|
Now, let’s feed this speech signal to an amplifier with
an output capability of 100 watts into an eight-ohm load, as shown
in Figure 8. We have labeled the left axis with
the actual output voltage produced by the amplifier, and we have
indicated the approximate average signal voltage at the right axis.
Figure 8: Speech envelope at the
output of a nominal 100-watt amplifier. |
It is clear in this figure that the average signal output is about
±10 volts, while the full voltage output capability of the
amplifier is ±40 volts. The difference here is 12 dB, which
corresponds to a power difference of 16 to 1. 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:
A. 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.
B. 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 they
correspond to full output of the amplifier. Values of + 15 volts
now correspond to normal signal levels, resulting in a new average
power output of 14 watts for normal program.
Figure 9: New speech envelope with
3.5 dB of signal compression. |
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 15-10. Here, we have raised the power available
for normal signal levels to 25 watts.
If 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.
Figure 10: New speech envelope
with 6 dB of signal compression. |
|
