HeadRoom Crossfeed vs. Polk SDA

[nspindel]

Hopefully this catches Tyll's attention....

About 12 years ago or so, Polk Audio stopped manufacturing what I always thought was one of the greatest consumer-audiophile lines of speakers I had ever heard. By consumer-audiophile, I mean speakers in the $1000-$5000 per pair range. Above the amount that would be considered consumer, but not priced so far into the stratosphere that only the most serious audiophiles would be compelled to buy them.

This line of speakers used a technology called SDA (Stereo Dimensional Array). These speakers were very unique in that there was a distinct left and right speaker, they were not meant to be interchanged. The back of the speaker actually said "Left Speaker" or "Right Speaker". Most notably, in addition to the cables between the speakers and the amplifier, these speakers had another cable that ran between the two speakers.

The purpose of this wire was to facilitate each speaker putting out an inverted signal of the opposite speaker. I purchased a set of these speakers back in 1989, and I can still remember the discussion that I had with the salesperson who handled the Polk line in the store where I bought them. He described the fact that when a person listens to loudspeakers, the left ear can hear the sound from the right channel, and vice versa. The salesman used the specific comparison to headphone listening - "When you listen on headphones, only the left channel reaches the left ear and only the right channel reaches the right ear - SDA is meant to make the speakers sound more like headphones." The SDA technology sought to eliminate that crossover, with each speaker putting out a cancellation signal of the opposing speaker. I'm not a scientist by any means, but what I can say was that the resulting imaging that these speakers put out was absolutely stunning, unlike anything else I could find in that price range. The soundstage extended much wider than the two speakers, and you clearly perceived a depth to the music as well. You could detect that the drums came from behind the guitars, for example.

Here is how the Polk manual described this technology: "SDA, TRUE STEREO TECHNOLOGY: HOW DOES IT WORK? SDA Technology is a means of reproducing a much larger and more realistic sound stage than can be achieved by conventional speakers. The way that this is accomplished is to make sure that only the original recorded signal reaches the listener's ears. The original recorded signal is entirely contained in the direct sound of the left signal reaching the left ear and the direct sound of the right speaker reaching the right ear. Normally when a person listnes to a conventional pair of stereo speakers sound from each speaker reaches each of the person's ears. The extra signals, crossing the listener's head to reach the ear on the opposite side, can be thought of as distortion which causes the sound field to be constricted and shallow. SDA Technology uses the acoustically inverted dimensional signal to cancel the extra signals without affecting the original recorded signal reaching the listener's ears directly. The result is that only the correct original recorded signal reaches the listener's ears and the full width and depth of the sound stage are accurately reproduced."

Obviously, this technology is in complete opposition to the Crossfeed technology employed in the HeadRoom amplifiers. The Crossfeed, in layman's terms, is designed to blend a little bit of left signal into the right ear and a little bit of right signal into the left ear, in order to make the headphones sound a bit more like conventional loudspeakers.

The HeadRoom documentation says the following:

"Imagine you are listning to a pair of speakers. If you turn off the left speaker, both ears hear the sound from the right speaker. But because the left ear is slight farther away than the right ear, it hears the speaker's sound slightly after the right ear; about 300 miliseconds. This time difference is called the 'inter-aural time difference' and it is the main thing your brain listens for in order to tell where to place sound left-to-right."

"But in headphones if you turn off the left channel, only the right ear hears the sound. In headphones, if there is any sound that is only in the left channel, or only in the right channel, then only that ear hears the sound. This is not natural, and you brain becomes fatigued trying to figure out where the sound is coming from when only one ear is hearing it. This tends to create an audio image that is a blob on the left, blob on the right and a blob in the middle."

"HeadRoom amplifiers cure the problem by allowing you to cross-feed a little of the left and right channels across to each other through a short time delay using the crossfeed switch. The usefulness of the circuit varies depending on what type of recording you are listening to; mono and binaural recordings need no processor at all. Old studio recordings that have instruments panned hard left or right, benefit greatly from the processor. Live and classical recording miked from a distance benefit somewhat less, and can often be listened to without the processor quite comfortably."

