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Old 21st July 2015, 06:12 AM   #1
CLS is offline CLS  Taiwan
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Default 2 Pairs of Opposite Stereo (for a flavor of XTC)

Hi there,

This is actually a spinoff from this. It's already OT in that thread, so I think I'd better start a new one.

Many of you must have quite a few unused equipment collecting dust in the storage. Why not bring them back and have some fun?

In the link above, that is a '70 Hafler circuit. Outer channels play differential signals:

Click the image to open in full size.

As can be seen, there's -R portion at the far L; -L portion at the far R. What's this? It's cross-talk cancellation !! Wow !

Also, there're (duplicated) R & L at the outside, which may expand the stage width (pro), but can also smear the images (con).

With the same physical layout and rewiring, they can do this:

Click the image to open in full size.

Now it's pure physical XTC.


Possible benefits:

- closer main pair provide better pinna cue, thus better central imaging

- stage width easily goes beyond main pair, even goes beyond outer pair

- images are very solid and 3D with depth, feel like they're properly shadowed, or feel like engraved

- 3D imaging bring some more impact and dynamics to the sound, overall a very vivid and lively presentation


Setting tips:

- the distance from the right ear to the inner L and outer R should be the same, vice versa

- setting with pink noise, pan the signal to one side, adjust the volume of outer channels, start from 0 until it's just detectable (they should be much quieter than the main channels)

- listening test with any recordings with good sound stage, usually a slightly higher volume on outer channels is allowed with music (compared to mono pink noise)

- you'll know when it's overdone, the violins appear on the right, or the likes, and smeared images.

- in a correct combination of distances and levels, there'd be a feel of well-focused

- feel free to experiment with the locations of speakers

- if no good things appears, something must be wrong in the polarity, maybe in amps, maybe in speakers


Any drawbacks? Sure, like anything else. Good effects only appear along the central line. Off by a short distance, they're gone. Fear not. It's not going to be worse than normal. I feel the imaging of off center listening is still better than plain stereo.

Oh, you need 2 pair of speakers and 2 sets of amps with individual volume control to play with this, of course.


Click the image to open in full size.
(Oops, there're 3 pairs of stereo. No, the small monitors by the TV are not in the system)


Have fun
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Old 21st July 2015, 10:06 AM   #2
TMM is offline TMM  Australia
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A few observations:
-All four speakers should ideally have identical frequency response. In a real world, even if you use the same model speakers, the exact same frequency response can't be achieved from each position due to room effects.
-What is preventing the 'R-L' or '-L' signal being heard by the left ear, just as the 'R' signal is?
-Humans have two ears for a reason - we are very good at locating the direction of the source of the sound due to the difference in arrival times between each ear. Therefore we don't really hear 'crosstalk' as such, simply a difference in the location of the speakers. The aim for attempting this setup would seem to be to try to approach a speaker setup which mimics a headphone. In which case, why not just place the left speaker directly left of the listener and the right speaker directly right of the listener? It will achieve better rejection than this type of setup imho.

Last edited by TMM; 21st July 2015 at 10:10 AM.
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Old 21st July 2015, 11:55 AM   #3
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If the picture is anything to go by I would fix the room before fiddling with extra speakers.

From what is visible the sound in there should be terrible as all I can see are hard, reflective surfaces.
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Old 23rd July 2015, 05:36 AM   #4
CLS is offline CLS  Taiwan
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I feel sorry that you missed the whole point and can't share the joy of this thing.

Just in case, maybe my writing is awful and hard to read, here are some highlight:

Quote:
Originally Posted by CLS View Post

Many of you must have quite a few unused equipment collecting dust in the storage. Why not bring them back and have some fun?
...

Any drawbacks? Sure, like anything else.
...

Have fun

Oh, and this:

Quote:
Originally Posted by CLS View Post
...

- if no good things appears, something must be wrong in the polarity, maybe in amps, maybe in speakers
I should add: ... something must be wrong in the way of seeing the whole thing.


BTW, it's fine to pursue a perfect-everything audio system, or insist on staying with (plain) stereo in a mastering room. However, those are not in this topic.
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Old 3rd August 2015, 02:26 AM   #5
CLS is offline CLS  Taiwan
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I guess no one tried.

No problem, I still share what I've found.

Inspired by the paper:
http://www.princeton.edu/3D3A/Public...CHPaperV4d.pdf

I added some LP filters into the recipe.

Click the image to open in full size.

On the array itself, an LR contour filter gives it an overall downtilt response, with a slope slightly less than 1st order from ~200 Hz and up. (I'll keep fiddling with this later... )

This is for reducing the intended cancellation in HF.

Another LP element (a simple inductor) is added in the path of common ground of both speaker (and amp). The reactance of inductor makes a rising impedance with frequency. In LF, it's almost shorted, so both speakers play as usual. As the frequency rises, there'd be more and more differential portion ( L-xR / R-xL ). -- You may also refer to this for how it works.

Overall, it's doing XTC in LF and gradually turning into Hafler circuit (virtual ambio channels on the outside).

