Spectral band replication (SBR) is an extension for existing audio encoders with the main purpose of reducing the bitrate without a perceptible reduction in quality in order to reduce data while maintaining an equally perceived quality. This thesis approaches the functionality and impact of SBR.

By reconstructing high frequency signal components these frequencies tend to shift leading to tone shift effects, beat effects and noise.

When directly comparing mp3 or AAC, respectively, of the same bitrate as the SBR-enabled HE-AAC the result shows that the HE-AAC encoding can save more high frequency signal components. This results in a richer and broader frequency range that is more similar to the original signal at low bitrates compared to encoders without SBR.

A listening test between mp3 and HE-AAC with music and speech shows that HE-AAC is perceived as equal in quality, despite having lower bitrates. If the used bitrate of the mp3 file is much higher than the bitrate used with HE-AAC, mp3 is perceived as better in terms of quality.

SBR allows lower bitrates while keeping the perceived quality equal. The fewer bits are available, the more SBR increases in efficiency. If the bitrate is not an issue, other encoders such as AAC with the highest possible bitrate can achieve a better result approaching the quality of lossless signals.