Diffuse field equalisation

Some years ago I bought a second hand ED-1 equaliser, then later a ED-1 signature equaliser.  Unfortunately, I don't have a true Lamda Professional model any longer, nor a true Lamda Signature, and I doubt that it's even possible to obtain (in good condition, let alone new) the exact electrostatic elements for either of these models.

I have long since replaced the elements in my Lamda models with those for the SR-404; and I have listened carefully through ED-1/ED-1 Signature equalisation...

I used to wonder why Stax marketed these equalisers as "Diffuse Field" rather than "Binaural" equalisers - finally, I understand (a matter of diffuse-field equalisation as distinct from free-field equalisation Along with some mixing of left into right and of right into left).   As played through either the ED-1 or ED-1 Signature, the diffuse field equalised sound is quite something to get accustomed to...

Once past the dissapointment of even less bass than was present without the equalisation, the musical image can sound more credible, but this is not always reliably so.

At about the same time, I fell in love with the sound of the Sigma (again, my examples of SIgma and Sigma Pro have all had their elements replaced by SR-404 elements.   A note here:   I could not detect any visible difference between the elements of the Sigma Pro and the Lamda SIgnature - they appear identical.  Further, the capacitance of the elements were identical.   I have difficulty to believe that there is any difference at all between the elements of the early model Lamda Signature and those of the Sigma Pro - something of an academic question now anyway, since the whole lot have been replaced by SR-404 elements).

The Sigma has noteable faults:  even less bass than any Lamda, along with some muddiness of image.  However, the Sigma produces beautiful music.  After a few minutes, I can forget its shortcomings and enjoy the music (whilst making everyone laugh - even a Cyberman, I think...).

In the last couple of years, I have found an algorithm to convert stereo to binaural; and, after a few experiements, I made a binauralised copy of everything that I have in stereo.  I was delighted to find that I could return to listen happily through my Lamdas, and (moreover) through the Omega (which I hardly ever used, because the sound-image was so fatiguing).   When I discovered that the iPod can be loaded with WAV as well as MP3 files, I bought one, filled it with all of the "binauralised" material; and I have never looked back !

A true binaural recording (certainly since the advent of the Neumann KU81...) would be done with diffuse field equalisation, and such recordings should be heard with diffuse equalisation of the headphones

Does anyone out there know the exact parameters necessary to convert free-field to diffuse-field?

Merci par avance....

It's interesting that both you and John Buchanan are fans of the Sigmas (especially the Sigma/404) and diffuse field equalized Lambdas as well as showing a preference for these over the 007.    I am also a Sigma fan,  having the low bias, pro and 404 models.  I am not quite ready to ditch my 007A as John did, but the Sigma/404 is very close to it in my affection.  The 007's have more detail, drive (if given a powerful enough amp) but don't match the Sigmas in openess and naturallness of the soundfield . Some people call it gives a huge soundfield however I think it is basically just an "out-of-the-head" soundfield and it just seems huge compared to the compressed "in-the-head" soundfield you get with conventional phones.

BTW I think the Sigma/pro and Signature drivers are the same.

Any chance you could share your digital binaural files?

Bonsoir,

I bought my Omega_2 (SR-007) back in 2005; and, until recently, I would say that I had listened through them for less than 10 hours, altogether.  I have at times considered selling them on, but I wanted to be sure that I had explored the possibilities of binaural reproduction enough before I reached any conclusion.   It was soon afterward that I got the ED-1.  This was quite interesting in combination with the Lamda; but, as anticipated, it just doesn't quite work with the Omega_2.   In any case, I was very happy to live with my Sigma/404.

Since having converted (or "binauralised") material to binaural, I have found that the Lamda/404, the Omega and the Omega_2 can be as enjoyable as the Sigma.  In fact, rather moreso.   The Sigma was an attempt at a real sound-stage, using the outer-ear as headphones had hitherto ignored.   In a real sound-field, an event from, say, the right side does reach both the right and left ears; and the brain manages to perceive its position despite its arrival at both ears..   The Sigma precludes such cross-feeding, and this little shortcoming could possibly explain the very "wide" image that it portrays.   I love the Sigma, all the same; however, the curiosity continues....

In the last year or so, I have almost ceased using the Sigma, for profit of using either the Omega or Omega_2 with "binauralised" material.   It is fatigue-free, and I can sometimes place the position of instruments/performers in front of the head, as they were intended to be reproduced by a pair of loudspeakers  (keeping in mind here that the input to the Binauralisatoin algorithm is stereo materiel intended for Loudspeakers - one can hope at best to reproduce that pair of loudspeakers, apparently just in front of the head).

At first, I made a trial binauralisation of some of my music, then after I felt convinced that material actually began to sound like it was in front of my head (rather than concentrated within it), I went ahead to convert everything.   I found later that even an iPod with little phones can be quite pleasant with this binauralised material.   I have not, however, bothered to convert binauralised PCM files into MP3.   Nor have I tried to binauralise an existing MP3 file - both strike me as pointless exercises....

