DIG­I­TAL BITS

Reg­u­lar In­Car tech con­trib­u­tor Stephen Daw­son takes us on a voy­age of dis­cov­ery through the ex­pan­sive and trans­for­ma­tive ter­rains of in-car time align­ment and EQ.

InCar Entertainment  - - CONTENTS -

Tech writer/re­viewer Stephen Daw­son takes us on a jour­ney across time… align­ment that is. Stephen dis­cusses the im­por­tance of co­her­ent fre­quency ar­rival at the lis­ten­ing po­si­tion for best sound qual­ity in the car.

There is an in­her­ent prob­lem in car au­dio. One quite apart from power sup­ply lim­i­ta­tions and hav­ing to com­pete with the noise in a car. at prob­lem is the lo­ca­tion of the speak­ers. In a proper home stereo sys­tem the po­si­tion of the lis­tener and the two loud­speak­ers are some­where close to the three points of an equi­lat­eral tri­an­gle. In a car, that is sim­ply not pos­si­ble. In a right hand drive car the driver will be closer to the right hand speaker than to the left. Adding more speak­ers com­pli­cates mat­ters fur­ther.

As a re­sult, with­out cer­tain fa­cil­i­ties in the car’s head unit, e ec­tive stereo and there­fore ac­cept­able sound­stage and imag­ing prop­er­ties are es­sen­tially im­pos­si­ble in the car.

Be­fore get­ting into what these cor­rec­tive elec­tron­ics do to x the prob­lem, let’s un­der­stand the prob­lem a lit­tle bet­ter.

Stereo is fake

Two chan­nel stereo sound is ac­tu­ally widely mis­un­der­stood. Well done, with ne loud­speak­ers, qual­ity elec­tron­ics and care­ful loud­speaker place­ment and an ap­pro­pri­ate en­vi­ron­ment it can sound thrillingly re­al­is­tic and three di­men- sional. But this is at best a highly re­al­is­tic fake, just like 3D video.

Both stereo sound and 3D video sys­tems work by de­liv­er­ing sep­a­rate left and right sig­nals to your left and right sen­sory or­gans: ears and eyes re­spec­tively. One way that you see three di­men­sion­al­ity in real life is by your eyes see­ing things at slightly di er­ent an­gles. So two cam­eras are used to cap­ture views from di er­ent an­gles, and those are duly de­liv­ered to your eyes via, for ex­am­ple, a Blu-ray 3D sys­tem and the spe­cial eyewear which makes sure each eye sees only what it’s sup­posed to.

But that di er­ent an­gle thing re­ally only works for rel­a­tively close ob­jects. To make it work for dis­tant ob­jects, spe­cial 3D cam­eras are used with their left and right lenses much fur­ther apart than your eyes are. An­i­mated lms show much greater di er­ences in the di er­ent eye views than you would see with an equiv­a­lent scene in real life.

We de­tect the di­rec­tion of sound in a num­ber of ways, but per­haps the most im­por­tant one is in tim­ing. Let’s say that you are stand­ing in an open eld and look­ing straight ahead. Some­one is stand­ing ve me­tres in front of you, but a me­tre to your left. Even if your eyes are closed, when he speaks you will in­stantly know his po­si­tion fairly ac­cu­rately to the left or right, although you will only have a rough idea of his dis­tance. You can tell largely be­cause his voice will reach your left ear very slightly be­fore it gets to your right ear.

His voice will send sound waves more or less di­rectly to your left ear. But his voice has fur­ther to travel to your right ear, both thanks to the an­gle, and also be­cause the sound then has to curl around part of your face be­fore reach­ing your ear.

e time di er­ence is not much. If the air tem­per­a­ture is 20-de­grees Cel­cius, sound trav­els at 343.5 me­tres per sec­ond. Us­ing Mr Pythago­ras (and al­low­ing 20cm for the width of your head), that time di er­ence works out to be just 400 mi­crosec­onds. But you don’t hear the voice twice, once from each ear slightly out of sync with the other. Your brain fuses the two sounds into one, and merely notes that be­cause the left ear’s sound is slightly in ad­vance of the other ear’s, the source of the sound must be, well, just about there.

