Abstract
The soft clipper described in this White Paper was the result of an investigation into the performance of a new stereo encoder for the ΒιΆΉΤΌΕΔβs FM transmitters.
There are now tight regulations in place concerning the performance of broadcast FM transmitters. In particular, it is important that their peak deviation should not exceed 75 kHz. On top of that, the broadcasters need to maximise the available deviation, so that the programme material sounds as βloudβ as possible to the target audience. The current standard is for peak programme levels of +8 dBu to deviate the transmitter by 67.5 kHz. Adding the 19 kHz pilot tone and RDS to this brings this up to 75 kHz.
There is no longer any guard-band in which to absorb any errors that might arise in the transmission chain. As a result, the stereo encoder associated with the transmitter needs to incorporate a protective device to prevent possible over-deviation. The encoder being investigated used a hard clipper. Whilst fully effective, it generated distortion products falling within the stereo difference signal β something best avoided. Lack of processing power within the encoder posed a challenge when looking for a better solution.
The soft clipper described here provides an answer. In essence, it slows the change of instantaneous gain associated with clipping. By a suitable choice of gain reduction βwindowβ, distortion components above a certain maximum frequency can be largely eliminated. Below that frequency, the components are largely unaffected. Distortion can be made arbitrarily low by widening the window, but the available digital memory and processing power will impose a practical limit. The advantage of the soft clipper approach is that it is fully flexible, subject to this limitation.
Although the FM transmission chain has been used as a concrete example, the soft clipper could be useful in many other applications.
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