If only two mics... ?

[abbey road d enfer]

Just having L/R drivers very close together doesn't comb anywhere near as much as summing L/R signals together to a single driver.

That is in contradiction with all the tests I've conducted.
Acoustic summing is nearly as perfect as electrical summing, as long as signal path difference is less than a 1/4 wave.
Actually, two speakers reproducing the same sound do comb. Maybe that's why combing due to stereo signals is not perceived as distinctly.

 

[soliloqueen]

That is in contradiction with all the tests I've conducted.
Acoustic summing is nearly as perfect as electrical summing, as long as signal path difference is less than a 1/4 wave.
Actually, two speakers reproducing the same sound do comb. Maybe that's why combing due to stereo signals is not perceived as distinctly.

i also find that the brain quickly deletes lots of comb filtering and other such artifacts, because i mean, i don't personally hear any of the comb filtering from the shape of my ears except for a brief moment if i cover my ears and uncover them. the brain is quick to adapt to different forms of phase related distortion ime

 

[k brown]

That is in contradiction with all the tests I've conducted.
Acoustic summing is nearly as perfect as electrical summing, as long as signal path difference is less than a 1/4 wave.
Actually, two speakers reproducing the same sound do comb. Maybe that's why combing due to stereo signals is not perceived as distinctly.

Well, that's what I meant; two closely-spaced drivers, one reproducing the L ch and the other the R ch of a stereo signal (like the Bluetooth speakers I mentioned) do not comb to any significantly audible degree (not two closely spaced drivers reproducing the same signal).

 

[soliloqueen]

what would happen if you played back a ms recording on a coincident ms speaker? would it interfere into stereo in mid air?

 

[k brown]

what would happen if you played back a ms recording on a coincident ms speaker? would it interfere into stereo in mid air?

What's a 'coincident ms speaker'?

And, an MS recording before or after de-matrixing?

 

[soliloqueen]

What's a 'coincident ms speaker'?

And, an MS recording before or after de-matrixing?

what would happen if you played a raw ms recording out of a sideways ribbon speaker right behind a forward facing cardioid cone speaker. couldn't the whole process happen in reverse, with the two interfering in mid air to make a coincident stereo speaker that has zero delay between the channels?

apparently bose did this once and it sounded really weird.

 

[soliloqueen]

i remember now. it's essentially beamforming 2 channels from a single point, and you can only be in 1 beam at once, so it's not stereo until it starts bouncing off the walls. the imaging is apparently bizarre

 

[abbey road d enfer]

Well, that's what I meant; two closely-spaced drivers, one reproducing the L ch and the other the R ch of a stereo signal (like the Bluetooth speakers I mentioned) do not comb to any significantly audible degree (not two closely spaced drivers reproducing the same signal).

It all depends on the correlation between signals applied to the speakers.
With a mono-compatible signal, you would have significant combing.
With typical ORTF or similar signals, the inherent delay between mics introduces a combing effect that dominates over other combing effects.

 

[MicMaven]

Regarding quad and surround sound and all of that:

I figure audiophiles listen to their equipment, whilst the rest of us listen to the music. I do not mess with anything more than stereo because I am happy just hearing the different voices and instruments playing together; for example, a bluegrass band using just one microphone works well enough for me. I don't care how it is recorded (as long as it sounds realistic and I can discern the different instruments.

I see many younger folks are buying cheap audio players with little speakers separated by mere inches. They want to hear the latest tune, They are not particularly interested in how it was recorded or mixed. My wife and I play a lot of old time radio shows and music from the 30s, 40s, and 50s, and, while audio quality is atrocious, we like the shows and the music, so we do not worry about stereo, quadraphonic, surround sound, Atmos or any of that jazz. We just wanna hear the program. I play my guitar along with the stereo, so it only has to sound realistic and loud - I don't care if it is stereo or surround or whatever - I just like playing along as if I was a member of the band.

I figure similar considerations squelched quadraphonic and surround in the larger market. And I knew the inventor of the Who's famous Quadrophenia sound system! Man, what a crazy cat!

So, sue me for being out of step and dampening the group spirit ! James

 

[thor.zmt]

What's a 'coincident ms speaker'?

Well, NOT coincident, and not pure M/S (it doesn't really work actually) but somewhat close (closest that works ):



Here the "magic" exposed:


Case width is ~ 60cm, so chunky for a "bluetooth" Speaker.

Driven by 4 channels of 80W each, Class D Amplifiers clocked 1.411/1.536MHz, with a novel output filter that combines > 200kHz flat bandwith with > 100dB suppression of the switching frequency. Tube preamp for the ineffable je ne sais quoi of that Voodoo that Toobs do.

