Tech Tip: Helix DSP Software Tech Tip - Setting Up Differential Rear Fill & Using Real Center

       Helix DSP Differential Rear Fill is a commonly sought-after rear speaker setup for car audio hobbyists using any of the Audiotec Ficher DSP’s. It is used for its ability to help “enhance” the front speakers by simulating the late reflections of a larger room. Implemented correctly, this can result in a sound system that feels larger than the boundaries of the vehicle. How it works is simple. It is a Left minus Right/Right minus Left signal. This L-R/R-L signal is a sum of left and right, but the reversal of polarity of one side results in the cancellation of all of the correlated information and leaves you with only the left and right information. Once the signal is set, you can add extra delay (again, to help simulate the reflections of a larger room), adjust crossovers, equalize, level adjust, etc to fit your needs/taste. When equalizing differential rear fill you will either need to use uncorrelated (stereo) pink noise, or just temporarily turn off the differential signal as correlated (mono) signal will not play through the differential signal routing. 

Things To Note

. From personal experience, you want your rear speakers as high, wide, and far back as possible for this to work well. Down low in the rear doors does not work well, but can still be better than nothing for some.

. I recommend using 3″ or so wideband/full-range speakers. This allows for an easy installation that gets you the full 200-20,000hz spectrum needed.

. Before applying the extra delay, I still recommend setting delays to account for the path length difference to the listener.

. High pass this signal no lower than 250Hz or so. You do not want any bass or mid-bass frequencies in your rear fill as it is harder to de-correlate them to your front speakers. 

. Speaking of crossovers, if your front stage is a 3-way, it is best to just match your rear fill high pass filter to your front midrange high pass filter. This way you do not have to worry about possible phase issues and cancelation when playing with lower delay settings. Technically, the appropriate crossover to use would be 12db slopes, this only yields no issues if your front midrange crossover slopes are also 12db, or if there is enough delay applied to the rear speakers to decorrelate them from the front speakers. I personally use a 12db LR or 24db LR high pass, and a 6db BW low pass (if I decide to use a low pass at all).

. As you can tell, I am not speaking in definites. Nothing about differential rear fill is technically accurate to the source material. You are PURPOSELY adding something that was never meant to exist, so dialing this in by ear to fit your subjective wants/needs/taste is highly encouraged. As a starting point, I typically end up with at least 14ms of delay, usually more, and start by matching the high pass crossover to the front midrange, run the top end wide open, and go from there.

Things You Can Try

. Give Stereo Rear Fill a try instead of differential signal, with or without the extra delay. You might prefer it. I find these setups are better for those who prefer something that feels more “fun” and energized as opposed to perfect SQ. This is usually what I end up with when doing lower rear door speakers for customers.

. Instead of using Left minus Right/Right minus Left, you can use the Real Center and Virtual Channels of certain Audiotec Fischer DSP models to do Left minus Center/Right minus Center. What this will do is give you hard left on the left speaker, and hard right on the right speaker. With a traditional differential signal, you get left and right on both sides.

Speaking of Audiotec Fischer’s Real Center..

       In the vast majority of applications, only the sum of the right and left front channels is used to generate a signal for the center speaker. However, this means that the spatial stage image is restricted and the sound on the driver’s side ultimately only takes place between the left front speakers and the center speaker. The same applies accordingly to the passenger side, where the stereo panorama is reduced to the area between the center speaker and the right front speakers.

Audiotec Fischer’s proprietary “RealCenter” is a much more complex approach in which the center loudspeaker only reproduces the monophonic elements of the music, i.e. only the information that is simultaneously available on the right and left front channels. Stereophonic information that is only contained in the left or right channel is not fed to the center speaker.

The advantage: The width of the spatial stage image, which is generated by the stereophonic components, remains completely unchanged with the RealCenter, since these are still only reproduced by the left and right front speakers.

Advice:

Without Virtual Channel Processing:
       For a proper operation of the algorithm it is mandatory that the “Front Left” and “Front Right” signals must be present on channels A and B, as well as a sum of “Front Left” and “Front Right” on the actual center channel.

With Virtual Channel Processing:
       For a proper operation of the algorithm it is mandatory that the “Front Left” and “Front Right” signals must be present on virtual channels “Front L Full” and “Front R Full”, as well as a sum of “Front Left” and “Front Right” on the virtual “Center Full”.

NOTE: Real Center is technically not an upmixer, but instead a center signal extractor. There is a difference. Personal experience.. while possible, it is NOT EASY to get a great sounding 2-seat system, but it does help with getting something that is definitely better than a traditional mono center.