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Compression
I was listening to a
Telarc CD and read in the notes: The signal
was not passed through any processing device
(i.e. compression, limiting or equalization)
during production).
They were known for
their audiophile quality. However the dynamic
range of a full symphony orchestra is simply
not comfortable in the home listening environment.
I realized that what they were doing was manually
riding gain or asking the orchestra to play
louder in the soft places and softer in the
loud ones. The latter procedure could be seen
as interfering with the artistic qualities
of the recording.
So technically they
are correct, but it is still a form of compression.
Once upon a time I was
testing a VHS recorder and found that the
input followed the output—a dB increase
in input signal caused a dB increase in output--…up
to a threshold of 1 volt. Above that the output
stayed the same (without distortion) no matter
how much the input increased, at least up
to 5 volts or so when there was heavy distortion.
This is called limiting
and at least on sine waves it works fine.
With analog processing there is burst of distortion
at the onset of a loud signal. With digital
systems, including the internet, there is
a delay (up to 25 seconds!) so there is ample
time to smoothly decrease the gain just ahead
of the peaks.
I looked at the waveform
envelope of the Boston Symphony broadcast
stream, which sounds quite good without obvious
compression, and found that they use peak
limiting. The waveforms are normal up to a
threshold, then flat-topped. As long as the
overall system gain is not too high that works
well. In the glory days of BSO broadcasts
on WGBH the engineer would follow the score
and manually ride gain to bring the dynamics
within the capabilities of FM (and the analog
recorder). Too much work for producers today.
The waveform image is
shown below.
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