The new app "SCHOEPS Image Assistant v3" is available as a browser version and in the Apple app store.

The Image Assistant was originally created as a JAVA Applet v2.1 by Helmut Wittek in the year 2000. It is a tool to calculate the localisation curve of arbitrary 2- and 3-ch stereophonic microphone arrays. With the new app, even more helpful features are available:

  • Offline usage
  • Calculated curves:
    • Localization curves
    • Level and time differences
    • Coherency/Correlation in the diffuse field
    • Power sum (loudness)
    • Prediction of all measures in and out of the sweet spot
    • Auralisation of the stereo setup including a correct simulation of the localization and correlation properties in the direct sound and the diffuse field


Help Guide

to the IMAGE ASSISTANT v3.0.2

For the urgent user:

Press the Menu button in the top left corner:

Choose a Preset in the top dropdown menu:

Toggle between different graphs by pressing the buttons in the bottom right corner of the figure:

Localisation shows the directional function of the setup

Signal Difference shows level and time-of-arrival differences

Coherency shows the coherence between the channels in the diffuse field

Power Sum shows the function of the overall sound energy

Use the bottom menu bar to toggle between the Visualisation tab to view the function graphs and the Auralisation tab to listen to the results:

For the thorough user:

- Presets

- Graphical Preview of the setup

- Left, Center, Right

- Base

- Center Offset

- Epsilon

- Preferences: Off-Axis Listening Position

- Preferences: MISCELLANEOUS: Distance

- Preferences: MANIPULATION

Bottom menu bar





- POWER SUM view



Predefined 2ch and 3ch stereophonic microphone array setups.

By choosing a setup of the dropdown list, the values of the corresponding parameters are updated automatically and the graphs are recalculated.

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Graphical Preview of the setup

A to scale preview of the stereophonic microphone array setup is shown in real time during a change of the parameters.

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Left, Center, Right

The directional pattern of the microphones. Each microphone is routed discretely to one single loudspeaker channel from L/C/R. The Center channel can be skipped for a 2ch L/R stereo setup.

The directional patterns are all omni and first order gradients which can be described by the function s = a + (1-a) *cos(φ).

Where φ describes the input source angle, a the omni portion and s the output level of the microphone.

You can choose a directional pattern from these options:

  • Omni (MK 2) – a=1
  • Wide Cardioid (MK 21) – also called hypocardioid or subcardioid – a=0.66
  • Open Cardioid (MK 22) – a=0.58
  • Cardioid (MK 4) – a=0.5
  • Supercardioid (MK 41) – a=0.37. In the jargon often called hypercardioid. However a hypercardioid is defined with a=0.25
  • Figure-8 – a=0

Often people ask whether it plays a role that these patterns are calculated theoretically and not taking into account the frequency dependency of real capsules. In fact, there is a discrepancy, but this depends on the brand and type of the microphone. In principle, small-membrane microphones are more frequency-independent than large-membrane microphones. Moreover, all SCHOEPS capsules are optimized particularly for their frequency independency, so that virtually no difference exists between the theoretical values and the practical measurements (of each produced microphone capsule!) within nearly the whole frequency range. An exception is the omni which intentionally increases its directivity at higher frequencies due to the pressure built-up effect.

Here are the measured polar patterns of the SCHOEPS MK capsules corresponding to the above list:

Omni (MK 2)

Wide Cardioid (MK 21)

Open Cardioid (MK 22)

Cardioid (MK 4)

Supercardioid (MK 41)

Figure-8 (MK 8)

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The distance between the microphones L and R.

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Center Offset (only visible when a Center microphone is chosen)

The distance between the baseline which connects the two microphones L and R and the microphone C.

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The angle between the main microphone axis and the 0° middle axis of the setup. Note that the angle between the microphones L and R is twice this angle! Hence, for a classical ORTF setup epsilon is 55°.

The Center microphone is always oriented at 0° and cannot be rotated.

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Preferences: Off-Axis Listening Position

In the Localisation view you can toggle between the on-axis and an off-axis listening position which can be set here. In the on-axis mode the listening position is in the exact sweet spot and the loudspeaker setup is standard (“normal”) with all three loudspeakers located on an ideal circle with the sweet spot in the center of that circle.