It's interesting to me that HeadRoom describes this technology as a cure for the "problem". This would suggest that, at least in HeadRoom's opinion, Polk had things all wrong. I'll leave the opinions to the experts, but what I can say with 100% certainty is that if I were to disconnect the SDA cable with the Polk speakers, they did not image anywhere near as well. The sound stage would only be as wide as the distance that speakers were apart from each other, and there was no depth to the music at all.

I was hoping to solicit some informed opinions as to the validity of one theory vs. the other. Now I'm going to rest my weary hands.....

 

[Pete7]

I see your point, but Polk discontinued the SDA Technology in any of their speakers, right? I mean, I'm sure the price range had something to do with that, but then again maybe it was more like a failed experiment. Reverse crossfeed in speakersto to make them sound like headphones would? Sounds kind of hare-brained to me. I'm not sure an expert opinion is needed on that, just some common sense. I'm sure they produced a very unique sound, but it must've been a hard sell to potential customers. Maybe a little too radical.

 

[nspindel]

Actually, not true at all. I actually spoke to Polk about that question. The reason they discontinued the line was that the SDA line by default led to extremely large speakers. You needed twice the amount of drivers in them, as one driver would be putting out the actual signal for the speaker, the other driver would be putting out the inverted signal for the other speaker. So their flagship speaker in that line was almost 6 feet tall - there is a famous picture of Mathew Polk standing next to that speaker - 15" passive woofer, 8 midranges, 4 tweeters - it was as tall as him! What they told me was that in the early 90's and the advent of dolby pro-logic, the speaker market started shifting dramatically towards home theather/surround setups, where the push was for lots of speakers, not necessarily big speakers. They turned their attention to that market, and many Polk enthusiasts feel that the quality of their line has never been the same since. The SDA line continues to sell quite well as used equipment, be it on ebay or elsewhere. If you visit the forums on polkaudio.com, you'll see there is a whole section dedicated to vintage speakers, and there is still a very active SDA enthusiast community out there. I've spoken to Polk representatives who've told me that over the years they've toyed with the idea of bringing back the SDA line, but it's a matter of marketing to the masses - the volume is simply not in big huge speakers for a 2-channel rig any more, everything is about selling 5-7 (much) smaller speakers that are designed for a subwoofer/surround environment. But there are many at Polk who still to this day feel that the SDA line were the finest speakers that Polk ever produced. I'm in that camp as well, I think that company in many respects has "sold-out". One thing is certain, these are not hare-brained speakers....

 

[Oliver :)]

Well, you cannot completely eliminate signal crossover with speakers, you will always hear some of the left speaker with your right ear and vice versa. Unless you pump up the cancelling effect to insane levels, but then you would not hear anything anymore

and if you *could* (and you could, it involves two walls at a certain angle and you listening with your forehead pressed to a piece of foam at their joint and a acoustically "dead" soundtrapping wall all behind you...) it would sound as strange and unnatural as headphones without crossfeed. Although, employing crossfeed, you will always loose a certain amount of soundstage width, so maybe Polk managed to combine the best of both worlds?

 

[Tyll Hertsens]

I've been asked to comment here, so I will comply. Mods please don't read thi as advocacy but rather as an opinion contrary to the existing Polk design.

I'v long thought that this design was based on faulty thought bordering on being rediculous.

Lets assume that all recording engineers mixed on headphones without crossfeed or on these Polk speakers. In this case, it would be possible for the recording engineer to design a soundscape that was natural and immersive. But it's very likely that to do so they would have to synthesize what you hear on two normal speakers to some degree. Here's why:

Assuming the goal is to replicate what you would hear from 5 or 10 discrete sound sources. Sound from each source makes it to both ears with varying interaural time differences. Those time differences asociated with each sound source would be the primary localizing audiotor cue with which you would manifest a perception of an acoustic image.