Two Sources

Stereo mu­sic kind of repli­cates that. In­stead of your ears, the sound is cap­tured by two mi­cro-

phones (keep­ing it sim­ple for these pur­poses). is sound is then con­veyed through the record­ing and play­back chain to two loud­speak­ers, one to your front left and the other to your front right. e sound from the left speaker will reach you rst. If your head is pointed right be­tween the two loud­speak­ers, your left ear is go­ing to get the sound 400 mi­crosec­onds be­fore your right ear does.

So, of course, it’s just like you were hear­ing the sound di­rectly in the rst place, right?

Well, not ex­actly. Now there are two sources of sound in­stead of one. e left hand speaker emits its sound and it gets to your left ear. But the left hand speaker’s sound will also go to your right ear, de­layed by that same 400us. In fact, at just about the same time as the sound from the right speaker reaches your right ear. And then the right speaker’s sound will, 400us later, hit the left ear.

With all this go­ing on, it’s as­ton­ish­ing that the stereo e ect works at all. For­tu­nately, the brain does a lot of sig­nal pro­cess­ing. Your an­ces­tors didn’t live long enough to re­pro­duce if they couldn’t spin around and ing a spear in the right di­rec­tion when they heard a car­ni­vore ap­proach. A process called bin­au­ral fu­sion gath­ers all the de­layed sounds – which in real life would merely be re ec­tions and echoes from var­i­ous en­vi­ron­men­tal sur­faces – and pack­ages them up with the orig­i­nal sound to avoid con­fu­sion, adding only a sense of air or re­ver­ber­ant space from which one gets a feel of things like the en­vi­ron­ment and dis­tance of the sound.

ose sorts of con­fu­sions can ac­tu­ally be elim­i­nated us­ing bin­au­ral record­ing tech­niques and lis­ten­ing through head­phones, but that’s another story en­tirely.

Time is of the essence Amaz­ing as our brain’s sig­nal pro­cess­ing is, it can be tricked by tim­ing di er­ences, es­pe­cially arti cial ones. If the same sound is be­ing pro­duced by two di er­ent sources at slightly di er­ent times – up to 40 mil­lisec­onds di er­ence – the two sounds will fuse into one, sub­jec­tively, and that one sound will seem to be com­ing ex­clu­sively from the source of the rst sound to ar­rive. It will typ­i­cally do this even if the sec­ond source is up to 10 deci­bels louder.

Sound re­in­force­ment pro­fes­sion­als use this trick in large con­cert venues – loud­speak­ers fur­ther back in the au­di­to­rium o er a slightly de­layed sound which, as a re­sult, seems to be com­ing from the stage in­stead.

In the car So you’re driv­ing along in your right hand drive Aus­tralian car. Let’s keep it sim­ple: you have just front speak­ers, with the tweet­ers in­stalled where the

top of the dash­board meets the A-col­umn. Rep­re­sen­ta­tive dis­tances might be 860mm from your head to the right speaker, 1260mm to the left. The dif­fer­ence is 400mm. Your favourite singer’s voice is com­ing out of both speak­ers equally, but it’s reach­ing you first from the right speaker, which is closer, by a bit over one mil­lisec­ond in time.

That’s enough so that the singer’s voice doesn’t sound like it’s com­ing from the cen­tre of the dash­board, nor from di­rectly in front of you, but from the right speaker. That kind of spoils the ex­pe­ri­ence. Add rear speak­ers as well and the com­pli­ca­tions pile up.

It’s not just your sense of di­rec­tion that can be mucked up by the tim­ing of the sound from the speak­ers in your car. Got a sub­woofer? Its out­put may in­ter­fere with the out­put of your main speak­ers around the cross­over fre­quency. If the tim­ing is wrong, the sub­woofer might be mo­men­tar­ily pres­suris­ing the air in the car’s cabin just as the main speak­ers are de­pres­suris­ing it. Power into the sub­woofer be­ing spent on coun­ter­act­ing more power from the head unit! Both sub­woofer and main speak­ers should be push­ing at the same time at those crit­i­cal cross­over fre­quen­cies.

The fix

All these things are easily fixed ... if your head unit has the right pro­cess­ing built in. This is vi­tal for the best per­for­mance.

