Modulation
.jpg){kind=logo}
In audio engineering, modulation is the setting of the electrical signal level in a communications transmission channel with a fixed dynamic range . In order to be able to monitor the level during a sound recording or a broadcast on the radio , calibrated level meters are used. In a professional environment, this usually takes place in a recording studio , at concert events of all kinds at the mixer , or in an outside broadcast van.
technical basics
The limits of the dynamic range of such a transmission channel are given at a high level by the occurrence of high non-linear distortions and at a low level by the interference level present in the channel itself . A technically optimal modulation is therefore a compromise between the greatest possible signal-to-noise ratio and still acceptable non-linear distortion. To put it more simply, the level should not be too high, because then the loudest tones will be distorted, but also not too low, because otherwise the quieter tones will be masked by the background noise (e.g. when recording a tape ).
Level meter
The current measured value of the modulation (signal level) is displayed by a modulation meter . These can be implemented as mechanical pointers , light emitting diodes , liquid crystal displays , fluorescent displays or displays on an LCD or computer screen. The technical and physical properties of these displays - e.g. For example, the tendency to form a temporal mean value of the signal at pointer measuring machines - especially the intentionally specified by the designer also design significantly influence the display characteristics of the Aussteuerungsmessers. The measured values of different types of level meters cannot therefore be compared directly with one another, depending on the type of signal - e.g. B. depending on the speed of the level change in a music signal - you get different levels of dB displays with different types of level meters . These differences are particularly evident at levels that are constant over time (continuous tone).
Therefore, in the recording studio , radio , television technology and measurement technology, calibrated level meters with known and defined display behavior are used. 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). The specifications of the different types are set out in a number of international standards. In contrast, level meters for non-professional use, for example in hi-fi devices and music software products, usually display undocumented behavior. The manufacturer only knows the display properties if it has been tested at all.
Compliance with peak levels
In radio and television technology , the quasi-peak level is used to describe the level of modulation. It is based on an operationally determined level, which is defined as full modulation . This level value should, if possible, not be exceeded by the transmitted signal in analog and analog / digital mixed signal chains. In digital systems, the level is also checked with a peak level meter. The maximum modulation here is identical to the largest possible numerical value of the transmitted data words. This value should not be exceeded during the A / D conversion or in the following processing steps ( clipping ) .
Loudness compensation
Another aspect is to have successive program parts with the same quasi-peak level but with different content (e.g. music and speech; here the music must never be as high as the speech, rather a music level of −6 dB [50%] is opposite the fully controlled speech) to change the level so that a balanced loudness curve is achieved. This mostly contradicts the requirement for technically optimal modulation: Since raising the level to compensate for loudness is not permitted due to the risk of overdriving, only the program parts that are too loud can be adjusted in the direction of lower loudness by underdriving (based on full modulation).
Dynamic reduction
A third aspect of modulation is the task of a sound engineer to narrow a sound event with a large dynamic range in terms of dynamics. The signal level resulting from loud sound events must be reduced by means of a level control (fader), while the signal level of particularly quiet sound events must be increased in order to achieve sufficient distance from the interference level. It is not only necessary to adapt to the dynamic range of the electrical transmission channel, but also to take into account the listening conditions of the listeners, who are usually only allowed to set certain maximum listening volume , in particular the room volume , but on the other hand, for example, pianissimo passages of musical works despite the i. d. Usually you want to recognize the surrounding noises at the listening position. This regulation is partly carried out (but not always with sufficient precision) by special, automatically operating devices in the transmission path of broadcasting companies (" AGC ") .
"Automation" of level control
In the last few decades there has been a trend, some of which has been heavily criticized , in both music production and broadcasting . Various technical measures are used to intervene in the dynamic range and the loudness progression of both individual pieces of music and program progressions (especially the changing sequence of speech passages, music and commercials ) using largely automated , time-changing control and other means . The methods used for this are primarily so-called sound processing and automatic gain control . Among other things, devices such as compressors and limiters are used, which are combined or chained together.
In music production, but also in the broadcasting sector, this development is also known as the loudness war . In the music industry , this was particularly evident in the fact that music gradually increased in loudness levels over the years in order to create an overall impression that stood out from that of other artists. This compression of the audio signal leads to a subjectively more constant "audibility" of the music, but at the price of a high loss of dynamics . Many artists are critical of this development.
Similar to the development above, popular music radio stations use automatic normalization to match the loudness impression between different music tracks, for example so that the listener is not tempted or forced to readjust the volume on the receiving device during quieter tracks. This is often combined with a reduction in the dynamic range by means of an audio compressor , which automatically reduces the loudness differences within a piece of music. At the same time, the loudness of commercials is often increased compared to the main program.
See also
literature
- 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 .