inertia. The physical laws that apply to analog displays can be incorporated into the software as a mathematical model.
Peak level meter
Peak level meters measure the (maximum) amplitude values of electrical signals. This can be both positive and negative amplitudes. Before a sound signal is measured, it is "rectified", i. This means that the polarity of the negative amplitude values is normally reversed.
An analog peak level meter (True Peak Meter) measures the peak value of an electrical alternating voltage ( U SS ), i.e. either the positive or the negative peak amplitude. Analog peak level meters for audio technology should have a response time (integration time) of ≤ 1 millisecond for sufficient accuracy.
In the simplest case, a digital peak level meter shows the largest (single) sample value of an electrical signal (SPPM). True amplitudes are only represented statistically, because higher values can be hidden between the sampled values, which only become visible after a sampling rate conversion or D / A conversion . In order to make the true amplitude values exactly visible, the number of sampled values corresponding to an analog signal would have to be infinitely large. In practice, oversampling by a factor of 4 to 8 is sufficient to keep the display error small and to be able to classify the instrument as a true peak meter .
Peak level meters do not evaluate any frequency in the transmission range.
Rectifier level meter
The standardized pointer instrument vu-Meter (DIN IEC 60268-17) is a moving-coil measuring mechanism . It shows the rectified value . The scale applied is in the unit VU or decibel , but the angle of rotation of the needle is linear to the current intensity displayed. Therefore the scale is not equally spaced. A VU meter averages the measured values over the design-related measuring period of 300 ms. It therefore shows lower values for dynamic signals and especially for short signal peaks than for continuous signals ( sine ).
If you raise the reference point ("0 dB") corresponding to the rectified value of a sinusoidal signal by 4 dB in a rectified level meter, the level value does not have to be differentiated numerically from that of the peak voltage in the case of a conventional measuring tone (continuous sinusoidal signal). This situation is common in literature as lead (Lead) referred.
Root mean square (RMS) level meter
An effective value level meter averages the effective voltage values over a design-related measurement period. As the English term 'RMS' shows, instantaneous voltage values are squared, with negative components being reversed.
With a standard-compliant RMS level meter, the reference point (“0 dB”) is scaled according to the RMS value of a stationary sinusoidal signal ( · U SS ; corresponding to ≈ 0.707 · U SS ). In order to obtain the same display for the sinusoidal signal as on a peak voltmeter, the effective measurement value (RMS) must be increased by 3 dB. A normal measuring tone therefore provides the same dB displays on both instruments.
Music, speech or measurement signals such as square or sawtooth , on the other hand, result in measurement values that deviate from the display of a peak level meter. For one-off, very short signals, the RMS evaluation results in significantly lower display values. For the reason mentioned above, a continuous square wave signal is displayed 3 dB "louder" with an RMS meter than with a peak level meter. This display property leads to the display of '+3' dBFS for a full-level square-wave signal, which at first glance seems strange.
The RMS level meter does not carry out any frequency weighting in the transmission range.
Loudness meter
Measuring devices for displaying the volume work in principle like rms level meters, but a frequency evaluation is carried out in the transmission range, for example the evaluation according to the A curve .
Types
Pointer instruments in audio measurement technology are typically (dynamic) moving- coil measuring mechanisms . The light pointer was previously used in analog level meters because of its low inertia; it also works with a moving-coil measuring mechanism. There are also plasma, fluorescence , LED and LCD displays . Historically, the magic eye was also used.
Analog environment
Historical roots
With the beginning of the development of radio technology, the choice for a display instrument for electrical levels fell on the sufficiently fast and precise mirror galvanometer , which is also the reason for the characteristic dynamic behavior. Their scale was initially only calibrated in “percent transmitter control”; later came the now uncommon Neper and, via Bell USA , the calibration in decibels . The percentage scale - unique as a German specialty - has faded into the background more and more in recent decades, only to have been completely replaced by the internationally used dB in current devices. The exception is consumer electronics, especially the vu meter .
Level meter according to DIN
Level meters according to DIN 45406 ( ARD level meters) are predominantly used in consistently analog signal paths . They have a scale calibrated in dB and percent between −50 dB (0.3%) and +5 dB (180%). Their integration time (response time) is 10 ms, i.e. H. a single pulse of 0 dB and 10 ms duration produces a display of −1 dB (90%), a peak of 0 dB and 3 ms duration produces a display of −4 dB. The display of even shorter level peaks is dispensed with, since such short overloads (in an analog environment) are no longer perceived by the human ear .
The return time must be long enough for reliable reading; the standard provides a value of 1.5 s for a drop from 0 to −20 dB.
The 0 dB point on the scale (100%) is calibrated to an absolute level of +6 dBu (corresponding to 1.55 V eff with a sinusoidal level tone) (ARD broadcast standard level ) .
