Text 10354, 58 rader
Skriven 2006-03-27 11:15:48 av Don Hills (1:379/45)
  Kommentar till text 10345 av Rich Gauszka (1:379/45)
Ärende: Re: Questions Re: Audio Ripping
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From: black.hole.4.spam@gmail.com (Don Hills)

In article <4426edae@w3.>, "Rich Gauszka" <gauszka@hotmail.com> wrote:
>This may be a dumb question but I've seen some quotes like those at the link
>below that give priority to VBR. Is this a bit of misinformation?
>
>http://www.mp3machine.com/news/842/

Nope. It's good info, as far as it goes. The bitrate for MP3 can be set
anywhere between 32Kbps and 320Kbps. 256Kbps or 320Kbps usually sound pretty
close to the original CD sound, and it takes a high quality CD and playback
system (and some training) to be able to tell a difference. As the bitrate
drops, you will notice more and more effects - a dulling of percussive sounds,
continuous notes (flute etc) cutting in and out, and odd warbling, whistling
and chattering sounds.

One demo I've done for various people is to take a music track and mix it with
a steady tone from a signal generator, with the tone at a level that is lower
than that of the music but still clearly audible if you listen for it. I then
encode it at various bitrates. At lower bitrates, the tone starts to disappear
in places where the decoder judges the tone well enough masked by the music.
The lower the bitrate, the more ruthless the encoder has to be about throwing
away information.

Music varies in complexity throughout the track. A CBR encoder always has to
trim the music down to fit the bitrate, rewgardless of its complexity. A VBR
encoder can adjust the bitrate as required in order to accomodate the varying
complexity of the music. 128Kbps CBR was originally chosen as an acceptable
compromise between quality and file size for non-critical listening, especially
in environments where there was significant additional masking noise such as
in-car, on public transport and in public places. I'm sure you've noticed that
"road rumble" in your car effectively masks the bass notes, for example.
Although the MP3 encoding spec allows for a variable bitrate, most of the
original decoders and portable players didn't have the smarts or processing
power to handle VBR. Many portable devices originally only handled a couple of
CBR values such as 64 or 128Kbps.

Glenn uses 320Kbps CBR for radio work. File size is obviously less of a problem
to his customers than maximum compatibility with older equipment - broadcast
stations don't replace their players every year or two. For private use, it
depends on your requirements. If sound quality is important to you, *and* a
significant part of your listening time will be in a quiet environment using
good equipment, then go for either 256Kbps or 320Kbps CBR, or high quality
setting VBR if your decoding equipment handles VBR. I ripped a lot of my CDs to
MP3 back when I was working in an office, in a fairly noisy environment. Even
with the noise, I found that I could still hear coding artifacts at 128Kbps CBR
(the de facto standard then). I didn't have the disk space for a higher CBR, so
I experimented with VBR and found a quality setting that sounded acceptable and
still only increased the file size by about 10 to 15% over a 128Kbps file.

--
Don Hills    (dmhills at attglobaldotnet)     Wellington, New Zealand
"New interface closely resembles Presentation Manager,
 preparing you for the wonders of OS/2!"
    -- Advertisement on the box for Microsoft Windows 2.11 for 286

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