On Dec 1, 15:19, Cameron Kaiser wrote:
> The question is, does source => Dolby encoding => Dolby decoding ==
source?
> Dolby encoding, AFAIK, increases the volume of sounds >10KHz, and
decoding
> does the reverse.
There are three Dolby systems, all based on audio companders. Dolby A (the
"professional" one, used by some recording studios for master tapes) works
by splitting the audio into a number of frequency bands (like a graphic
equaliser does). On recording, each band is then fed through a compressor
to reduce the dynamic range, thus keeping the volume level fairly high. On
playback, it's fed through an expander to recreate the dynamic range of the
original signal. Dolby B is a simpler version, with a single filter for
HF; it uses a similar compander for the upper frequencies only. Dolby C is
a n "improved" Dolby B, which (IIRC) uses a different corner frequency and
different compander.
None of them use any form of frequency shifting. The reason Dolby B and C
work so well (for tape, which is what they were designed for) is that tape
hiss is largely made up of HF noise. You don't hear the hiss if the signal
is at a sufficiently greater level than the hiss (which is more-or-less
constant) -- in other words, if the signal-to-noise ratio is high enough.
So Dolby B/C boost the level of quiet sections of the high frequency signal
before recording, but leave the louder passages as-is, to avoid saturation.
On playback, the amplitude of the quieter passages is attenuated again,
and the hiss is attenuated with it.
> However, that doesn't mean that there won't be subtle
> differences between one particular encoder and a particular decoder's
> respective frequency responses, so you can't be sure they'll be precisely
> the same. Your ear won't care, but I bet the computer might.
Unlikely to make much difference, unless the filter corner frequencies and
rolloffs were quite different, which Dolby Labs were quite picky about
before granting licences.
> I came up with, during my days when I was too cheap/poor to spring for
prefab
> audio equipment, a superamplification system that was Dolby compatible.
I'm
> not saying that *that* was how Dolby worked (in fact, I'm sure it isn't),
but
> on the treble section of the sound, which is where Dolby NR operates, it
was
> pretty #$%&ing destructive. I wouldn't risk it.
Then it wasn't Dolby-compatible :-)
--
Pete Peter Turnbull
Dept. of Computer Science
University of York
Received on Tue Dec 01 1998 - 20:04:54 GMT