The algorithm can be found here:   http://www.hydrogenaudio.org/forums/index.php?showtopic=42916.   It should be noted that the author/creator did eventually develop a VST Plug-in (which is most handy for converting hundreds or thousands of files as a batch....)

(scroll down to the 6th post....)

I forgot to say as well that I will get around to testing the ED-1 and ED-1 Signature for the presence of cross-feeding of signal (I felt so "sure" that this was actually being done that I didn't bother to try to prove the point!)

I have opened up both boxes to take a good look around.  To start with, the ED-1 Signature is a bigger box than the ED-1.  This bigger box is exactly the same size as the SRM-T1, which it was designed to accompany.  The ED-1 is exactly the same size as the SRM-1 (or SRM-1/Mk2), which it in turn was designed to accompany.

The contents of the ED-1 box occupy the box quite nicely.  The contents of the ED-1 Signature appear to be near identical to those of the ED-1, transplanted into a bigger box....

The ED-1/ED-1 Signature contains a collection of op-amps (normal stuff AND which could be replaced by directly pin compatible superior updates!) along with a rather interesting, vertically mounted board featuring some 8 or so trimmer potentiometres.   I have been meaning to get around to observing these pots on an oscilloscope with some white noise...   My guess is that the ED-1 is manufactured, then tested and adjusted via these trimmers.   There seems to me to be one series of trimmers, rather than there to be a pair of series (one per channel).   Intéressant....

Bonjour Kevin,

Thank-you for those schematic diagrammes!   Would they be be diagrammes that you made, perhaps?   I have a couple of questions about them....

Speaking of the schematics for a moment....

In the upper plot of the ED6 schematic, there is a blue line for R51; and there is another green plot, also for R51.   The second plot depicts R51 again, looking this time like the first blue plot in greater detail (?).   There's a similar representation in the ED-1 schematic - what is the difference between the two R51 lines in the second plot?

Is R51 the output point of the schematic?

What is the difference between the blue and green plots labelled R51?

Speaking about the ED-1:   No, I haven't listened carefully to orignal Audiostax binaural recordings (done via the Neumann KU81) through equalisation.

In the next couple of days, I will retrieve these two equalisers; and I will listen to some Audiostax binaural material.

Within a couple of weeks, I will be "looking after" a Neumann KU100 microphone as well.   I will try to make not only some recordings with it, but I will try to measure the response of the Lamda 404 and the Omega.

Alright, thank-you for that.   I take it that R51 is the output point of the signal?

What about channel cross-feeding in the ED-1?

And, speaking of DSP, that is something that I was planning to do:  emulate the ED-1 by DSP, in order to see how it compares with the Bauer Binaural DSP.

Assuming that something interesting comes out of that experiment, I would then try to make some equivalent DSP for the Lamda/404, then perhaps the Omega.   Have you looked into either of these, at all?

Digital Signal Processing is not limited to any one process:  any transformation imaginable of audio is possible via DSP.   Multiple tap equalisation, such as may be found in the ED-1 schematic (see above) , can be achieved easily enough with software (Soundforge, Wavelab, Audition, etc).   I hope to do this soon for myself.....

Speaking of DSP, I have just made a first stab at an idea for the Sigma....

In a respect, the Sigma is something of an antithesis of the "in-head" headphone sound - it produces an excessively wide reproduction of stereo, for want of some cross-feeding of the channels (as heard from stereo speakers).

I have taken a copy of a live stereo recording, swapped the channels, then reduced the level by 20 dB.   I have mixed into this the original stereo recording, at original level.

The result is interesting:   I have no idea of how much the level should be reduced in order to reflect the true cross feeding of channels, nor have I attempted to delay the cross-fed signal, nor applied any slight low-pass filtre. 

From this quick experiment, I notice that the width of the image sounds a little more "normal".   The image is still quite out of head, and it sounds credible without sounding quite Sigma-wide.

This could be an avenue well worth exploring !

It would be worthwhile to calculate exactly how much:  (1)  the level of the cross-fed signals should be reduced; (2) delay should be applied to the cross-fed signals; and (3)  low-pass filtration to apply (in order to mimic the natural filtration imposed by the human head).

a practical note about living with DSP:   I have since a few years moved all of my audio material to file-servers (carefully backed-up, of course...).   I no longer keep any of the original compact discs (or whatever other source medium).  Having  taken a copy of this original stereo material, I have applied the "binauralisation" DSP, then put the the result into another directory; and, depending upon which type of headphone I feel inclined to use (or speakers), I play from either the directory containing the original stereo, or the directory containing the binauralised material (using a network media player capable of reading PCM and which puts the signal out to a SPDIF terminal which feeds a decent D/A converter).

With material so stored, it becomes easy to experiment with any other DSP treatment/algorithm:  diffuse equalisation, for example...