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Stereo Tool 7.83 - Help - Stereo

Stereo section

Several filters that repair and change stereo separation.

AZIMUTH section

Reparation of AZIMUTH (phasing) errors.

AZIMUTH errors are often present in tape recordings, and also on some cheap CDs. Phasing problems causes playing a recording in mono or through a surround system to result in very ugly artifacts. But even normal stereo playback may sometimes sound a bit unpleasant. The phasing offset is automatically detected and removed by this filter.

This filter only works properly if the sounds at the left and right channel are similar. If this is not the case for a longer period, the azimuth correction will slowly be reduced.


  • Azimuth correction enabled
    Enables AZIMUTH correction.

  • AZIMUTH limit
    Maximum tape head displacement (assuming cassette tapes) to be detected and resolved.

    0 disables this filter. Suggested value: 40 μm.

  • AZIMUTH change speed
    Maximum speed at which the filter follows detected phasing errors.

    This slider controls how fast the azimuth correction can change if the measured displacement is differs from the correction that is taking place.

    Use a very low value ( 0.1 m) to correct for a constant azimuth correction.

    Use a higher value (~ 0.4 m) to also correct for rapid changes.

    To avoid getting too much effects from measurement noise, use the lowest possible value where the blue line (the actually performed correction) can keep up with the red dots (the measurements).

Stereo Boost section

Stereo Boost is the preferred filter to add stereo separation to music.

It basically just uses a multiband compressor to increase the level of the L-R channel. It can be configured to never reduce stereo separation (in tracks with completely separate channels) and it does also not create too much (anti-phase) boost of the L-R channel.

Stereo Boost panel

  • Stereo Boost enabled
    Enables the Stereo Boost filter.

  • Protect against excessive reverb
    Removes sounds (usually reverb) made louder by a strong boost of the L-R channel.

    Reverb often gets increased a lot when using strong Stereo Boost settings. Some people like this - but if you want to keep the audio closer to the original, turn this on.

    In extreme cases, this can cause artifacts.

  • Never reduce below original level
    Never allow the stereo separation to get reduced by Stereo Boost.

  • Stereo Boost strength
    Configures how strongly Stereo Boost works.

    Higher values sound 'wider', but also increase certain sounds (see Protect against excessive reverb) and may cause multipath problems with FM reception (see Stereo multiplier, Multipath clipper and Stokkemask FM).

  • Stereo Boost maximum amplification
    The maximum increase of the L-R channel.

  • Stereo multiplier
    Multiplies the L-R channel with this value.

    This can be used to reduce extreme stereo separation. For example, if Stereo Boost strength and Stereo Boost maximum amplification are increased and this slider is decreased, the stereo separation for most tracks should not change much, but extremes will be removed.

    Values below 1.00 can be beneficial when using Multipath clipper and Stokkemask FM.

Stereo Image section

Stereo Image gives independent control over phase differences and instrument placement. But it can easily create artifacts.

The most interesting feature is to convert stereo to mono without cancellation (loss) of sounds. This creates a sound that is as full as the stereo image, but it is in fact mono. This can be done by setting both Image phase amplifier and Image width amplifier to 0.00. If you are not located between the speakers, when you press the MONO switch on your audio system you often still hear a big difference, because a lot of the sound disappears. After setting these two sliders to 0, that doesn't happen anymore. Example uses are radio stations (FM, AM, streaming) that broadcast in mono, and people who are deaf in one ear who want to listen with headphones.

Several sliders of the Stereo Image manipulator cause severe artifacts and should only be used as described. They are marked as 'partially deprecated'. Please read this section carefully if you enable the Stereo Image filter.

Stereo Image panel

  • Stereo Image enabled
    Enables the Stereo Image filter.

  • Center bass
    Prepares the bass to be played on a system with a single subwoofer.

