Datasheet

HH
VS1053B OGG VORBIS ENCODER VSMPG
3 The Ogg Vorbis Format
This chapter gives a brief introduction to the Ogg Vorbis format and presents recommen-
dations for basic parameters when transcoding MP3 files or compressing CD content
into Ogg Vorbis files on a PC. The intent is to give VLSI Solution’s customers informa-
tion on how to create high-quality Ogg Vorbis content.
Before using the information in this document, you should get the free Ogg Vorbis en-
coder “oggenc”, minimum version 1.0.2 (2005), or a version with aoTuV tunings. While
the instructions will work with older versions, sound quality will not be as good. Down-
load oggenc at http://www.vorbis.com/ .
It is also strongly encouraged to get VorbisGain, a program that tags files with loudness
information (Chapter 3.3). For this example, vorbisgain v0.36 has been used. Download
VorbisGain at http://www.sjeng.org/vorbisgain.html .
3.1 Introduction to Ogg Vorbis
Ogg Vorbis is a license-free audio codec that allows for high-quality sound around a
large gamut of bit-rates, ranging from roughly 10 to 500 kbit/s.
With a given bit-rate, Vorbis is capable of surpassing the quality of MP2, MP3, WMA
and AAC LC, particularly in low-bit-rate applications.
An Ogg Vorbis file is a Vorbis compressed file that has been encapsulated inside an
Ogg stream. The default file extension for Ogg Vorbis files is “.ogg”.
An Ogg Vorbis file consists of two major parts: header and actual data. Every Ogg
Vorbis file begins with a header which contains compression tables that the decoder
needs to correctly decode first. After the header has been fully received, Vorbis audio
may be decoded. The size of the header information is around 4 KiB for most encoders.
(However, for VLSI Solution’s VS1053b encoder, header size is only roughly 1.5 KiB.)
After the header has been decoded, full random access is available in an Ogg Vorbis
file. As opposed to many other formats, an Ogg Vorbis file contains its own time code, so
absolute position in a file can be displayed after random access operations (fast forward
/ rewind).
Ogg Vorbis is not particularly well suited for encoding very short, low bit-rate samples.
E.g. a 16 kbit/s audio sample that lasts for one second would take 2 KiB for the actual
audio data, so the default 4 KiB header triples the file size. However, for normal appli-
cations where audio lasts for more than 10 seconds, this header overhead is negligible.
3.1.1 Variable Bit-Rate
For a long time almost all MP3 files had a constant bit-rate, typically 128 kbit/s. While
constant bit-rate (CBR) makes it easy to calculate a relation between file size/position
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