Waveflex Caraudio

DSP-NX concept

The DSP NX is a totally new type of processor when compared to products available on the established automotive DSP market.

If fills a gap in the automotive acoustic correction.

The automotive passenger’s compartment is indeed far more hostile to audio signals than for instance your living room. Being the sole invisible element between your speakers and your ears it’s also most probably the last one you would think off to correct.

Nonetheless the passenger’s compartment is the most degrading factor for the sonic result of your installation. To obtain a high fidelity sound in your car, it takes much more than properly installing high-end devices. It is absolutely necessary to identify the acoustical flaws of the passenger’s compartment in order to correct them.

Enter now the world of digital correction and take the quintessence of your installed devices.

  1. Foreword:
  2. In addition to the hostile behavior of your car interior, your head is not centered between the speakers. The sonic result is heavily degraded both in amplitude and phase response. In our cars it’s always difficult to obtain a realistic sound stage for every frequency range.
    Mostly achieving a proper impact in the low-mid band reveals difficult while respecting the center sound stage.

    Even after equalizing both sides separately, some frequency bands will still not be centered properly. It’s impossible to correct this using usual IIR processing. This is due to phase conflicts between left and right speakers. The concept of phase is not an easy task to understand. First of all, it’s important to understand that phase is relative to a listening position (or the microphone used during the measurement protocol). The phase response of a loudspeaker will vary depending on its position and/or the positions of our ears.
    Very simplified we can say that phase represents the moment of arrival of the different frequencies at our ears for a given speaker position.
    Let’s have a look how phase is represented with the help of some curves and why it causes problems in the stereophonic sound stage and why it is affecting timbre and dynamic.

    Red curve: Amplitude response of right loudspeaker.
    Blue curve: Amplitude response of left loudspeaker.
    Dashed red and blue curves: Phase response of left and right speakers.

    You will note that the amplitudes curves are totally different for the left and right speakers. This is due to different causes like reflections and resonances. If the processor used offers a high enough precision we can correct most of it applying dissymmetric equalization for left and right. Phase curves in our example above also diverge, mainly between 70 and 700Hz right where we find the fundamental frequencies of instruments and human voices. If we look at the black boxed area, we can see differences up to 180 degrees!
    This equals to saying that in this frequency band the left speaker is in total phase opposition to the right speaker thus purely and simply cancelling the concerned frequency band.
    We all have an experience with an electrically out of phase wiring. The results are highly degraded dynamics, a lack of timbre and an incoherent sound stage.
    When looking at the curves you can see the biggest gap in phase at 120Hz. In this frequency band we find the impact of the upper low range.
    If we trace the curve representing the difference between the phase response of the left and right loudspeaker we obtain the green dashed line hereunder.

    Dashed red and blue curves: Phase response of left and right speakers.
    Green curve: differential phase curve.

    Looking at the green dashed curve, you will see that the most important phase deviation is in the 70 to 700Hz frequency band. In this area, the resulting sound is altered (timbre and sound stage).
    We can confirm this by summing the left and right speakers taking into account the phase response (complex summing):

    Red curve: Amplitude response of right loudspeaker.
    Blue curve: Amplitude response of left loudspeaker.
    Green curve: Complex summing of both loudspeakers.

    The resulting curve shows some areas (green box) where the summing of the two loudspeakers is nearly perfect. In this box the amplitude of the green curve is 6dB higher than the red and blue curve.
    You can also see serious alterations (red boxes), going as far as total cancellations (phase opposition) at the frequencies of 120 Hz and 220 Hz.
    For this pair of loudspeakers and for the given listening position sound reproduction is heavily compromised.

    Now we will have a look at how the DSP NX will help us to solve these issues.

  3. Using the DSP NX :
  4. Due to its FIR processing capabilities (see application notes FIR / IIR ?), the DSP NX is capable of modifying independently frequency response without altering phase response and vice versa.
    The DSP NX allows you to solve one of the major and at the same time the most overlooked issue in automotive sound reproduction: the acoustical phase of loudspeakers.

    1. Amplitude correction
    2. We will start to equalize both left and right amplitude.
      It’s common practice to realize the equalization on the main DSP (DSP A8) which is used for filtering and time alignment purposes. The input of the multichannel DSP (DSP A8) is connected to the output of the DSP NX.

      Hereunder is a direct comparison of raw and corrected amplitude curves for the left and right loudspeakers.
      We have used a maximum of 5 parametric equalizing bands.

      Right loudspeaker:

      Left loudspeaker:

      Red curve : raw amplitude response.
      Blue curve : equalized amplitude response.

      Overlay of the equalized left and right speakers amplitude response curves:

    3. Simultaneous phase and amplitude correction
    4. Here is what we get after applying phase correction to our equalized amplitude response.

      Right loudspeaker:

      Left loudspeaker:

      Red curve: raw amplitude response.
      Blue curve: equalized amplitude response.
      Red dashed curve: raw phase response.
      Blue dashed curve : corrected phase response.

      Hereunder the acoustic summing of both corrected loudspeakers:

      Red curve: equalized amplitude response of right loudspeaker.
      Blue curve: equalized amplitude response of left loudspeaker.
      Red dashed curve: corrected phase response of right loudspeaker.
      Blue dashed curve: corrected phase response of left loudspeaker.
      Green curve : acoustic sum of the equalized left and right loudspeakers.

      In the curve plot above, we notice the added value of the DSP NX and its abilities to solve phase conflicts issues. It allows after equalization and phase correction to obtain a near perfect summing over the total frequency spectrum. This is confirmed by the overall gain of +6 dB (green curve) in comparison to the curve of each speaker.

      Now let’s have a look at the differences between the summing curves with and without phase and amplitude corrections.

      The overall sonic performance is literally transfigured. All the fundamentals sound fuller. Sound stage is now stable and centered. No more dull sounding speakers. The overall efficiency of the lower audio spectrum goes up and reduces the need for amplification power. Overall distortion of the system is reduced as well.

  5. Conclusion
  6. The DSP NX allows to recover the stereo summing by a precise and totally free correction of phase and amplitude. This makes the DSP NX a unique product on the Caraudio market.

    - increased efficiency
    - increased dynamics
    - reduced overall distortion
    - reduced needed power
    - recovered sound stage stability
    - recovered timbre