Martin Walker wrote:The special feature of the Waves S1 Imager for me was its Blumlein shuffling, as I described in this SOS feature:
"S1 also has a very useful Shuffle control that lets you further increase stereo width at lower frequencies (typically below 600Hz, as set by the associated Frequency control). This compensates for the fact that the ear is less sensitive to stereo bass effects..."
Ahem... Kindly take your seats gentlefolk, and pay attention... l have a short history lesson for you... ;-)
Although the end result is much the same, the issue being compensated for in the Waves approach is actually the other way round! It's not that the ear being less sensitive to stereo bass effects, it's that the sense of hearing is actually more sensitive to HF stereo effects when listening over stereo loudspeakers.
And it's not Blumlein Shuffling either -- that's a similar technical process but performed on a different type of source material and for a very different reason. I'll return to that in a moment...
The function you've described and which Waves is trying to emulate in the S1 is actually called Stereosonic Shuffling!
In the 1950s the EMI boffins (many of whom had worked with Blumlein before the War) worked out that there was a fundamental flaw in Blumlein's format of conventional stereo from two-loudspeakers arranged in an equilateral triangle.
The stereo loudspeaker system basically works by converting inter-channel level differences between the signals fed to the speakers into inter-aural timing differences at the ears. However, the discovered 'flaw' was/is that in practice the perceived stereo image width is inherently exaggerated for frequencies above about 700Hz -- mostly because of the shadowing effect of the head. So this flaw effectively blurs the stereo image slightly, degrading the perceived imaging precision and focus.
Consequently, EMI's boffins developed a process which they subsequently called the Stereosonic Shuffler and it was later built into several of EMI's REDD mixing and mastering consoles.
The Stereosonic Shuffler was designed to compensate for the HF image exaggeration effect by attenuating the Sides signal above 700Hz, thus narrowing the stereo image for high frequencies slightly and realigning the spatial positions of low- and high-frequency components. EMI's implementation involved converting the left-right stereo signal to Mid-Sides, introducing a gentle -3dB shelf cut above 700Hz in the Sides signal, and then reconverting to left-right for replay over the speakers.
Obviously, attenuating above 700Hz has much the same effect as boosting below 700Hz... except that it has the benefit of not risking running out of headroom which the bass boost approach definitely does!
Note, the 700Hz figure is not an absolute -- it's a smooth crossover process and the the exact frequency depends on personal physiology, but it's generally somewhere in the 600-750Hz region for most people, and it's not that critical.
Also note that David Griesinger (of Lexicon fame) has made extensive studies of the significance of the relative stereo width at low frequencies and its effect on perceived imaging precision and the sense of spaciousness in stereo recordings.
Sadly, Stereosonic Shuffling never caught on as an 'essential process', mostly because the effects are relatively subtle and only really appreciated by someone sitting in the correct listening point between the speakers -- so great for mixing engineers, but pretty unlikely to get noticed by most domestic listeners! And EMI decided to drop the idea as the benefits weren't worth the efforts and technical challenges involved with the available electronic circuitry of the day.
However, as an accidentally discovered benefit, the EMI boffins also found that the same process improved the sound from coincident stereo mic arrays, and particularly crossed fig-8 (Blumlein) arrays.
The polar pattern of most directional mics naturally narrows with rising frequency, and this is often particularly noticeable on vintage ribbons... and EMI were fond of using coincident XY fig-8 ribbons for much of their classic music recording in the 50s and early 60s.
The polar pattern narrowing in an XY array increases the inter-channel level differences and thus exaggerates the stereo width at higher frequencies. Consequently, by using the Stereosonic Shuffler to reduce the Sides signal from the XY array above 700Hz, the stereo image width at HF was brought back into alignment with the image width at LF, thereby improving the overall image precision and focus.
Modern mics tend to have much more consistent polar patterns than those of 90 years ago, of course, and its really only the low bass that causes a problem as the mic degrades towards omnidirectionality -- thus reducing the interchannel level difference. So I've often found that often narrowing from above around 100Hz or thereabouts (or widening below 100Hz if you have the headroom) by 4-8dB is usually quite effective.
Historically, the Stereosonic Shuffler was used quite extensively by EMI on many of its classical recordings of the late 50s and 60s... but it wasnt adopted by anyone else and it quietly faded into obscurity as the REDD consoles evolved and were later replaced with more modern commercial third-party designs.
So... Stereosonic Shuffling to improve the precision and focus of stereo loudspeaker listening is a real thing, and the benefits can definitely be appreciated by those sitting at the correct listening position. Modern digital signal processing makes it a perfect and loss-less process too, which is more than can be said for EMI's original implementation with lots of transformers and passive balanced filters!
Next... Blumlein Shuffling.... Same term but different implementation and application!
Back in the 1930s when Blumlein was developing his stereo recording techniques, he came up with the idea of using closely-spaced omni-directional microphones, ideally with an acoustic baffle placed between them. This concept is better known today as the Jecklin Disc (and variations thereof), but Blumlein got there first, and then discounted the idea in favour of coincident arrays which gave better stereo imaging accuracy when auditioned over loudspeakers. (The close-spaced microphones idea worked well on headphones, though, and we know that today as Binaural recording -- Blumlein got there first!)
So Blumlein's close-spaced array involved mics 22cm apart, ideally with a (head-sized) baffle between them, although that seemed to be optional. Most of his tests used omni mics, although he also used forward-facing ribbon fig-8s -- an idea which Tony Faulkner re-invented in the 80s and is now known as the Faulkner Array.
When the outputs from these mics are auditioned on stereo loudspeakers the imaging is rather vague and the main reason is the lack of inter-channel level differences between the channels -- it's nearly all small timing differences at high frequencies, and only very small phase differences at low frequencies.
So Blumlein invented the 'Shuffler' to correct that, turning small LF phase differences between the channels into amplitude differences. Again, the process involves attenuating the Sides signal above about 700Hz, thereby expanding the stereo image at low frequencies relative to high frequencies... and it works quite well.
I've used it on recordings made with a Schnieder Disc (very similar to a Jecklin disc, but with an absorbent 'bulge' in the middle to better replicate the absorption of the human head!). it also works with ORTF recordings, bringing a little more focus and definition into the sound. It's subtle, and in no way a game-changer, but I think it can be beneficial....
Both forms of Shuffling are easy to implement and experiment with. You just need a pair of complementary LR-MS and MS-LR converters, and a shelf equaliser in the Sides path between the converters.
It's generally best to reduce the HF above about 600-750Hz, rather than boosting below, simply because bass signals are usually quite loud and boosting risks overloading the signal path. The amount of HF cut is largely a matter of taste, but I find 3-6dB is usually the right area. Try a shelf at around 350Hz for the ORTF array.
Okay... lesson over... thank you for your attention....now go out and play in the sunshine.... Matron has ice lollies for everyone :D