Oh dear, and I've been trying to unravel muddled thinking in this
thread.
"Compression" means both audio volume compression where the gain is
varied according to the input level, and a way of reducing the bit
count on a digital recording (digital compression). Totally different
from each other.
The original recording is highly audio compressed which is a complete
explanation to what is heard in the original recording and what is
seen in the spectrogram. Doubters please believe a pair of ears which
have professionally listened to his stuff for decades.
The white patches on the spectrogram are when the parts of the
spectrum which are at high level, and audio compression from these
peaks has reduced the entire volume level leaving white patches where
the background level used to be. End of. Tha's what spectrograms do.
Digital compression is a quite different techique. WAV format digital
compression compresses the digital version down to 16 or 24 bits. This
compression is inaudible and we tend to regard WAV as "uncompressed"
which it is not. Just so good that it is inaudible.
MP3 and other types of digital compression offer a wide range of
qualities. A bit rate of 320kb/s is very high and in practice
indistiguishable from WAV digital compression. You can test this by
subtracting a WAV compressed file from an MP3 version by sutracting
one track from the other. The difference is lower than than a good
background noise level and as such is inaudible.
BBC radio MP3 podcasts are digitally compressed at 64kb/s which they
regard as acceptable for speech, but it does sound a bit crunchy with
audible artifacts and a "reverberation" effect. Check this out by the
subtraction test between a WAV version of a good recording and a
64kb/s MP3 version..
The one thing which is easy to refute is that MP3 digital compression
does not alter the amplitude levels which is what is clearly happening
on the original recording. Even people with cloth ears can hear the
background noise jumping down and back up.
As far as spectrogram displays go, even high definition ones will not
show up any artifacts the ear cannot hear. If you want to check this
out try audio compressing a random noise like wind in trees at 32kb/s
or 24kn/s. The artifacts sound like running water.
> Is clipping the same thing as aliasing (referred to in the Raven
> manual)? If so, then I know what you're referring to, but if not
> then I'm still not sure what exactly that is.
No. Clipping is distortion of the peaks of the waveform by clipping
them off and will appear to reduce the volume level because most
meters only measure the peaks. Clipping makes any clipped recording
sounds rough and it cannot be undone. The answer is to record at a
lower level, nothing else.
Technical note: All digitisers clip or audio compress the input level
just below 100% to avoid the digital signal form turning "inside out".
100% plus just 1 bit digitises as 0% plus 1 bit.
To be boringly repetitive, digital recording is so good, bringing up
the volume later will not produce more noise as tape recording would.
Old habits die hard.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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