Signal summing / cancellation Question / Challenge .... - Car Audio | DiyMobileAudio.com | Car Stereo Forum

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Old 06-09-2019   #1
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Question Signal summing / cancellation Question / Challenge ....

I'm trying to understand if the phase inverting and signal summing capabilities of some DSPs (Helix?) might help me deal with a challenge I'm facing ….

Background
  • > 2019 Honda Ridgeline RTL-E (with ANC disabled via module disconnect)
  • > OEM amp/DSP has 8 output channels, one for each speaker (FR L&R Tweeters, FR L&R Woofers, FR Center, Rear L&R, Sub)
  • > Most vehicle generated voices and tones are on FR Center along with a mono music signal apparently consisting of simple L+R. So, using "V" for vehicle voices and tones distinct from music, let's call the OEM FR Center signal (L+R+V) for this discussion.
  • > There is no method within my capabilities to 'extract' "V" as a discrete analog signal from the OEM system ("V" is generated and combined with the L+R music within the OEM amp/DSP module based on several digital CAN inputs + analog music input from the "HU" and I'm not an electronics hacker)

Objective

Try to 'cancel' the L and R (music) component of the (L+R+V) FR Center OEM amp output so that the FR Center can be used as a DSP input>output to retain vehicle generated voices and tones without screwing-up music imaging / staging.

My (maybe wrong) Understanding of Theory
  • In theory, this phase inverting and signal summing scheme would achieve my objective: [(L+R+V)+([email protected]*)+([email protected]*) = V] , is that correct?
  • For that to work, each instance of L and R must be the exact same amplitude (as well as frequency of course) at every instant in time, is that correct?
If my above understanding of theory is flawed, please let me know, stop reading, and thanks for your time (any other approach for solving my challenge would be most appreciated)

Real World Implementation (?)
  • I believe that in my case the Rear L and Rear R signals 'match' the frequency of the respective FR Center L & R components (but maybe not close enough to cancel if phase-inverted due to minute differences in the amp/DSP circuitry?).
  • I have no reason to expect the Rear L&R signals to match the amplitude of their FR Center counterparts, and I'm skeptical that I can force an 'exact amplitude match' sufficient to achieve cancellation using level-setting controls in either the OEM system or the external DSP.

Bottom Line Question - Am I on a "Fool's Errand" pursuing this line of thought for upgrading my Ridgeline system in a way that offers 'no compromise' for both music and vehicle features?

Anyone see a different possible / better way to accomplish the 'no compromise' objective given the constraints descibed?

Thanks In Advance


Last edited by FordEscape; 06-09-2019 at 12:00 PM..
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Old 06-11-2019   #2
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Default Re: Signal summing / cancellation Question / Challenge ....

Sounds solid to me. You're right that the frequency and amplitude must match but you must also match the timing too. If the front center is delayed relative to the front left and right channels you might get excellent cancellation at lower frequencies and mild or no cancellation at higher frequencies. Should be really easy to test one way or another.

Depending on how sophisticated your new DSP will be, you could also think about ducking either the main inputs or some combination of the output channels so that the chimes/voice content can play over the top without making much interference. Sometimes a DSP will have an auxiliary input channel that can do this. I think my old Mosconi 6to8 had a feature like that but I don't remember exactly.

If you can do your mixing and summing on the input side of things, that might be best? You could also get creative by looping outputs back to inputs and inverting channels and so on. Maybe something like a surround-sound decoder could be helpful in a pinch since it also does summing of reverse-polarity signals.

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Old 06-11-2019   #3
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Default Re: Signal summing / cancellation Question / Challenge ....

Thanks for the reply Justin.

OK, so maybe not totally hair-brained.

I guess the next step is to pull 'taps' from all of my OEM amp outputs to a 'breadboard' of sorts (terminal strips on a piece of wood I'm thinking) just for further testing.

I'm a semi-competent user of the 2-channel PicoScope bought a few years ago. Perhaps I can overlay the Center with various L&R output plots to see if I get a 'match' in all respects needed when only playing 'music' (test tones / noise).

Unlike my Escape the Ridgeline has no adjustments for 'seating position / occupancy' so I'm hopeful timing will not be an issue (suspect it makes no attempt at TA at all).

As far as tones/voices over music … I think that would be taken care of by the OEM amp's muting scheme:


The notion being (IF I can work through this center channel/vehicle tones separation thing) to use an 8Ch in > 8Ch out DSP with all channels routed to their OEM speaker positions. Use the DSP to allow XO, EQ, and TA tuning with new drivers in a 2-way front + 1-way rear-fill + sub arrangement for music with the center dedicated only to the vehicle tones & voices currently on the center. Tune the system without any vehicle voices/tones then let the 'muting' that occurs on the input take care of the voice-over-music muting.
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Last edited by FordEscape; 06-11-2019 at 08:56 PM..
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Old 06-12-2019   #4
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Default Re: Signal summing / cancellation Question / Challenge ....

Quote:
Originally Posted by FordEscape View Post
....[*]In theory, this phase inverting and signal summing scheme would achieve my objective: [(L+R+V)+([email protected]*)+([email protected]*) = V] , is that correct?[*]For that to work, each instance of L and R must be the exact same amplitude (as well as frequency of course) at every instant in time, is that correct?
...
Yeah (L+R+V) -L -R =V
So it is the same...

A possible problem could be is the channels do not have the same filtering, and say a LR24 and a butterworth is used...
Then at the band edges, the phase would not be 0, so 180 would not relate in subtraction.
(But just try it)

If the amplitude is off by 10%, then you will only get 10dB of suppression.
If the amplitude of -L or -R is 1% then you get 20dB.
(Which may be -20 and -40dB as wattage goes as voltage squared)
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