Mp3Pro / Spectral Band Replication (SBR)

What is Mp3Pro?

Mp3Pro is the name of the combination of two things: Mp3, and an audio coding enhancement tool from Coding Technologies named Spectral Band Replication (SBR).
The goal of SBR is to enhance the audio quality of highly compressed sound files.

SBR with 64kbps MP3

While a conventional Mp3 encoder at low bitrates (as an example 64 kbps) will not be able to maintain full bandwidth, giving you only the grey output, using spectral band replication you will obtain a full bandwidth signal once decoded (grey + orange signal). The advantage is that SBR only uses a small fraction of the bitrate in the encoded file.

How does Spectral Band Replication works?

SBR does not really encode the highest part of the frequency spectrum; it reconstructs this part. In a 64 kbps Mp3Pro encoded file, around 60 kbps are used for the conventional Mp3 data. The Mp3 encoder encodes the music signal up to 8kHz in a conventional way. This part of the resulting Mp3 file can be decoded by any Mp3 decoder, so compatibility is kept with conventional decoders.

The SBR technique will use this part of the decoded signal to reconstruct the missing high frequency part, from 8kHz up to 16kHz, by duplicating the spectrum. This kind of trick was already used by Kenwood (D.R.I.V.E.) and Pioneer (Legato-Link). So what does make SBR better than those other solutions? A simple fact: while previous solutions were blind, SBR knows better how to reconstruct the missing parts.

During the encoding stage, the SBR encoder stores some information to in order to try reconstructing properly during the decoding stage. It stores from which part of the original band-limited signal the upper frequencies should be replicated, and also the scale of those frequencies. That information is very small compared to a conventional encoding, and are stored into ancillary data of the Mp3 file. This way, an SBR-enhanced decoder will be able to use this info.

The results:

The first thing is that this SBR reconstruction needs some processing power. So current SBR-enhanced Mp3 decoders need 3 times (yes, 300%) the power of a conventional Mp3 decoder. If this is not a problem for a computer, it could be a problem for hardware decoders, like those found in portable devices.

For low bitrates, the sound result is quite good (The interest of SBR with high bitrates is limited). Of course Mp3pro won't give you CD quality at only 64 kbps. Such a 64 kbps file gives a result similar to a 100 kbps Mp3 file, but according to the bitrate, this is a good result. With SBR, Mp3Pro is now able to compete against Windows Media Audio for low bitrates.

© 2001 Gabriel Bouvigne for MP3'Tech - www.mp3-tech.org