Digital transmission signal processing
To increase the signal-to-noise ratio for FM transmission, pre-emphasis will usually be applied to the audio signal before it reaches the transmitter, and corresponding de-emphasis is applied at the receiver. As a result, pre-emphasis will increase the level of high frequency components in the transmitted audio signal and therefore increase the amount of deviation in the FM carrier frequency. The output signal level of the Model d05 digital transmission processor can be limited taking into consideration the need for pre-emphasis usually applied to an FM-transmitter system. The unit offers two different methods of taking the pre-emphasis into account whilst maintaining a limited audio output. In the "fixed mode" the Model d05 itself applies pre-emphasis with fixed parameters to the audio signal before carrying out processing to limit the output signal level. After the limiter the unit applies a corresponding de-emphasis to the audio signal. Since the limiter acts on a pre-emphasised signal this ensures that when the audio signal has pre-emphasis applied again at a later point in the signal chain, the signal is still limited to a level at which over-deviation in the transmitter will not occur. The disadvantage of this method of processing is a lowered maximum level for the low to mid frequency portion of the signal since the (broadband) limiter decreases the overall signal level based on the increased level of high frequencies which dominate the limiter activity. This situation can be overcome with "adaptive spectral processing" specially developed by Jünger Audio. When used in adaptive mode the signal does not have pre-emphasis applied before the limiter. Instead, the limiter is followed by an additional dynamically controllable low pass filter. The characteristics of this filter are variable, and adaptive control of the filters' characteristics is used to reduce the level of high frequency signals only at those points in the programme material where the pre-emphasised audio signal would otherwise cause over-deviation of the transmitter. The reduction in the level of high frequency signals to prevent over-deviation is effective only for a very short duration, the Model d05 having an extremely fast release time for the low pass filtering process. The result, in practice, is completely inaudible. The advantage of this method of processing is that signals at low to mid frequencies are not affected. Output level can be maximised at all times without risk of over-deviation.The dynamic range processor principles developed by Jünger Audio enable compressors, limiters and expanders to be produced with exceptionally high audio quality, without colouration, pumping, breathing, distortion or modulation effects sometimes associated with this type of processor. In short, almost inaudible processing - with ease of use.
fig.1: deviation statistic for typical FM radio station audio signal controlled by d05 using adaptive spectral processing