For the off-axis mode, the Speaker Setup can be changed to be a cinema-like “one-line” setup with the Center located in the middle of the connection line between loudspeakers L and R.

For the off-axis mode, also the width of the speaker setup (Speakerbase) and the Vertical Offset and the Horizontal Offset of the listening position can be set. By clicking Rest Speaker Settings the default is reset: the default off-axis listening position is the chair positioned on the left of the chair in the sweet spot.

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Preferences: MISCELLANEOUS: Distance

The distance of the sound sources used for the calculation of the Localisation curve. This parameter plays a significant role if the distance of the sound source is not much bigger than the distance between the microphones. In this case, two effects are taken into account:

  • the paralaxis which means the angles between sound source and the three microphones differ
  • the 1/r-law makes the sound level higher for closer microphones. For example, a Decca tree using big distances between the microphones (1-2m) and a usual sound source spacing (3m) actually relies much more on the level differences than on the time of arrival differences!

By clicking Reset Miscellaneous the default distance (5m) is reset.

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Sometimes it is required to apply an electronic level change or a delay of a channel in order to create a suitable stereo microphone array setup. This can be set here.

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Bottom menu bar

Toggle between the VISUALISATION tab to view the function graphs and the AURALISATION tab to listen to the results.

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The view button in the bottom-right corner toggles the view between Localisation, Signal Differences, Coherency and Power Sum.

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Localisation view

The function of the perceived phantom source direction vs. the angle of the input source in the recording stage is shown. The ordinate shows the phantom source direction on a scale between -100% (in the Left loudspeaker), C (in the Center resp. exactly between L and R) and +100% (in the Right loudspeaker). The abscissa shows the input source angle between -90° (left edge of the stage) and +90° (right edge of the stage).

The Legend tells whether the stereo image between L and R (Left-Right), L and C (Left-Center) or C and R (Center-Right) is shown and depicts the Recording angles:

  • the Recording angle 100% (light-gray box) is defined as the angle between input source angles which are perceived between the loudspeakers. The Recording angle 100% is commonly used by the tonmeister for matching the directional image of the stereo microphone array with the geometry of the ensemble of sound sources.
  • the Recording angle 75% (dark-gray box) is defined as the angle between input source angles which are perceived in a range between -75% and +75%. The Recording angle 75% is particularly useful to compare similar stereo setups as most of the input sources are located in this range of input source angles and also most deviations between level and time stereophony happen between 75% and 100% phantom source shift.

The On-axis/Off-axis button in the bottom-right corner toggles the listening position. On the Off-axis listening position the perceived directional image is distorted due to the changed levels and time-of-arrivals of the loudspeaker signals. This function is useful to compare the robustness of a stereophonic microphone array with respect to listener movements and to estimate the size of the listening area produced by this array.

Only in 3-Channel mode: The LR/LCR button in the bottom-right corner toggles the view of the stereo pairs. You can view the stereo image between L/R (Left-Right) or the two stereo pairs L/C (Left-Center) and C/R (Center-Right).

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The level and time-of-arrival differences.

The LEGEND depicts that you see the Level differences ΔL (green bars) and the time-of-arrival differences Δt (blue bars). Both add up to create the phantom source direction. Conflicting level and time-of-arrival differences result in a blurred phantom image.

Also here, you can toggle the listening position with the button in the bottom-right corner.

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The Coherency functions in the diffuse field between the microphones LR and - in 3-Channel mode - between the microphones L/C.

The diffuse field coherency is a measure of the spatial image created by the stereophonic microphone array. The lower the diffuse field coherency, the better is the spatial image.

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The power sum of all channels vs. the input source angle.

With that measure you can predict the loudness distribution of your stereophonic microphone array.

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Listening to the directional and the spatial image predicted and simulated by the Image Asisstant v3 by convolution.

Set the desired input source angle and press PLAY to listen to the simulated sound image.

Both the correct directional pickup of the stereo array is simulated as well as the correct diffuse field coherency. Use this function e.g. to assess the differences in the spatial image between a narrow XY stereo pair an a wide A/B pair.

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