    If there is a phase difference in the bass between the left and right channel, and the sound is played using a single bass speaker, the bass will get deformed and lowered in volume. If this filter is turned on, phase differences for bass sounds are removed completely, which solves this problem. This occurs only very rarely. Note however that when the Phase shift slider is set to a high value, this will occur much more frequently. When listening with headphones, this somewhat reduces the stereo effect.

    In some cases this filter can cause severe artifacts.

  • Phase shift
    Adds a constant phase offset between the two channels.

    This slider can be used to add a phase offset between the channels. Both -180 and +180 cause the channels to be the opposite of each other, 0 is normal output.

    Image phase amplifier is performed first!

  • Image phase amplifier
    Increases or decreases phase differences between the left and right channel, without touching instrument placement.

    Moves between 0 (no phase differences between the channels), 1 (no change) up to 8 (8 times as much phase difference as in the original signal). 0 is VERY useful for converting to MONO, the resulting sound can be downmixed to mono without any distortion or loss of sounds, which occur in normal stereo to mono conversion. This creates a much fuller and undistorted mono sound. Note that "0" does not mean that the output signal is mono, because the instrument locations are not affected by the phase slider. To get mono sound, also set Image width amplifier to 0.

    This filter creates artifacts, mainly for values above 1.00. When playing compressed audio, especially lower (< 192 kbit/s) bandwidth MP3 files, setting phase to a high value will also very strongly amplify the already present MP3 encoding artifacts, which results in a very poor sound quality.

  • Image width amplifier
    Changes the placement of sounds without changing their phase differences.

    Moves between 0 (all sounds in the center, 1 (no change) up to 8 (the sounds are moved 8 times further away from the center than in the original signal, if possible of course). Note that "0" does not mean that the output signal is mono, because the phase differences are not affected by the width slider. With this setting, for someone who hears both speakers it still sounds like stereo, but if someone hears only one speaker all the sounds are present.

    Setting width to a very high value will almost always introduce artifacts, so it should be used with care - or not at all.

  • Image phase amplifier maximum separation strength
    Sets the maximum allowed level separation per frequency.

    Could be useful for example for FM radio stations, this can ensure that the maximum phase difference stays below a certain level, which reduces signal loss when a receiver switches to mono.

    This setting currently introduces artifacts. It should not be used unless it is really necessary. Changes may be made to it in later versions.

  • Image phase amplifier maximum angle
    Maximum phase difference between the left and right channel.

    When the total phase difference between the two channels gets above this level, the Image phase amplifier is reduced temporarily to keep it below the level that is set here.

  • Mono or stereo only
    • -100 %: Play ONLY the mono sounds.

    • 0 %: Don't do anything
    • +100 %: Play ONLY the stereo sounds.
    If an instrument is only present on one channel, -100% will completely remove it. If an instrument is present at the center, +100% will completely remove it.

    Note: Image width amplifier is performed first!

    Use 'stereo only' with care: High values can cause annoying artifacts.

ACR Stereo section

ACR Stereo is to date the most effective stereo enhancer in Stereo Tool.

ACR Stereo adds a copy of the L-R (stereo, difference between channels) information to the original stereo signal. This creates a sparkling stereo effect. In cases when the stereo effect in the input signal is already very strong, or almost not present, this filter switches off automatically. On top of these things, the stereo effect is stronger for transients.

Several users have reported that the bass also sounds much better with this filter enabled.

This type of stereo widening has very little impact on reverb (which often gets boosted by other types of stereo widening). When used for FM, it also has very little impact on multipath distortion (see also Multipath clipper).

ACR Stereo panel

  • ACR Stereo enabled
    Enabled ACR Stereo.

  • L-R Level Boost
    Maximum boost of the L-R signal that's added to the original stereo signal.

    This is only the maximum, the effect can be reduced by Limit below, Stop all action if input is wider than and if there's almost no L-R signal present, Or smaller than.

  • L-R Delay
    Amount of delay of the L-R signal that is added to the original stereo signal.