The devices have a button to increase the sensitivity by 20 dB, so that z. B. external voltages can be read more precisely in pauses in modulation . In addition, at −9 dB (35%) there is a calibration mark for the metrological check of audio frequency lines.
Digital environment
The main contrast in audio technology is not “analogue or digital”. Since digital recording devices were created in direct competition with analog processes and digital mixing consoles and effects devices also appeared with them , it only looks like this at first glance.
The technical difference is rather due to the - from a purely technical point of view - inadequate linearity of analog recording methods and tube technology . What they have in common is that they achieve the optimal compromise between (high) signal-to-noise ratio and (low) distortion factor in the full-scale range. Above this, the saturation effects of the analog tape technology and the tube technology ensure the "pleasant sound". From a physical point of view, this is a non-linear signal change similar to a compressor , mixed with pleasant-sounding distortion through integer multiples of the fundamental (= odd overtones ). This effect is called "tape saturation" for tape and "overdrive" for guitar amplifiers .
In contrast, there is digital audio signal processing , together with analog transistor technology . What they have in common is that the overload limit of an audio input or channel is a fixed value. In the case of digital signals, it is system-related at 0 dBFS, based on individual samples. In the case of analog transistor circuits, it is at the upper limit of the supply voltage, more precisely for technical reasons it is slightly lower in practice. If a digital device is overdriven , a considerable increase in non-linear distortion occurs almost suddenly ( clipping ) . But this also applies to the transistor circuit.
With the advent of digital technology , it was also possible to implement completely inertia-free modulation meters that can display the highest modulation precisely to the bit. In practice, however, it quickly became apparent that these inertia-free displays are unsuitable for modulation, the eye is much too sluggish for that. The digital displays were also introduced to an artificial inertia, at least they have a more or less strongly damped return time so that the eye can still register the level peaks.
Level meters for digital environments are standardized in DIN IEC 60268-18 . The display, which is between −60 and 0 dB, is calibrated in dBFS; a display of positive dB values is not absolutely necessary because of the high response speed. These instruments are useful for detecting amplitude peaks, especially in audio production, to avoid clipping. However, it has been shown that modulation of different audio genres according to 'true' peak values can cause a strong imbalance between very peak-containing and dense (compressed) audio signals, which does not occur with the quasi-peak level meter according to DIN. This imbalance has an impact in very different volumes.
An additional phenomenon of digital level meters is that they have so far only displayed statistically true peak values in the form of sample values (SPPM), because higher values can be hidden between the sample values, which only become visible after a sample rate conversion or D / A conversion. Newer level meters try to avoid this disadvantage by at least 4-fold oversampling (oversampling) before the measurement.
In radio and television technology , digital level meters are used that precisely simulate the ballistic properties of the standardized analog QPPM level meter. This way, the way of working could be transferred from the analog to the digital world . The headroom of 9 dB comes into play here, as the peak values of speech or music can be +3 to +6 dB above the display value of the standardized level meter. In rare cases up to +9 dB can occur. The 9 dB headroom derived from EBU recommendation R.68 applies specifically to level meters with this ballistic behavior. This way of working ensures that neither analog nor digital audio transmission is distorted.
The display behavior of analog and digital level meters was specified in 1998 by the radio operations manager conference in the document specification 3/6 . This document expressly points out that the level meter specified there is not to be confused with the devices standardized in DIN-IEC 268-10 Type II and DIN-IEC 268-17.
Execution in other countries
Traditionally, level meters with different scale divisions and display characteristics are used in various European countries. Their properties are described in DIN IEC 60268-10 . Accordingly, the following types can be distinguished:
| DIN IEC 60268-10 | DIN IEC 60268-18 | ||||
|---|---|---|---|---|---|
| Type I (DIN) | Type I (Nordic) | Type IIa (British) | Type IIb (EBU) | Digital | |
| scale |
 |
 |
 |
 |
 |
| Reference display | "0 dB" | "Test" (0 dB) | "6" | "+9 dB" | "0 dB" |
| Reference level | +6 dBu (≈ 1.55 V) 1 | 0 dBu (≈ 0.775 V) 1 | +8 dBu (≈ 1.94 V) 1 | +9 dBu (≈ 2.18 V) 1 | 0 dB fs |
| Integration time | 5 ms / 80% | 5 ms / 80% | 10 ms / 80% | 10 ms / 80% | ≤ 1 ms |
| Return time | 1.5 s ("0 dB" to "−20 dB") | 1.5 s ("0 dB" to "−20 dB") | 2.8 s ("7" to "1") | 2.8 s ("+12 dB" to "−12 dB") | 1.7 s ("0 dB" to "−20 dB") |
| 1 with sinusoidal level tone | |||||
Level meters according to DIN IEC 60268-10 I correspond to DIN 45406, which expired in 2000, with the difference that its integration time is now given as 5 ms on 80% display (−2 dB). This corresponds to the earlier DIN behavior (10 ms to 90%). Devices commonly used by Scandinavian broadcasters have the same properties, but a different scale (“Nordic Scale”) with a reference point at 0 dBu.