Four identical 4.5 Inch "wideband" drivers, two forwards and two to the sides.

Around 60Hz-12kHz usable bandwidth, 87dB/1W/1m. Bass is extended to 27Hz in room with EQ and passive Radiators. Custom drivers designed for this, including progressive spider, paper cone and very soft (and well damped) rubber surround.

Add "Supper Tweeters" at ~ 45 degrees angle, they really should be at the sides, but that restricts placement too much. They come in at ~ 8kHz with a single cap, they get the side signal. Both 4.5"Drivers and tweeters are custom designs to work really well together this way.

This of course is modified M/S, as I found a true M/S Speaker a bit wired and extremely hit/miss on how they work. Maybe a bit more research work could bridge the gap.

Low bass is Mono across all four drivers. So definitly "M" for part of the range. Sliding Peaking Highpass Filter to boost the bass flat at 27Hz in room at lowisch SPL.

As the filter "slides up" and cuts first bass boost and later ends up with an 80Hz 2nd HPF when playing very loud. The "cut off" parts of low bass actually produce harmonics exploiting the "missing fundamental" effect.

This is a purely analogue variant of Waves "MaxxBass" which uses EQ to boost low bass instead of synthetic harmonics, as long as the LF SPL doesn't come close to overloading the system. Once we dial back bass boost and cut out low bass, we use the modulating element (J-Fet) to create low order H2 dominant harmonics that create a "virtual subwoofer " effect.

Upper bass and midrange are mixes of L/R & L-R / R-L including delays. In the unit shown this also done all analogue. I'm working on an all digital system now.

The ultrasonic distance meters (there is a third to measure rear wall distance) determine distance to walls, which affects the LF EQ and the amount and frequency response of the "Side" signal added to L/R.

All coefficients were implemented based on the empirical method. For comparison a variety of Hai Enten systems were used.

It works remarkably well. For demo's I always parked it between really big Hai Enten Speakers, like shown here:



It always created massive disbelief when the (Hai Enten crowd) listeners were told they were listening to this overgrown clock radio selling for 1,500 USD RRP and not 50,000 USD worth of Hai Enten Gear.

Like that Thai Monk who claims he saw the Face of Jesus Christ in his Margerine and exclaimed "I can't believe it's not Buddah!"

Ok, back to normal programming and "Waz ist echtes M/S?" (was: Waz ist ein echter Dekka Baum?")

Thor

 

[thor.zmt]

what would happen if you played a raw ms recording out of a sideways ribbon speaker

Cannot get enough bandwidth and SPL at lower frequencies.

right behind a forward facing cardioid cone speaker.

All cone speakers are cardioid above a few 100Hz.

And perhaps the dipole should be in front. Or on top, like Xenia Onatopp.



The 3.5" NXT Cone Hybrid drivers from the Cambridge Minx speakers that float around the usual DIY sales sites seem to be ideal for around 159Hz and up for both a mono line source and dipole.

Below 159Hz we can probaly manage with a cardiod mono wuffer that transitions to an omni supp wuffer...

I tried something similar. It works if the dipole has a wide baffle (> 1m) and the cardioid also is "wide".

couldn't the whole process happen in reverse, with the two interfering in mid air to make a coincident stereo speaker that has zero delay between the channels?

There are limitations.

It's the old logical fallacy of equating microphones with ears, speakers with microphones in reverse and human ears in a room with measurement mic's and concluding what makes a "correct" chain from such fallacies.

Kinda like Bauer/Linkwitz Crossfeed was derived also (green table numbers, as opposed to using a generalised HRTF.

apparently bose did this once and it sounded really weird.

Surprised they never commercialised it.

I mean B*se always sounds weird. It's their hallmark, no bass, one note midbass, no treble, strange imaging.

Every B*se is full in your face on extended logical fallacies.

Like that because in a concert hall 89% of sound that reaches the listeners ear is indirect (which in itself is a logical fallacy, it doesn't, there is more direct sound in MOST locations than reflected) a loudspeaker in a domestic room should also have 89% reflected sound.

I actually owned 901's for a while in the early 90's and always played them "the wrong way around" which tended to fix a lot (but not all) of the weirdness for imaging.

Deleting the EQ box, adding a Fostex wooden radial horn with 2425 Driver and Fostex slot radiator tweeter (active X-Over), blocking the ports and adding a mono 18" JBL Subwoofer fixed the response weirdness, but now the 901 was basically just a big, fairly efficient lower midrange speaker in a 4-Way active system and all that made it "B.O.S.E" was eliminated with extreme prejudice.