In audio engineering there are ways to not only pan a signal from left to right with volume changes when sending the signal to left and right channels, but to move it left and right by delaying the signal on one channel. Because the left speaker is louder in the left ear (even though the right ear still hears some of it) and the right speaker is louder in the right ear the added delay to one chanel will move the image of the virtual source toward the undelayed side. However, it will be somewhat smeared by the acoustic cross talk of the speaker-to-ear relationship that is actually there---despite what the sound engineer is trying to paint. Therefor the holy grail of speakers is to somehow dissapear, and recreate the whole of the origional acoustic wavefront of 5 people playing instruments.

There is evedence to these ears that what you really want more speakers to recreat this wavefront as opposed to supply isolated channel signals to each ear. I will point to Bob Stuart's Meridian products that synthesize the appropriate signals for 3 or 5 speakers as an array across the front of the room. His stuff (as I recall!?) supplies the normal left and right signals to the left and right speakers, and creates a signal for a center and extreme left and right speaker channel. You would have to research the details, but I can tell you that I have been uniformly impressed by the stability and refinement of the image his systems make. Not only in the exact center but from your entire walk from the door and pretty much anywhere in the room.

Which bring me back to the Polks. Correct me if I'm wrong, but I understand they feed an inverted phase to an an additional speaker array in the opposite speaker in hopes of cancelling out the acoustic cross talk signal from the far speaker. Well what about the mono-componant of the stereo signal. That part that is the same on both channels. Left and right channels can easily be converted into sum and difference signals (with the left and right direction stored in the phase of the difference signal; this is also how "mid-side microphone techniques work with a omni or wide cardiod and figure 8 microphone) and the "sum" signal is large. In Polk speakers you have the
sum signal comming in phase from the left and right channel and out of phase from the compensating speakers. This causes chaotic (because all these speakers are spacially arrayed and also interact in a reflective and reverberant acoustic environment) cancellation of the central image and an unrepresentative of the origional intent broadinging outward.

Also, the Polk principle ONLY works when your head is at the exact center between the speakers. I don't like sitting there with my head in a clamp. (With headphones I can walk around wherever I want

) Any where else in the room the image is completely askew.

I think it's bull$#1t.

The trick, whether it's on speakers or headphones, is to recreate a wavefront that's natural and believable to your ears. I think a significant and accute understanding of the acoustics/psycoacuostic involved and subtle and exquisite tweeks to the audio signal are the right way to go.

 

[Tyll Hertsens]

I guess I didn't talk about the HeadRoom cross feed in particular, I would have to have a moderators OK to talk about what crossfeed is relative to the conversation.

If a mod wants to OK it, I will try to stay more on what headphone crossfeed is, as I undertand it, as opposed to what HeadRoom actually does in it's crossfeed to mimic the real acoustic space (which is yet a separate difficult compromise).

 

[nspindel]

Tyll, thanks so much for posting on this comment. I've been very curious to hear the opinion of the Polk SDA design from people like yourself since I started reading about (and listening to) headphone crossfeed, given that they are diametrically opposed viewpoints.

One thing I am curious about - have you ever had the opportunity to listen to this line? One thing I can say for certain is that they definitely image better with the SDA connection than without.

I'd love to see a debate between yourself and Matt Polk on this topic. It would be fascinating, although I'm sure I'd only understand 10% of what you'd say. But the profit I'd make selling the tickets would certainly fund an upgrade to the Desktop line!

Your posts are so informative!

 

[Tyll Hertsens]

Another way to answer the question of wheter it's a reasonable way to go is look how many other manufacturers are doing anything remotely similar.

Zero.

 

[nspindel]

Well, that may not necessarily be a valid argument. I'm sure there were patent issues preventing others from doing this during the time when Polk was selling this line. Nowadays, even though the patents would have expired, even Polk is out of the business becaue of the reasons I described earlier.