In Great Britain, level meters with a scale division from “1” to “7” (“British Scale”) are common; there is 4 dB between the graduation marks. The reference point is at “6” at a reference level of +8 dBu. These devices are standardized in DIN IEC 60268-10 IIa.
Devices in accordance with DIN IEC 60268-10 IIb with a scale from −12 to +12 dB (one division corresponds to 2 dB) are used for international audio transmissions within the framework of the EBU . The reference point is +9 dB, the reference level +9 dBu; a reference mark is attached at 0 dB (u).
practice
The level meter must enable precise and fatigue-free reading . In principle, the technical design is of secondary importance: "Pointer instrument" does not necessarily mean " vu meter "; " LED chain" does not necessarily mean "peak value display ". The type of measuring amplifier (rectifier) connected upstream is decisive. Moving- coil measuring mechanisms with tube pointers were quite common in German recording studios - devices with light pointers were more common - which were replaced by devices with LED chains or gas plasma displays from the late 1970s . While the measuring amplifier (U70, U270, U370 ...) and display device (J45, J47, J645 ...) used to be housed in separate housings, today only integrated devices are on the market. Some of these have a built-in correlation meter to indicate monocompatibility .
Level meters are not suitable for a volume measurement or a volume comparison. Only with a little experience can approximate conclusions be drawn from the display about the volume ratios. If different sound programs are controlled at the same level, different loudnesses result .
The different ballistic properties of level meters for analog and digital environments lead to difficulties in the practical assessment of the average level. Modulation with only one device with a short integration time tends to lead to undermodulation, since the display of the shortest level peaks indicates an apparently higher average level. Some level meters are switchable in their display characteristics; For correct control, however, a fast and slow display must be used at the same time - either with two devices or with one device with a combined display.
The vu meters installed in many devices from the USA or Japan have a fairly long integration time of 300 ms according to the standard to display signal peaks. These VU meters are therefore sometimes provided with an additional single LED to indicate the peak value, but this is an inadequate solution in practice.
In the case of hi-fi and home magnetic tape recorders, for example, it is very difficult to adhere to sensible distortion limits. This can be compensated a little by non-standard level displays. These displays deliberately show frequency response distortion and ideally a combination of VU meters and peak value displays. With most home sound devices, however , correct level control is hardly possible in terms of sound technology, neither in terms of volume, nor in terms of maintaining a satisfactory sound quality (exhaustion of the distortion limit).
In addition to the level meters calibrated to the ARD standard level (+6 dBu), devices calibrated to the international value for full level (+4 dBu, corresponding to 1.228 V eff with sinusoidal level tone) are increasingly coming onto the market. Modern level meters therefore have a switchable reference level.
As manufacturers of level meters, the companies RTW (Cologne) as well as DK-Technologies (formerly NTP , Denmark; brand name "DK-Audio") and Dorrough (Woodland Hills, California, USA) are represented on the market (formerly e.g. . also AEG-Telefunken , Siemens or EAB ).
In addition to devices, there is also an increasing number of software-based level meters that can be used for measurements in real time as well as for file-based workflows. They are available in the form of software plug-ins and / or as standalone software. Some of the most famous manufacturers for this are Dolby Laboratories (San Francisco, USA), NuGen Audio (Leeds, England), Pinguin (Hamburg, Germany) and TC Electronic A / S (Risskov, Denmark). Most of these software-based level meters can be set for different integration times and reference levels, many now also measure the loudness according to ITU BS.1770-2 or EBU R128 in addition to the electrical signal level.
literature
- DIN 45406: Level meter for electroacoustic broadband transmission.
- DIN-IEC 60268-10: Electroacoustic devices - Part 10: Peak voltage level meter.
- Michael Dickreiter, Volker Dittel, Wolfgang Hoeg, Martin Wöhr (eds.), "Handbuch der Tonstudiotechnik", 8th, revised and expanded edition, 2 volumes, publisher: Walter de Gruyter, Berlin / Boston, 2014, ISBN 978-3- 11-028978-7 or e- ISBN 978-3-11-031650-6
Individual evidence
- ^ Michael Dickreiter: Handbook of the recording studio technology. Saur-Verlag, Munich.
- ↑ Technical Guideline 3205-E of the EBU: The EBU Standard Peak-Program Meter for the Control of International Transmissions
Web links
- Level problems - HiFi stereophony , 1979 May, Arndt Klingelnberg
- EBU / IRT document (English) on loudness and level meter ( Memento from January 5, 2011 in the Internet Archive ) (PDF; 809 kB)