David,
While it is true that audio compression could as you say leave, "white patc=
hes where the background level used to be," MP3 also does that, however by =
a different mechanism.
Here is an article I googled that exlains it pretty well.
http://www.soundonsound.com/sos/may00/articles/mp3.htm
Look down the page under "MP3 Encoding":
"The encoder examines the contents of these frames, and attempts to determi=
ne where masking in both the frequency and time domains will occur, and thu=
s which frames can safely be allowed to distort."
If a sound is loud enough to completely perceptibly mask the frequencies be=
low or above it, those frequencies are altered, and in some cases completel=
y deleted - and in this case leaving the white patches. The lower the bitra=
te used, the more coarse and audibly noticeable such artifacts become.
This approach is also common to the Sony ALTRAC compression scheme.
John Hartog
rockscallop.org
--- In "Avocet" <> wrote:
>
> 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
>
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
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