Back to overview Documentation version 9.23


Delossifier section

The DeLossifier attempts to clean up the effects of lossy compressed (MP3 etc.) input.

Lossy compression (such as MP3 compression) reduces the amount of data needed to store or stream audio by 'throwing away' things that it deems 'nearly inaudible'. Especially at lower bitrates, this causes very clearly noticeable artifacts that are typical for lossy compression.

Beside the fact that these artifacts are often already noticeable, the processing that is done by Stereo Tool (or any other processor) invalidates the assumptions that the compressor has made on which effects are noticeable and which are not.

The DeLossifier attempts to detect these artifacts, and to clean them up.

Main panel

Attempts to remove pre-ringing from lossy compressed audio files.

One of the typical artifacts in MP3 and other lossy compressed files is pre-ringing. This means that a sound that should start suddenly, has a sort of 'pre-echo': Instead of starting suddenly you first hear it come up softly, and then there's a sudden jump when the sound was really supposed to start. This gives the typical 'smeared-out', non-punchy sound on lower bitrate MP3s.

This filter attempts to detect pre-ringing, and reduce it.

Important: Because to be able to determine whether a sound is caused by pre-ringing or it is a sound that should be there, this filter needs to look ahead a lot, which adds a lot of extra latency.

  • Delossifier
    Enables the Pre-Ringing Killer

Pre-ringing killer (big latency!) panel

Settings that control MP3 pre-ringing removal.

Pre-ringing is the effect that a suddenly starting sound, after MP3 encoding, doesn't start suddenly anymore, but "fades out" in both directions in time. Post-ringing would be similar to reverb, which is not really annoying, but pre-ringing is very unnatural (a sound is audible before it actually starts).

Removing pre-ringing means that an analysis is needed where both the pre-ringing and the actual sound are taken into account. Because the actual sound starts later in time, pre-ringing requires a relatively large lookahead, and causes extra latency.

  • Pre-ringing killer
    Enables the pre-ringing killer.

    This causes a lot of extra latency.

  • Difference
    Plays only the sounds that are determined to be pre-ringing.

    If Left channel only (testing) is enabled, the pre-ringing sound is played on the left channel, and the audio on the right channel is unaffected. This makes it easy to hear if only pre-ringing is removed (the sounds stop at the moment when a kick is played), or more sounds are removed.

  • Maximum relative volume reduction
    Determines how strong the pre-ringing killer works.

    At 100%, the audio that is determined to be pre-ringing is removed completely. At higher levels, more audio is removed. This may cause 'gaps' before punchy sounds, but it also improves the dynamics further.

  • Left channel only (testing)
    For testing: Pre-ringing is only removed for the left channel.

    This also affects Difference.

Spectral hole filler panel

Controls the Spectral Hole Filler part of Delossifier.

MP3 and MPEG2 encoding at lower bitrates causes holes in the spectrum - typically, the louder sounds are encoded until there are no bits left, anything that's not encoded yet at that point is turned into silence.

This is a very unnatural effect which normally never occurs in natural sounds, and the Spectral Hole Filler attempts to put reconstruct the audio that was removed, and puts it back.

  • Spectral Hole Filler
    Enables the Spectral Hole Filler.

  • Strength
    The relative maximum amount of audio that the Spectral Hole Filler can add.

  • Threshold
    Detection threshold for detecting sounds that are removed by MP3/MPEG2 compression.

  • Detect MPEG
    Detects MPEG compression, and disables Spectral Hole Filler if the input signal doesn't appear to be compressed.

    In some rare cases, the Delossifier could add artifacts on audio that wasn't even MPEG compressed. This setting analyses the input, and turns the whole filter off if the input doesn't appear to be MPEG compressed.

    While it's not 100% accurate, the meter next to the button can be used as an indicator of MPEG compressed audio. If it's on during most of a track, it has probably been MPEG compressed.

  • Maximum background noise
    Maximum noise level, added after MPEG compression.

    The Spectral Hole Filler assumes that audio that has been thrown away by MPEG encoding is completely silent. If the MPEG compressed audio is sent through an analog studio, for example, some noise might be present, and this setting can be used to make the Spectral Hole Filler ignore that noise in its determination of whether something is an MPEG compression artifact or not.

  • No reconstruction below
    Disable any Spectral Hole Filler action below this frequency.

    Typically, low frequencies are normally encoded very well by MPEG codecs, and wrongly reconstructed audio at very low frequencies can sound bad.

    This setting controls above which frequency the Spectral Hole Filler works.

  • Do not reconstruct top part
    Disables reconstruction of very high frequencies.

    The Spectral Hole Filler is good at reconstructing small gaps in the audio, but less good at reconstructing large areas. In many cases, in lower bitrate MP3/MPEG2 encoded files a lot of high frequency content is removed. If the Spectral Hole Filler reconstructs this audio it can sound weird.

    A better solution is to use Spectral Hole Filler in combination with Absolute Highs - a link between the two still needs to be made...

  • Slow rise
    Controls how quickly the maximum frequency at which Spectral Hole Filler works follows a burst of highs.

    In many MP3/MPEG2 encoded files, the highs (especially above 16 kHz) are removed, except at some spots where they were very loud. If we were to suddenly reconstruct the whole area upto the highest frequency that was reached for a very short moment, the highs could become very restless (present - not present - present - not present) - which is already an issue with these files, but it would actually get worse due to this.

    By not responding instantly, but only when the high frequencies are present for a longer period, this restlessness is avoided.

Input spectrum panel

Displays the input spectrum of the Delossifier.

  • Show histograms
    Shows the input and output spectrum as histograms instead.

Output spectrum panel

Shows the output spectrum of the Delossifier.