It’s usu­ally called ‘Time Cor­rec­tion’ or ‘Time Align­ment’, or some­thing sim­i­lar, and it al­lows you to de­lay the sound in­di­vid­u­ally to each speaker and the sub­woofer to bring them all into co­her­ent align­ment.

Set it cor­rectly and the left front speaker will pro­duce your favourite singer’s voice while the right speaker waits a mil­lisec­ond and then is­sues its con­tri­bu­tion. Be­cause of that de­lay, the sound from both speak­ers reaches your left and right ears at the same time. Now the voice sounds like it is right in front of you, rather than dragged off to the right.

Like­wise, both front speak­ers need to be de­layed by a few mil­lisec­onds to al­low the bass from the sub­woofer to reach your ears at the same time as the bass from the front speak­ers. Rather than sub­tract­ing as the other speak­ers are adding, the speak­ers and sub will be co­op­er­at­ing in pump­ing air at those cross­over fre­quen­cies.

The Time Cor­rec­tion setup will be some­where in the au­dio set­tings of your head unit and will typ­i­cally al­low you to set the dis­tance each speaker is from the driv­ing po­si­tion. Just de­ploy a tape mea­sure.

Sub­woofer 2nd step

Ah, a tape mea­sure. If only that’s all it took! In gen­eral, that will do the trick in the time cor­rec­tion of the main speak­ers, but the sub­woofer may be a dif­fer­ent mat­ter. The sub may be from a dif­fer­ent man­u­fac­turer.

If an ac­tive sub­woofer then it will have its own am­pli­fier, or you could have a ded­i­cated am­pli­fier with pos­si­bly dif­fer­ent char­ac­ter­is­tics to the other am­pli­fi­ca­tion used within the sys­tem. And in many cases the acous­tic path isn’t straight­for­ward (such as when it’s in­stalled in the car boot). Dial in a naive dis­tance mea­sure­ment, and the re­sults may be unim­pres­sive, with weaker than ex­pected bass around the cross­over fre­quency.

Just wind­ing up the EQ prob­a­bly won’t fix that. If the weak­ness is due to the sub­woofer be­ing out of phase with the other speak­ers, boost­ing the ef­fected re­gion will just waste power as the speak­ers will con­tinue to work against each other.

So a fi­nal step for sub­woofer time align­ment is to do it the old fash­ioned way. Put on some mu­sic with plenty of mid-bass fre­quen­cies – say the higher strings on a bass guitar. (Re­ally deep bass won’t help here be­cause that’s be­ing pro­duced by the sub­woofer only. What we want is con­tent han­dled around equally by both the sub and the main speak­ers.)

So put on this style of mu­sic, go to the setup panel, and ad­just the time de­lay on the sub­woofer, step by step. Try both di­rec­tions. What you want is the po­si­tion where that mid-bass is loud­est. That will mean that all the speak­ers are work­ing in phase at those fre­quen­cies.

Con­clu­sion

Have fun with the en­tire time align­ment process but don’t be com­pletely self­ish. If your head unit pro­vides mem­o­ries or pre­sets, then it can be worth set­ting up sev­eral dif­fer­ent ar­range­ments. One would be op­ti­mised for the driver, one for the front pas­sen­ger, and per­haps have one op­ti­mised for the cen­tre of the car, in case you want to share a some­what com­pro­mised sound with a car full of peo­ple.

Time align­ment, and in­deed EQ in gen­eral as well as other sound­tai­lor­ing func­tions are now avail­able on smart­phone plat­forms — this one is from Alpine.

A mis­aligned 3D im­age can serve as a good il­lus­tra­tion of what hap­pens with in­co­her­ent sound sources. Time align­ment al­lows the sound from dif­fer­ent sources to fuse into one.

Aud­i­son’s bit Ten (and the bit One too) of­fers sub­stan­tial EQ, fil­ter/cross­over set­tings and time align­ment ca­pa­bil­i­ties. Rock­ford Fos­gate’s 3sixty.3 in­ter­ac­tive sig­nal pro­ces­sor’s ex­ten­sive on-screen EQ con­trols.

This re­cently re­viewed Alpine head unit fea­tures time align­m­net ad­justable at the ‘Front Left’, ‘Front Right’, ‘Rear Left’, ‘Rear Right’ and left and right for the sub­woofers too.

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