This topic explains how the Relative System Level, defined by the ON-AIR Designer configuration, affects the operation of the dynamics processing.

Overview

The dynamics elements of the Power Core use relative decibels, or dBr, as opposed to dBFS. So when you are using the dynamics elements (compressor, expander, gate, limiter, de-esser), the threshold values for those elements are in dBr, not dBFS.

How dBr is referenced to dBFS is dependent on the value you choose for the Relative System Level (RSL) in ON-AIR Designer. This value is calculated using the following formula:

  • RSL = working point - 18dBFS

The working point (or reference level, or alignment level), is a level that represents a normal audio level. In the US, the SMPTE RP155-2004 standard specifies that the working point should be -20dBFS. In Europe, EBU R68-2000 specifies the working point should be -18dBFS, and in Germany the working point is -9dBFS.

The -18dBFS offset exists because the Power Core has an additional 18dB/3bits of digital headroom in the DSP section. This allows the user to compensate for very dynamic input signals that require low gain to avoid clipping the microphone pre-amp. It is very important that the level of the signal is reduced below 0dBFS before being connected to an audio output.

RSL Values

The default Relative System Level is set to -27, which corresponds to a working point of -9dBFS, which is valid for German broadcasters.

For the US, if you want to adhere to the SMPTE RP-155-2004 standard, you should choose an RSL of -38dBFS. This will ensure that your working point, or 0dBr, is at -20dBFS. This will also ensure that the level of the internal tone generator is set to 0dBR/-20dBFS.

For Europe, if you wish to adhere to EBUR68-2000, you should choose an RSL of -36dBFS.

What this means in practice

As a practical example, if you set the threshold of the limiter to -10dBr on a source using the internal tone generator, you have a pre-limiter level of -20dBFS, and post-limiter level of -30dBFS.

If the RSL is set to -38dBFS, then the dynamics processors have a maximum threshold of +38dBr/+18dBFS; this is because of the additional 18dB of headroom available within the DSP processor. 

The de-esser uses a fixed threshold of 0dBr. This means it will activate anytime a signal reaches 0dBr/-20dBFS or greater.

The compressor in the Power Core operates slightly differently to traditional compressors. In a traditional compressor, the output gain of the signal will be reduced anytime the input signal level is above the desired threshold. This reduces the dynamic range of the signal, but also means that the overall level of the signal is lower, and therefore you must apply makeup gain to compensate.

Power Core also reduces the output gain of a signal anytime the input level is above the specified threshold. However, if the threshold is set below the working point of the system, the Power Core Compressor automatically increases the output gain anytime the input level is below the working point of the system (in correlation with the ratio). Practically speaking, this allows a user to very quickly and easily compress a signal without dramatically changing the on-air sound. Additionally, it ensures that the output signal level will always be automatically compressed around the defined working point of the system.