 

[Tyll Hertsens]

Alright, I was being too harsh.

It just seems so antithetical to what I understand of the problem that I just want to shreak.

 

[nspindel]

Quote:

Originally Posted by Tyll Hertsens Alright, I was being too harsh. It just seems so antithetical to what I understand of the problem that I just want to shreak.

Exactly the reason why I wanted to hear from you!

Sure you come across divergent opinions all the time when it comes to "this sounds better than that" kind of things. But it's not very often that you find two such completely opposite theories with respect to the science behind all of this.

As for me, I don't claim to be an audiophile - all I can say is that they both sound great!

Now if we could only get Matthew Polk to join this thread....

By the way, if you want to see what the flagship of this line looks like, here they are:

http://www.audioweb.com/Ad/AdInfo.asp?adid=157991


Wow, for sale in NJ for only $1000. Hmmm.......

 

[elrod-tom]

This is one of the more interesting threads that I've come accross in some time. Thanks to nspindel for bringing it up, and to Tyll for adding his opinion.

I've used Polk speakers since I was in college (by that I mean undergrad...a LONG time ago!!), back in the day when they made what many thought to be near-audiophile grade speakers for a reasonable price. I owned a pair of tiny Polk 4's for 20 years before they finally pooped out on me last year, and still own two pairs of vintage Polks (4's and 5's) to this day. I find them to be excellent for what I would call noncongested music (for God's sake, whatever you do, don't listen to Oasis' What's the Story, Morning Glory on them!!

).

A very good friend of mine had the smallest SDA speakers (sorry, the name escapes me right now), and I thought that they were very nice indeed. He used the umbilical cord to link the two together, and I thought he had pretty good results. I've always wondered what they would have sounded like without it....

BTW - I've tried from time to time to bid on a pair when they come up on eBay and are within reasonable driving distance. So far...no luck.

It's an interesting notion...removing the crosstalk. I suspect that SOME of that might prove beneficial, perhaps compensating for poor room accoustics at least somewhat...that was always my take on it. You know...make our headphones more like speakers, and our speakers more like headphones.

Thanks again everyone...

 

[nspindel]

Quote:

Originally Posted by elrod-tom A very good friend of mine had the smallest SDA speakers (sorry, the name escapes me right now

SDA-CRS+ were the smallest, they were a bookshelf version of the line with the passive radiators in the back of the speaker. I had these, just sold them (my babies, I can't believe I sold my babies) I had them for 17 years, and recently grabbed a pair of SDA-2's on ebay, but they were banished to a small room in the basement that just was not big enough for them to do any kind of imaging - the speakers were too close together. When my amp died, I decided to go the headphone route and sell the speakers, which is what brought me here.

By the way, I'm haven't sold the SDA 2's yet - where do you live? I bought them for $200 about 6 months ago, just trying to get my money back for them. I'll toss in the terminated monster cables that they guy gave me. I live in the NYC area....

 

[Spad]

Nspindel, I wonder if you recall Julian Hirsh's "Ultimate Stereo System" from mid-80s? He and his cohorts put together a cost-be-damned system at the then huge cost of $30-35,000. The speakers they chose were the Polk SDA SRSs. This prompted me to rush right out for a listen. Twenty years later I've yet to find anything I like better, and they're still happily dominating my listening room.

 

[nspindel]

I've only known one person to have the SRS's and this was back in college about 17 years ago. All I can say is that you are a lucky man. I'd go grab those SRS's for sale in NJ at $1000 in a heartbeat, if it weren't for the fact that my wife would murder me when she saw the size of them. And I've got nothing to power them with. But man are those things sweet.

Spad, what do you think of the SDA technology? Do you agree that they image much, much better with the connector attached? Any comments to share on the comparison between Polk SDA and headphone crossfeed?