Headroom (audio signal processing)
Headroom in English headroom, is a term used in radio and television technology. It refers to the difference between the nominal level and technical maximum level. Must be distinguished from the coined in German literature term headroom.
The inputs, outputs, and storage media of analogue and digital audio engineering systems ( amplifiers, speakers, mixers, digital computer ) are designed only for a finite range of values of the signal. For analog devices, the boundaries are down by the external voltage level defined at the top by the distortion. In digital systems (especially fixed-point -based ) is on the lower side, the quantization noise (the resolution ), the limiting factor, on the other hand there is an upper limit, above which the signal values can be reduced to the maximum value (clipping).
The headroom is the safe distance of the nominal level or reference level to the technical maximum level.
In the radio and television technology the program level ( Nominal level = full scale) is measured with a QPMM, the technical maximum level can only be measured with a ( nearly ) the inertia-free SPPM or True Peak meter.
In principle, the required size of the headroom is both signal-dependent and dependent on the inertia of the Aussteuerungsmessers. However, this would mean that you could - depending on the dynamics of the signal - every time would have to determine a new reference level. If you want to accommodate all types of program material without distortion, without any time to change the reference level, so you take the source signal with the highest dynamics as a benchmark. Typically particularly high dynamics have for example, voice recordings close to the microphone as well as drums and percussion instruments.
Inertia -contact measuring instruments, peak level meter ( true-peak meter ), do not need headroom, as they show the highest peaks of the signal. However, most non-standard VU integrate over short or longer time intervals and therefore do not display these brief tips. The carrier indicating the meter, the greater must be the associated headroom.
Headroom of standardized instruments
For the VU meter, a reference level of 4 dBu is set, the maximum level is usually at 22 dBu. Thus, for the VU meter headroom of a 18 dB.
Within the ARD as a standard measuring instrument that QPPM ( Quasi Peak Program Meter) will be used. The reference level for full scale ( 100%) at 6 dBu (-9 dBFS ), the maximum level is 15 dBu (0 dBFS ). Both analog and digital headroom ( headroom ) is therefore at 9 dB.
Broadcasting
In the processing chain of broadcasting, there is a difference between the technical maximum level of the digital signal ( 0dBFS ) and the maximum possible transmission level. The maximum transmission level is defined by the limits of the modulation of the frequency modulated analog transmitter, it would be exceeded, there would for adjacent channel interference. To comply with these limits, the imaging companies are legally obliged. Of course, the studio equipment before the broadcast limiter allow maximum technical level to 0 dBFS, the 100 % modulation of the transmitter but most commonly achieved already with -9 dBFS. The headroom of 9 dB is therefore ineffective due to the broadcast limiter, the still existing by the signal dynamic headroom is removed from the broadcast limiter.