    When using more delay, the sound appears to 'open up', however it might start to sound like an echo, especially for high frequency transients.

    When using 0, there is no delay and the existing stereo information is boosted.

  • Limit below
    Do not add more than this amount of L-R stereo.

    This level is relative to the mono (L+R) audio level.

  • Stop all action if input is wider than
    Turns the widener off completely if the input is already very wide.

    If the input signal already contains a lot of stereo content, the ACR Stereo is turned off.

    This is particularly important when playing audio with 100% channel separation such as old Beatles tracks where voices are often recorded on one channel, and the instruments on the other. If a delayed (L-R Delay) version of the signal would be added, it would sound like an echo on the complete signal. And if there's no delay, it would cause anti-phase signals to appear in the audio.

  • Or smaller than
    Switches stereo enhancement off when the signal is nearly mono.

    If the left and right channel levels are not perfectly equal, sounds such as a DJ speaking through a microphone would constantly be moved off-center even more - and if a delay (L-R Delay) is used, an echo would be audible all the time. To protect against this, the filter switches off is the signal is nearly mono.

Input, Extra Stereo & Limiter panel

Shows the input (top bar) and added (bottom bar) amount of stereo signal.

The bottom bar also shows if the effect is reduced (gray bar) by Limit below, Stop all action if input is wider than or Or smaller than.

Bandpass panel

Controls the frequency range on which the stereo widener works.

Widening deep bass sounds is useless and may actually reduce the reception quality. The human hearing cannot distinguish where bass comes from (unless you're using headphones).

Widening high frequencies in combination with a delay may cause an echo-like effect. Some people like this, others don't.

  • Highpass from
    Frequencies below this frequency are not widened at all.

  • Highpass to
    Frequencies above this frequency are widened by the maximum amount.

  • Lowpass from
    Frequencies below this frequency are widened by the maximum amount.

  • Lowpass to
    Frequencies above this frequency are widened by End level.

  • End level
    The amount of stereo widening for frequencies above Lowpass to.

Punch detection panel

Controls if transients are widened more than constant sounds.

Widening transients more helps to protect against boosting reverb. It also tends to keeps voices in music more audible.

  • Volume effect
    Controls how much effect punch (transient) detection has.

    At 0%, everything is boosted maximally regardless of detection of punch. At 100%, the amount of widening for constant tones appoaches none at all.

    Setting this too high may reveal some unwanted effects and cause the total effect of the stereo widener to be less than desirable.

  • Fadeout
    Controls how fast the volume drops down to Volume effect after a transient.

  • L+R Contrast
    Increases effect of detected transients (punch).

    Punch detection often doesn't reach 100%; this setting boosts the effect to reach maximum widening. The detected punch will never exceed 100%.

    For punch detection, the minimum of L+R and L-R Contrast is used.

  • L-R Contrast
    Increases effect of detected transients (punch) in the stereo (L-R) signal.

    See also L+R Contrast. To protect against unwanted effects (a mono-only sound which is not present in the L-R stereo signal but would still affect the stereo separation), the minimum of L+R Contrast and L-R Contrast is used to detect punch.

  • Attack
    Attack speed for punch detection.

  • Release
    Release speed for punch detection.

  • Relative volume release
    Controls the difference in release speeds between 2 compressors to determine punch.

    Punch is detected by looking at the difference in amount of attack between two compressors. This slider determines how much slower the release works for the 2nd compressor.

Response speeds panel

  • Lookahead time
    Lookahead time for the total behavior.

Fake Stereo section

Adds a fake stereo effect. Intended to make mono recordings sound more like stereo.

Fake Stereo panel

  • Fake Stereo enabled
    Enables the Fake Stereo effect.

  • Channel delay (left)
    Delay between the left and right channel.

    This introduces a (very rudimentary) stereo effect. Note that this also changes recordings that are already in stereo, that it also makes sounds like voices stereo (which is generally considered bad), and that the result will sound very bad when played back in mono.
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