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FiX™: The Importance of Factory Time Correction

A central feature of the FiX™ OEM Integration Processors is their ability to automatically detect and correct time delay in factory audio signals. Many vehicle manufacturers are implementing time delay in their stock audio systems. In a complex 4-way system for example, the mid-bass, midrange and tweeters for the front stage and subwoofer in the rear may all have their own delays, allowing the different signals to reach the driver or a specified position at the same time.  This may work well for the stock audio system, allowing for better staging and an improved listening experience, but it can cause major issues when trying to rescue a clean signal to use with an aftermarket system build.

In order to successfully add an aftermarket full range amplifier to an OEM multi-way system, whether two-way or three-way with the FiX™ 82 or 86, or four-way (FiX 82 only), the individual band-limited channels need to be summed into full range (20 Hz - 20 kHz) left and right signals.  Any delays that are present on the various band-limited signals become troublesome in the frequency ranges where these signals overlap (the crossover regions). Attempting to sum (combine) these time-shifted signals into a full-range signal, results in what is known as “comb filtering distortion” in the crossover regions.  Below is an example of a four-way system with separate subwoofer, mid bass, midrange and high frequency channels, at different levels and often with individual time delay, which is not visible in a read out of the frequency response.

JL Audio FiX 82 Time Correction
Example of a complex four-way OEM audio system

What is “comb filtering”? Essentially, when two signals with common frequency content attempt to combine, one of which is delayed relative to the other, the result is narrowly spaced constructive and destructive interference (in-phase/out-of-phase in adjacent frequency ranges). The resulting response looks like the teeth of a comb on a frequency response plot, which is where the term “comb filtering” gets its name. See the image below for an example of comb filtering. 
JL Audio FiX 82 Time CorrectionSummed signal showing comb filtering distortion prior to equalization

In this real-world example of four-way factory audio signals being summed, the major dips in the summed frequency response can be seen at the factory audio system’s crossover points, where two signals with shared frequency content are meeting in the same frequency range at different times. Comb filtering can be observed extending beyond the major dips. 

These dips and comb filtering effects caused by delayed input signals cannot be effectively corrected by adjusting the appropriate frequencies on an equalizer. In the image below you can see the frequency response can be improved by the EQ where the signal is free of comb filtering effects, but the comb-filtered areas remain largely unimproved. These are the best results you can expect with conventional summing preamplifiers and EQ correction.

JL Audio FiX 82 Time CorrectionEqualization can't correct comb filtering as it's an issue with time

The FiX™ difference: In order to properly correct the frequency response seen above, the time delay between the factory audio signals must be corrected before summing and equalization is attempted. This is exactly what the FiX™ OEM Integration Processors do. When the calibration begins, the FiX™ measures signal delay in each input channel and then calculates and implements additional delays to bring all of the individually delayed channels back together in time. Once the channels are time-coherent, they can be successfully summed into a complete full-range signal, without comb-filtering at the crossover points. The final step of the FiX™calibration process measures the resulting summed frequency response and applies a powerful 30-band equalizer in each channel to remove the factory system’s equalization from the signal. The end result is a flat, full range signal to send to a tuning processor or amplifier(s).

JL Audio FiX 82 Time CorrectionSummed signal from the FiX™ after Time Correction is applied

JL Audio FiX 82 Time CorrectionThe final corrected response from the FiX™ after equalization is applied.

The FiX™ processors are the first products to automatically correct delay prior to summing and equalization. Thanks to these correction capabilities, the FiX™ OEM integration processors provide the best possible audio experience when using factory audio signals.

 

 

 

 

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