In the paper mentioned above, it says a perfect XTC is not linear (flat) in FR. It's more on LF. I think this makes sense, because the head of listener blocks the XT of HF above certain frequency anyway. So it's not necessary. OTOH, striving for perfect cancellation in HF is not practical. The wavelengths are so short and hard to be aligned perfectly, or the 'sweet' spot would get so small. (could be smaller than the head)

That's why, in the previous setting, the SPL of outer channel should be kept very low. Or the images change sides. With these LP filters, such effect is reduced. And the level can be raised somewhat, thus more cancellation.

The increased cancellation in LF brings more feel of envelopment & widening, without negative influence in central part. However, there's still a clear threshold. Once crossed, the images also change sides and there can even be some effect of 'double vision". Funny and messy.

Forgot to mention, the tweeters in the diagram above are put together at the center, against the wall to be as diffused as possible:

Click the image to open in full size.

The two on both sides of TV are part of the main channels, support only the very top octave.



Some more good read:

The binaural performance of a cross-talk cancellation system with matched or mismatched setup and playback acoustics. - PubMed - NCBI

http://sound.media.mit.edu/Papers/gardner_thesis.pdf

Last edited by CLS; 3rd August 2015 at 02:30 AM.
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Old 3rd August 2015, 01:06 PM   #6
zelgall is online now zelgall  Canada
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CLS, is that a diffuser in the right hand corner of the room? It looks interesting. Would you comment on that please?
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Old 3rd August 2015, 01:09 PM   #7
Elias is offline Elias  Finland
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Quote:
Originally Posted by CLS View Post
Hi there,

This is actually a spinoff from this. It's already OT in that thread, so I think I'd better start a new one.

Many of you must have quite a few unused equipment collecting dust in the storage. Why not bring them back and have some fun?

In the link above, that is a '70 Hafler circuit. Outer channels play differential signals:

Click the image to open in full size.

As can be seen, there's -R portion at the far L; -L portion at the far R. What's this? It's cross-talk cancellation !! Wow !

Also, there're (duplicated) R & L at the outside, which may expand the stage width (pro), but can also smear the images (con).

With the same physical layout and rewiring, they can do this:

Click the image to open in full size.

Now it's pure physical XTC.


Quote:
Originally Posted by CLS View Post
Hi there,

Let's have a revisit to this. In post #299, there's a 4-ch proposal:

Click the image to open in full size.

I seems simple, just an ordinary stereo amp and 4 speakers without any fancy filter. Has anyone tried it?

Hi,

See post #300 Try Ambiophonics with your speakers in the ambiophonics thread, for my interpretation of the concept

I believe it is primarly not for cross talk cancellation but for envelopement.

"55 degrees" is referred also in ambiophonics site as the optimum space angle for minimising IACC (inter aural cross correlation). Means maximising envelopement that way.


.
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Old 3rd August 2015, 01:24 PM   #8
Elias is offline Elias  Finland
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How about moving the cancellers to sides ? That way the cross talk generated by themself is minimised because of maximal head shadow.

The level of the side speakers must be adjusted properly of course.

Click the image to open in full size.



.
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File Type: png PassiveXTC_zpsijloa2es_sivulla.png (18.4 KB, 46 views)
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Old Yesterday, 03:09 AM   #9
CLS is offline CLS  Taiwan
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Thanks for the responses

@zelgall,

Sort of. What I hope for is actually reflection. The room is not symmetrical. It's much more open on the right side, so I need more reflection here. It's helpful in HF, to some extent.

Practically, I can't build a solid wall there, so here it is. It's just a sparse fence made in unfinished wood strips with minimal building effort. Those strips are not even fixed, staying in the frame just by their own resilience. There're additional two pieces of cardboards in the fence for more reflection, barely visible in the photo.



@Elias,

Finally, you came. (Where's graaf & other psychoacoustics guys?)

I wish I could. 180 degree positions are not allowed practically. Or I could have tried real OSD. Even if I can add delay to the elements on sides, the space is still not enough to place any physical speakers there. The only exception I can think of is DML on ceiling, but it's still difficult to get enough LF from available (allowable) dimensions.

The condition shown in the photo above is already the largest angle I can have. It's (only) +/- 40 some degree on the outer channels. Nevertheless, it feels very wide already in recordings of orchestra or chorus (or any other genres with wide soundstage). There's a sense of being at front row seat, facing a wide stage. Sometimes, it's even overly so...

TBH, I'm not quite sure about the XTC here, thus the conservative title, but I guess there's at least some effect of it, because it sounds different from Hafler circuit (R-L / L-R). The best part of it is not adding ambio/envelopment/widening, but a strong sense of well focused and sculpture-like imaging.

The latest setup is a mix -- opposite sides/polarity in LF, gradually turning into (some kind of) Hafler circuit as the frequency rises. The previous sculpture-like imaging seems diluted a bit, with some more envelopment. (makes sense, that's what the hardware is doing)

I'll do more listening and fiddling until I'm tired or enough...


Eventually, like many, I think I'll add a PC into the system, to play with all those DSP things. Maybe I'm old fashion, too slow in adopting new stuff.

Anyway, now I'm happy to put those extra speakers in interesting usage and have some fun along the process.
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