Swapping out the B*se 901 after selling them for a pair of JBL 4312 was even mo betta and allowed to put the horns into my Car (Yes, IN MY CAR!)...

Anyway, a famous live guy once told me B.O.S.E is an abbreviation for Buy Other Sound Equipment.

Thor

 

[thor.zmt]

Here is the Russell "Schoktava" schem. I don't know (I'm not enough experienced with electronics) if it's good, great, or just so-so. I've build up 2 boards but never tested them... I could be tempted to get 2 hyper caspules (brand new from their online german shop)

The original Soviet Octava (not sure if they kept it to today) and the Microtech Gefell circuits it seems derived from was a direct consequence of Eastern Europe's "Mangelwirtschaft" (an east-german word that describes an economy that is suffering a persistent lack of everything, across the board).

Active Part's like J-Fet's were already uncommon and most went to the military.

Here a Microtech Gefell design:



I have reasons to believe that Octava in Russia copied earlier Gefell designs. Under the Comecon treaties Gefell had to fulfil a quota of several TONNES of studio microphones supplied to the Soviet Union at prices well below manufacturing cost.


Octava is basically a bootstrapped J-Fet follower with the Output taken from the Bootstrap PNP Transistor. As is the Gefell variant. Gefell is a bit smarter by having what ammounts to a Darlington arrangement for the follower, plus a built-in 2nd order HPF.

Back to Octava.

The 13k Resistors and 47uF form a power supply filter. The FET is a simple follower, with the 7.5k resistor "bootstrapped" by the 2N5487 (another hard to get item, an import PNP Transistor).

The PNP Follower in effect recycles the FET drain current and operates with the 7.5k resistor and J-Fet as load, which is another bootstrap. Kinda opposite than Schoeps does. There the frontend recycles the Emitter current of the follower.

In my view a Schoeps derived circuit (P-Channel Fet for the followers instead of PNP and fil coupling cap's is the vastly superior choice across the board over Octava variants. Inputs and outputs are DC coupled and the remaining interstage coupling cap's can be SMT Film or C0G Ceramic.

If we ABSOLUTELY MUST do something Octava/MTG style, just to be different, annoying and individualist (all of which traits I applaud), why do oldthink and oldtech is we don't have to...

I dub thee Sir Mocktava:



We get ~ 40V to polarise the capsule, +6dBu out @ ~ 0.03% HD (H2 dominant), noise depends on Capsule Capacity and self noise. Frequency response < 7Hz - >10MHz (-3dB).

Still, I fail to see merit. Same for a modded B1 BTW, except one could use an LSK389 to make a single package follower (hardwired) without offset and +/- Supplies (if we already abandon 3-Pin XLR for more complex and unstandard cable solutions. In that case, perhaps...

It still is not much simpler than the Schoeps with P-MOS followers and a J-FET / P-MOS Sziklai frontend. Or just an OPA1642 OpA.

Thor

 

[thor.zmt]

For discrete sound sources. Surround can be very nice when it provides a realistic diffuse rear ambience, which you don't get with front speakers in a domestic size room.

The solution I used repeatedly is the classic old "pseudo Quadro" Effect. With the audio out from whatever source (in my case a PC for serious movies etc.) output Dolby Pro Logic II.

I originally used Canon speakers (S-35) which as main speakers were totally weird. Sound was, interesting.

I actually flipped them upside down and put a compact Halogen Spot on top (well, the original bottom) so it looked like a funky wall lamp:



These are basically kinda Pseudo Omni.

https://www.stereophile.com/standloudspeakers/695canon/index.html

So if you want to try this, get some cheapo wireless amp boards, some 4" wideband drivers (I notice the NXT drivers from the Cambridge Minx BT Speakers still float around in volume) and some wooden Bowls from Idea, bodge together something similar, integrate some nice RGBWWCW LED Spots and camouflage as Lamp's.

Ideally 4pcs side and rear wall in the rear corners maybe 1m from the actual corner.

Connect the speakers on each Amp Board from L+ to R+ with the right polarity.

While you are at it, make a soundbar to park under the TV and give it LS/L/LM/RM/R/RS drivers, use the same ones. for M connect L+ to R- and R+ to L- and place the drivers near each other. L/R is obvious and LS(ide) and RS(ide) just get same Side signal as the rears. Use the full width of your 75" TV (if you are poor like me, all 100" if you are not) and angle the "S(ide)" drivers outwards.

Feed the signal to all the AUX Crap speakers from the speaker feed of your main 2-Channel Rig.

Adjust levels to a point where all the auxiliary speakers do not become distinctly audible, instead the center under the TV just helps anchoring dialogue solidly to TV and the rears should just add to immersion.

