Headroom
Headroom , in English headroom is a term used in radio and television technology . It describes the difference between the nominal level and the technical maximum level. This is to be distinguished from the term overdrive reserve, which is coined in German literature .
The inputs, outputs and storage media of analog and digital audio systems ( amplifiers , loudspeakers, mixing consoles, digital computers) are only designed for a finite range of values for the signal . In the case of analog devices, the lower limits are defined by the external voltage level and the upper limit is defined by the distortion factor . In digital systems (especially those based on fixed points ), the quantization noise (the resolution) is the limiting factor on the lower side ; on the other hand, there is an upper limit above which the signal values are reduced to the maximum value ( clipping ).
The headroom is the safety margin between the nominal level or reference level and the technical maximum level.
In radio and television technology , the program level (nominal level = full modulation) is measured with a QPPM, the technical maximum level can only be measured with an (almost) inertia-free SPPM or TruePeakMeter.
In principle, the required size of the headroom is both signal-dependent and dependent on the inertia of the headroom meter . However, this would mean that - depending on the dynamics of the signal - a new reference level would have to be determined every time. If you want to be able to record all types of program material undistorted without changing the reference level every time, you take the source signal with the highest dynamic range as a yardstick. Voice recordings close to the microphone and drum and percussion instruments, for example, typically have particularly high dynamics.
Inertia-free measuring instruments, true peak meters, do not require any headroom, as they display the highest peaks of the signal. Most non-standardized level meters integrate over short or long time intervals and therefore do not display these short-term peaks. The slower the measuring device displays, the greater the assigned headroom must be.
Headroom of standardized measuring devices
A reference level of +4 dBu was specified for the Vu meter , the maximum level is usually +22 dBu. This results in a headroom of 18 dB for the VU meter.
The QPPM (Quasi Peak Program Meter) is used as a standard measuring instrument within ARD. The reference level for full scale (100%) is +6 dBu (−9 dBFS), the maximum level is +15 dBu (0 dBFS). Both analog and digital, the headroom is thus 9 dB.
broadcast
In the processing chain of broadcasting there is a difference between the technical maximum level of the digital signal (0 dBFS) and the maximum possible transmission level. The maximum transmission level is defined by the modulation limits of the frequency-modulated analog transmitter. If it were exceeded, neighboring transmitters would be disturbed. The issuing institution is obliged to comply with these limits. The studio devices in front of the transmission limiter naturally allow technical maximum levels of up to 0 dBFS, but the 100% modulation of the transmitter is usually already reached with -9 dBFS. The headroom of 9 dB is therefore ineffective by the transmission limiter, the signal dynamics still present through the headroom are removed by the transmission limiter.
See also
Individual evidence
- ↑ 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 .