Audiotec Fischer Tech Talk #3

The quality of the input signal is essential for a perfect sound experience and an optimal signal processing. However, especially in OEM integrations the signal is mostly already manipulated by the OEM head unit.
In these cases our Input Signal Analyzer (ISA) with its integrated InputEQ comes into play – It allows to analyze and compensate such a manipulation of the input signal before the signal is processed by the DSPs sound optimizing functions.
In addition the InputEQ provides amplitude-linear allpass filters to ensure an in-phase addition of the left and right channel which is especially important if subwoofers shall be added to installations where the OEM head unit is using allpass filters for their manipulations.

But why should I set an amplitude-linear allpass filter?
Allpass filters are often used in French and American vehicles instead of a time alignment to realize a wider stage image.
If the channels are summed up, typically the summation of the channels leads to a broadband cancellation in the lower frequency range and therefore a poor bass reproduction.
This issue can be solved by using an allpass filter as a correction in the InputEQ and can directly be verified through the ISA.


The ISA and InputEQ benefits at a glance:

Input Signal Analyzer:

  • Tool for the frequency response measurement of an analog input signal.
  • Adjustments of the InputEQ can be seen in the measurement with the ISA as well. This allows to instantly verify all InputEQ adjustments.
  • Multiple input channels can be summed up before measurement to detect phase differences between each input signal.
  • The summation can be adjusted in level and polarity for the optimum summing ratio in multi-channel active speaker systems. The result of the summation is directly visible in the ISA.
  • The proportional summation of the measured signal can be transferred as "routing" directly into the IO channel routing to optimize the work-flow.



  • Used for the correction of the frequency response of input signals which have already been manipulated (generally in OEM head units). The goal is to linearize the frequency response before the signal is processed in the DSPs sound optimizing functions.
  • Allows the use of amplitude-linear allpass filters to ensure an in-phase addition of the left and right channel for subwoofer integrations e.g. in special applications where Peugeot OEM head units are used.


The following illustration shows the most important steps for correcting a frequency response:

  1. Open the Input Signal Analyzer (click on the controller symbol in the top right corner of the IO menu).

  2. Select an input channel – multiple selections for a summation is possible via drag & drop as well.

  3. Playback a correlated pink noise test signal via your OEM head unit.

  4. Start the first signal measurement – the result will be displayed after approx. 20 seconds.


  5. Adjust the InputEQ and wait for another measurement – the optimizations will be visible after a short period.

  6. Repeat steps 4 and 5 until the input signal is as linear (flat) as possible.

The Input Signal Analyzer is part of our new ACO technology and therefore included in all current Audiotec Fischer products with ACO platform: