An Examination Of Bandwidth, Dynamic Range And Normal Operating Levels

Stated differently, in order to provide peak output capability of 100 watts for speech signals, the amplifier in question can only deliver an average output of 6.3 watts for normal speech signals.

In order to handle the occasional speech peaks, the amplifier is operating at an average power output of 6.3 watts.

This may not be enough power output for effective system operation, and we can solve the problem two ways:

1) Use a larger output power amplifier. For example, a 200-watt amplifier would provide a new average operating level of about 12.5 watts (-12 dB relative to 200 watts).

While this might get the job done, it is still an inefficient mode of operation.

2) Peak-limit the input signal so that the normal peak-to-average signal ratio is less than 12 dB.

If we do this, a higher average output from the amplifier can be attained.

SIGNAL PEAK LIMITING, CONDITIONING
Figure 9 shows the result of limiting the input signal by about 3.5 dB, while retaining the 100-watt amplifier. When this is done, the new peak signals may now be raised so that they correspond to full output of the amplifier.

Figure 9: New speech envelope with 3.5-dB of signal compression.

Values of + 15 volts now correspond to normal signal levels, resulting in a new average power output of 14 watts for normal program.

We can extend the process a little further by adding another 2.5 dB of limiting for a maximum of 6 dB signal limiting overall, as shown in Figure 10. Here, we have raised the power available for normal signal levels to 25 watts.

Figure 10: New speech envelope with 6 dB of signal compression.

It you study Figures 8, 9 and 10 you will notice that, at each step, the amount of useful “signal space” has effectively doubled. The dark area under the curve is roughly proportional to signal power, and thus relates to perceived loudness.

At the same time, peak levels have remained the same, and this invariably raises the questions: Is the signal limiting we are applying deleterious to the signal? Can you hear it in operation? The answer is mixed; an experienced listener may be able to identify the signal limiting as such, but it will not sound unnatural if it is properly done. The limited signal is louder and as such permits an improvement in intelligibility.

In normal speech applications 12 dB would be about the maximum amount of signal limiting that would be employed. However, for music applications it is customary to provide for a higher degree of signal limiting, plus some degree of compression. Compression and limiting are related operations, and a combination of both enables level manipulations to be made over a fairly wide dynamic range.

An example of the need for both limiting and compression would be a speech reinforcement system in a house of worship where both clergy and lay persons may be called upon to talk. Both experienced and inexperienced talkers will present a wide range of levels at the microphone which can be safely processed by a limiter and compressor in tandem.

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