Polk SDA effect vs. headphone crossfeed

nspindel

I started a thread on headfi a few years ago to debate the differences between the theory behind the SDA effect and the crossfeeds that are built into many headphone amplifiers.

The idea behind headphone crossfeed is that under normal conditions, music mastering in a studio uses monitors, not headphones. As we SDA-heads all know, this means that a little left signal makes it to the right ear, and a little right signal makes it to the left ear of the recording engineer. Since that interaural crosstalk, to put it in Polk terms, is present during the mix session, a certain amount of it is designed to be heard, or at least that's what was heard by the recording engineer doing the mix (unless they were using Polk SDA's in the recording studio). So headphone crosstalk in headphone amplifiers is designed to blend a bit of left signal into the right ear, and vice versa. The blending happens at about a 300ms delay, since in the loudspeaker world the signal from the right speaker takes slightly longer to reach the left ear than it takes to reach the right.

You can see that this is the exact opposite of what the SDA's are trying to accomplish. As I said in my headfi thread, when I bought my original SDA's, the sales pitch on the technology was something along the lines of making speakers sound like headphones - making sure the left ear only hears the left channel, and the right ear only hears the right channel. Headphone crosstalk makes headphones behave like speakers - the left ear should hear a bit of right and the right ear should hear a bit of left.

Here's the thread on headfi:

SDA vs. Crosstalk

I'm curious to hear this same discussion take place on this forum. Please feel free to comment, but PLEASE let's keep this civil! I'd like to make sure that this thread stays on topic and is an intelligent debate on the different theories, and doesn't degrade to insults if someone disagrees with what you think!!!

As for my own thoughts, the jury's still out. I've listened to both, and think both sound great. Even though these two theories are completely juxtaposed, I'm nowhere near a point where I can label one as "right" and one as "wrong". Let's hear what others have to say....

ShinAce

Correction, the time lag between ears is more like 0.300 ms. Speed of sound is roughly 1000 feet per second and the distance between the ear pathlengths can be estimated at 4", or in this case, 1/3000th of a second.

Also, the crossfeed applies to certain frequencies. Frequencies below about 1 KHz are attenuated a tiny bit before being fed to the opposite channel. As the head becomes a better screen at higher frequencies, there is a roll-off. The crossfeed acts as a low-pass filter(12 dB/oct, IIRC).

I'm not 100% familiar with the SDA technology, nor have it heard it myself. That's the most I can add for now.

p.s. Crossfeed helps get rid of the 'sound inside your head' feeling of headphones and lessens listening fatigue. It restores the soundstage as it should. I have also noticed a number of recent songs that appear to have an inherent crosstalk which lessens the value of the crossfeed. User beware.

lightman1

Well fed SDA sounds astonishing, a good set of headphones sounds just as good. A great set of headphones sound Ok with a so-so amp and SDA's sound nice with the same.

Open acoustic vs. psyco-acoustic. Both have their benefits.


My .02

hoosier21

WTF is this, an actual audio discussion. GREAT post


Let me get some thoughts together, just wanted to say Thanks

nspindel

Funny, I thought there'd be more takers for this thread. I think everyone's too tired from arguing whether or not to put a cap in series with the SL2000...

Marty913

nspindel wrote: »

Funny, I thought there'd be more takers for this thread. I think everyone's too tired from arguing whether or not to put a cap in series with the SL2000...

You may be right Neil, or it could just be the Easter thing, Spring Thing, etc.

Speaking for myself, I wasn't aware that crosstalk was even built into headphone amplifiers. But then again, I have never liked headphones and don't listen to music that way (I could volunteer my services as the ignorant contributor). It seems that Polk's SDA-effect efforts do make it the odd man out approach to speaker design and would be an opposite to those that would want headphones to sound more like "traditional" speakers.

A worthwhile question, just not one that affects me one way or the other beyond the curiosity / general knowledge level.

Schurkey

I bought a Headroom portable headphone amp (Total Bithead: http://www.headphone.com/products/headphone-amps/the-mobile-line/ ) about six months ago. I had to have the salesman demo the crossfeed circuit because I couldn't hear a difference just flipping the switch while playing my music. I though it might have been broken. No, with the right demo material the crossfeed circuit was clearly audible.

Have had SDA-1Bs since '86 or '87. There's NO DOUBT when the cable is disconnected or re-connected.

Both play games with cross-feed, and both crossfeed-circuits are bandwidth-limited. Polk restricted the bandwidth more and more as the SDA-series was refined.

One wonders if the two systems don't kind of "meet in the middle" between fully-separated listening (normal headphones, and the Headroom system adds some "blend";) and fully-open/unrestricted crosstalk of normal speakers, and Polk added "anti-noise" to enhance the separation.

nspindel

That's actually a very interesting point. While it is true that normal headphone listening does achieve 100% separation, it's highly unlikely that the SDA effect eliminates interaural crosstalk, it merely reduces it. So there's still a bit of that crosstalk present, and the headphone crossfeed then introduces some of it. So the "meet in the middle" concept may really be what's happening, which is why they both sound good!

Joe08867

nspindel wrote: »

That's actually a very interesting point. While it is true that normal headphone listening does achieve 100% separation, it's highly unlikely that the SDA effect eliminates interaural crosstalk, it merely reduces it. So there's still a bit of that crosstalk present, and the headphone crossfeed then introduces some of it. So the "meet in the middle" concept may really be what's happening, which is why they both sound good!

It would be almost impossible to eliminate the crosstalk. you would still have the problem of reflected sound. Make in the polk Chamber they has total elimination but in most living rooms and even listening rooms there is still going to reflection.

Very interesting discussion. I don't listen to alot of music via headphones but I may do a little study of my own SDA vs Headphones.

lightman1

Sennheiser headphones, any source (!) and a nice HP amp...Y'all know what I mean.;)

George Grand

There is still a little cross-talk conducted through your jawbone when listening to headphones, so it is not eliminated 100%.

Schurkey

Schurkey wrote: »

Have had SDA-1Bs since '86 or '87. There's NO DOUBT when the cable is disconnected or re-connected.

As a sidebar to this thread: I pulled the interconnect cable on these speakers last night--first time I've listened to them without the cable for something like fifteen years. I'm starting a new thread on my experience.

http://www.polkaudio.com/forums/showthread.php?t=66082

ShinAce

George Grand wrote: »

There is still a little cross-talk conducted through your jawbone when listening to headphones, so it is not eliminated 100%.

Chances are that the brain would ignore such a signal in the opposite ear. I'm not going to research acoustic perception to prove it though. The speed of sound by conduction is very fast and the signal will reach the other ear so quickly that the brain will see it as just that, a conducted signal. It also risks being extremely attenuated.

Another signal that your brain ignores is a fast echo. Imagine you're in the forest and an animal about 20 feet away lets out a shriek upon seeing you. The direct sound to reach your ears will be first, followed by numerous and quick echos which should dither the first sound. Luckily the brain is adapted to substract these echoes. It's something along the lines of the first 10 ms of echoes are ignored.

Marty913

"Another signal that your brain ignores is a fast echo. Imagine you're in the forest and an animal about 20 feet away lets out a shriek upon seeing you. The direct sound to reach your ears will be first, followed by numerous and quick echos which should dither the first sound. Luckily the brain is adapted to substract these echoes. It's something along the lines of the first 10 ms of echoes are ignored."

I'm thinking that if this happens, the echo's are gonna have to catch up with my ****.

F1nut

Nice try putz. You've been reported!

nspindel

Huh? Where'd this thread get dug up
from???

zingo

I never caught this thread the first time around, but it's an interesting thought. Do all headphone amps and jacks do this?