Actual full surround outside a THX certified Theatre with a seat centre 1/3rd back from the front row but instead at home annoys me intensely.

FWIW, the "Main Rig" ATM is Technics SB-E-100 12" + Horns 3-Ways, phase linear design with controlled directivity and low objective distortions, 95dB/W/m and a Marantz PM-75 Amplifier, 100W/8R Class AA (non-switching Class AB) with build TDA1541 based "Crown DAC".

Thor

 

[ricardo]

I just follow the practice of Tony Faulkner, who uses 26" spaced SDC omnis on his 4-mic 'phased array', and they are aimed 90 degrees.

By that do you mean they are pointing straight ahead?

There's no combing with ORTF (or any other near-coincident pair); only a teensy bit when summed to mono.

Perhaps your idea of 'teensy' is different from mine

Spaced techniques are not "out of phase"; that term referrs to signals that have a 180 degree phase (or polarity) difference, that will null when mixed together.

I thought I said "sorta out of phase". If I didn't, I should have said so.

 

[Barry Hufker]

Probably what should be said is that spaced microphone techniques are less "phase coherent" than coincident ones.

 

[k brown]

By that do you mean they are pointing straight ahead?

Perhaps your idea of 'teensy' is different from mine

I thought I said "sorta out of phase". If I didn't, I should have said so.

No, 90 degrees to each other - as in ORTF is 110 degrees, NOS is 90 degrees, etc.; pointed straight ahead would be 0 degrees.

 

[k brown]

Probably what should be said is that spaced microphone techniques are less "phase coherent" than coincident ones.

And all the better for it !! (for Classical music, anyway)

 

[thor.zmt]

Probably what should be said is that spaced microphone techniques are less "phase coherent" than coincident ones.

Probably what should REALLY be said that is humans, when listening to a classic equilateral triangle stereo playback set-up in a well treated room with both relatively directive speakers and reasonably constant directivity (or better a 1dB/8ve increase of DI above ~ 700Hz), tend to locate sounds direction more by sound intensity as frequencies increase above 700Hz and more by time of arrival difference as frequencies drop below 700 Hz.

Sorry for this extreme concatinations, but unless we have all qualifications in place, the statement would be in part wrong.

Inductive reasoning, neither pure intensity based "coincident" or "near coincident" microphone arrangements nor pure time of arrival based systems (spaced mic's) can create a convincing illusion of the spacial aspects of the original acoustic event.

Worse, if we modify pure intensity systems for more time of arrival difference, or pure time of arrival based systems for more intensity difference, we effectively destroy the systems ability to create a convincing illusion across a limited frequency range.

In effect, we trade "limited perfection" for unlimited imperfections.

The inductive reason from all these facts is that to create a recording that provides a convincing illusion of the original recording space requires a widely spaced pair of Omnis for the range below appx 700Hz and a coincident pair for the range above 700Hz, using time alignment to the acoustic center of the ensemble being recorded and zero phase error mixing and "crossover" between the two distinct, "limited perfection" microphone arrays to achieve a situation where we may tweak the overall setup to, by successive approximation asymptotically, ever close approach "unlimited perfection".

What a goshdarn muthafunkin wordsalad. OMG. But I have no idea how to say in a different way that is less salad and words and still absolutely and unassailably true.

Thor

 

[pmfalcman]

@Emmathom - you wanted to simplify things with "only two mics", right?
And look what you have done, man! Does it look simple to you?


Since we already have an "Alice in Wonderland" reference in our mic's universum, now it's time to introduce Winnie the Pooh:
"The more I read, the less I know"

Fantastic discussion, guys! Truly! Great reading, a wealth of information. Superb stuff.

(In a very quiet voice) Never mind it brought more questions, uncertainities and doubts than I had before

I sincerly thank you all for all the discussions, fights and disagreements you share here. The knowledge, informations and points of view. Invaluable and keeps my gray matter from getting rusty.

 

[Emmathom]

@Emmathom - you wanted to simplify things with "only two mics", right?
And look what you have done, man! Does it look simple to you?


Since we already have an "Alice in Wonderland" reference in our mic's universum, now it's time to introduce Winnie the Pooh:
"The more I read, the less I know"

Fantastic discussion, guys! Truly! Great reading, a wealth of information. Superb stuff.

(In a very quiet voice) Never mind it brought more questions, uncertainities and doubts than I had before

I sincerly thank you all for all the discussions, fights and disagreements you share here. The knowledge, informations and points of view. Invaluable and keeps my gray matter from getting rusty.

You're x100.000 right ! Gosh...
I've learnt that "only two mics" is an utopy and yes I harvested more interrogations than answers !