Re: Apple and EMI

Jerome O'Donohoe <jerome@xxxxxxxxxx> wrote:

Rowland McDonnell <real-address-in-sig@xxxxxxxxxxxxxxx> wrote:

Jerome O'Donohoe <jerome@xxxxxxxxxx> wrote:

Rowland McDonnell <real-address-in-sig@xxxxxxxxxxxxxxx> wrote:
[snip]

Take the frequency range divided up into bands. In each band, you run a
mapping so that quieter sounds are made louder in a fashion that keeps
the quiet stuff quieter than the loud stuff: you just compress the
dynamic range, squish things up so that all the signal is at the `high
volume' end (IYSWIM).

multiband compression does indeed exist...however it is not the only
kind of compression. Multiband compression is used to make adverts
louder, for sure. Audio compression per se is much simpler than you
describe!

My explanation of `audio compression' is as I have had it described to
me by a sound engineer.

It's the basis of old-fashioned Dolby noise reducation, as it happens -
but with a Dolby rig, you uncompress on playback. Mismatches between
the frequency bands and suchlike is why Dolby recorded on one machine
sounds bad when played back on another.

Now, when you talk about `audio compression', what do you mean?

just so i don't look like a muppet, I am a sound engineer...

Fair enough. Good.

audio compression is, at its most basic form, the reduction of the level
of a signal based upon a peak reading of any part of its frequency
spectrum, with a threshold, ratio, attack and release.

Eh? Surely reducing the level of a signal is simple attenuation? Or
have I missed something.

(this `threshold, ratio, attack, and release' lark - wossat?)

This is how a
compressor works in, say, an input channel on a mixing desk,

What you've described is an attenuator to my mind. Please explain more.

What I understand from audio compression is that it's meant to reduce
the dynamic range of the signal, and I don't see that you're describing
something that does that job at all.

and has
done for decades, way before Mr Dolby got involved.

Mr Dolby was heavily involved in the mid 1960s. Can you provide a
better description of `audio compression'? And when did it start being
used?

They're still used
all the time today on individual instruments, voices, dialogue, etc.,
or across a mix of elements of anything...multiband compression comes
into play when you desire to make a whole final mix signal as loud as
possible to its audience by manipulating the bands, boosting them where
possible.

And as I understand it, that's what they use on the output of typical
pop music radio stations (and Classic FM). Is that right?

A TC Electronic Finalizer (or DBMax in tv & film world) is a
good example of a box in a rack. Optimods are used in radio to do the
same thing to final stage radio output.

Good - so you can spout product names. What do these products do to the
signals?

Now, Dolby, from A through to SR, is multiband companding, compressing
and expanding the freqs as required to reduce the noise in the "usual"
noisy areas of film, tape, cassette tape, etc. An encoding and decoding
process, as the dolby circuit at the other end knows what to look for
and behave appropriately.

Which is what I said. Except that the Dolby circuit at the other end
usually doesn't behave appropriately if it's from a different firm.

Rowland.


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