View Full Version : Difference between front and rear outputs

03-31-2011, 10:56 AM
Someone posted a question about a head unit in the modifications forum and I thought it would be great to pose it here as I have wondered what the difference is.

The question concerned a particular HU, but could really apply to any unit:

Does anyone know if the Front and Rear pre-outs get the same signal (assuming the fade & balance is neutral)?

Please weigh in on what part of the recording is sent to Front and Rear, if they are the same, if they should be the same, etc. Thank you to all contributors!

03-31-2011, 11:21 AM
Unless you are in the realm of a video soundtrack, there is only L & R material. The deck will have the identical material on both front and rear outputs unless you are dealing with a processor which should not apply here.

Earmark Marine

03-31-2011, 11:44 AM
That's what I thought. That's why they call it "stereo", right? Unless you are going all digital with 5.1 or something and putting in a surround sound speaker system there are only 2 channels, L and R.

ian ashton
03-31-2011, 12:17 PM
Thats also what I thought, but I figured before I made the assumption I'd ask, especially with all of the digital media options, I didn't know if iPod docking had front/rear or something crazy like that :)

03-31-2011, 12:55 PM
Typical mobile head units distribute left and right signals equally front to rear. Waveform-wise, left-front is the same as left-rear electrically.

There are some mobile decks, (Sony MEX-DV2200 comes to mind) where you can have the rear outs be "surround channels" but this tyoe of head unit is exceedingly rare.

Now for something completely off topic. Don't read unless you are curious about audio. I apologize in advance for pulling this down this length path. If there is interest, I will start a new thread.

Off-topic-expansion-of audio discussion here:

BTW, in a most simple explanation, surround sound is usually only encoded in a two-channel format. Ever notice that when you used to hook up a video player, (DVD, or tape format) there were only two audio cables, despite being Dolby, Dolby Pro Logic, Dolby 5-bazzillion??? :) It is simple math folks... (I know, how does math come into music... let me explain)

For surround sound, you already have the concept of left and right down. That takes care of your left front and right front speakers. For the center speaker, the basic formula is L+R=C We take the left and right recorded information and sum them electronically. Anything that was recorded at equal levels in the left and right speaker gets summed to 2X and anythig that is not recorded at equal levels dues not sum so completely.

Now for the fun stuff:
Left Rear Surround
Electrically, the equation is Left -Right = Left Rear Surround. The information in the right channel is flipped to 180-deg. out of phase and then mixed with the left channel. Anything that is common to both the left and right channels at equal volume is cancelled, i.e. all center channel information... All that is left is the stuff that is recorded way to the left or the right, the ambience and echos and effects that fly over head in a hard-panned effect.

Right Rear Surround
Electrically the equation is Right - Left = Right Rear Surround. It is the same information as the Left Rear Surround, yet at 180-deg. phasing....

The magic happens in our brain when we put the 4 or 5 speakers around our heads properly, (I don't believe in center channel speakers, I will share the history if anyone is curios, but won't write it here).

The Left Rear Surround channel is largely in-phase with Left Front so the common left sounds tend to pull your brain that way when there is a left effect. The RIght Rear Surround channel is identical to Left Rear, but out of phase. However, it is largely in-phase with Right Front so the combination of Rght Front and Right Rear Surround gets your head pulled to the right for right-tilted effects....

You can play with this on your own pretty easily. There are several ways, but the easiest way is with a dual voice coil subwoofer. take a stereo amplifier, running full range and connect the left channel to the first coil +to+ and -to- (normal connection). Playing a song with vocals, you can easily hear the vocal. Now connect the right amp channel to the second coil, but do it +to- and -to+, basically backwards. When you make the connection the vocal will almost completely disappear, and what will be left will be echos and reverb of the voice, with other musical sounds kinda hovering and echoey rather than coming thogh cleanly. The two voice coils aer both working playing each of their information, but the stuff that is recorded at teh same level for both left and right goes away, as one coil is trying to go forward with "x" strength, while the other coil is trying to go backwards with "x" strength also. It cancels out and the vocals go away.

I am using this simple surround method in my sailboat to get surround sound, although I am mixing signals electronically in the front-end of the amp via remote programming. It is pretty neat... :) There is a way to do the same thing by using a simple 4-channel amplifier if anyone wants to know how though... :)

In summary:
Basic surround sound follows this simple math:
L = Left Front
R = Right Front
L+R = Front Center, (and sub)
L-R = Left Rear
R-L = Right Rear

And Now I return you back to your regularly scheduled programming. Thanks for indulging this propeller-head off-topic audio diversion...


03-31-2011, 01:42 PM
This is good stuff, Phil. Can we assume that "digital" surround sound is just the same formula encoded digitally (and likely with excess noise removed)?

03-31-2011, 02:12 PM
This is good stuff, Phil. Can we assume that "digital" surround sound is just the same formula encoded digitally (and likely with excess noise removed)?

I don't know that answer. If I were guessing, and I think I am at least half-right, that the digital component talks more to some digital computing logic applied to further isolate and steer the audio signals, (based on this basic math and taking it a little further) rather than converting the signals to digital and then back. David at Earmark might be able to elaborate more to what goes on with digital...