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Whilst it is immediately obvious that left/right channel intensity differences result in high frequency interaural intensity differences when listening to a stereo loudspeaker system, it is far from obvious that these same level differences do, in fact, translate into low-frequency interaural phase differences as well. Indeed the common misconception that bass frequencies contribute little to stereo imaging is largely due to misunderstanding this very important principle. What happens when two spaced loudspeakers produce different intensity, yet identically phased, sounds is that the sound-waves from both loudspeakers travel the different distances to both ears and therefore the signals arrive at each ear at different times.
Fig. 1 illustrates the principle involved: The louder signal travels the shorter distance to the right ear and the longer distance to the left ear. But the quieter signal travels the shorter distance to the left ear and the longer distance to the right ear. The result is that the sounds add vectorially to the same intensity but different phase at each ear. The brain is able to interpret this information in terms of interaural delay.
Two channel intensity-derived stereo is more subtle in its operation than is generally supposed. A purely intensity controlled stereo signal produces a continuum of simultaneous and coexistent stereo images: A set of predominantly high-frequency interaural intensity-derived images and a set of predominantly low-frequency delay-derived images. As far as the brain is concerned the signals it receives from the ears are very similar to those it expects to experience when listening, for instance, to live music in a concert hall. But only "very similar" because conventional stereo cannot produce a perfect illusion, the problem being that these simultaneous, and coexistent, stereo images are not in exact perceptual register. So that, when two-loudspeakers reproduce a stereo image from an interchannel intensity-derived stereo music signal, the high frequency components of each instrument or voice emanate from one place within the stereo image and the low-frequency components emanate from nearly - but not quite - the same place. The result, to use a visual analogy, is a slight smearing or blurring of the stereo image. The figure below illustrates this. (After Wendt, reproduced in Blauert "Spatial Hearing" (1983).)
In 1931, a young engineer at EMI's research laboratory wrote a paper entitled. "Improvements in and relating to sound recording and sound reproduction systems". Even today. this patent is breathtaking in its vision and engineering scope. In it its author, Alan Blumlein, describes a two channel 'stereo' recording system and its possible application in the production of gramophone records and film soundtracks. It sounds tame today, but bear in mind this application was written a couple of years after the first 'talking picture'!
Blumlein described an intensity controlled two channel stereo signal used to produce a convincing wideband stereophonic illusion. He appreciated too that the system was not perfect. His remedy involved matrixing the left and right stereo signals into sum and difference signals (in exactly the same way it is done today for stereo radio transmissions) and inserting in the difference channel a filter of special and unique characteristics before matrixing back to the normal left and right signals.
Sadly. there Blumlein's work stopped. It was soon whisked away on secret radar work. He was during trials of a radar system that the plane in which he was flying crashed and he was killed. It was only after the war that the EMI team working on a practical stereo system implemented his serial filter known as the 'Shuffler' circuit. Unfortunately. this circuit was found to introduce distortion and tonal colouring and was eventually abandoned.
Blumlein's theoretical work was correct, it was only the practical implementation of the 'Shuffler' that was flawed. More modern and sophisticated derivatives of the 'Shuffler' have appeared throughout the years. Others, like the stereophonics researcher Edeko, have taken a more unusual approach to solving the 'bluffing' problem. It is a fundamental characteristic of the 'blurring' problem that the brain perceives the high frequency, intensity derived stereo image as generally wider than the low-frequency delay derived image. With this in mind, Dr. F. Edeko conceived a way of solving the problem acoustically.
His system involved the construction of two special three-way loudspeakers which were positioned on their side. In this way, the angle between the left and right woofer, the left and right midrange unit, and left and right tweeter respectively decreased at the listening position. This clever idea, combined with carefully designed crossover units, enabled Edeko to construct a stereophonic system in which stereo image quality could be improved without tonal distortion.
I can vouch for Edeko's methods because I built a prototype system and was surprised by the improvement in stereo image quality it brought about. Indeed, it was Edeko's articles which first alerted me to the fact that 'drawing room' stereo could be improved. That, and one other unforgettable experience.
Several years ago I was involved in my first recording which appeared on vinyl and CD. It gave me a unique opportunity to compare analogue and digital reproduction because I also had a copy of the original master tape. Unhesitatingly, I should say, the CD was 'nearer' the master, but vinyl produced a 'better' stereo image - in fact, better than the master tape! A conundrum indeed.
After much deliberation and armchair theorizing I set about doing some experiments. Late nights with an oscilloscope eventually uncovered that electrical and mechanical crosstalk within the cartridge and pre-amp were causing a stereo image manipulation which was similar to that brought about by the Blumlein 'Shuffler' circuit and Edeko's loudspeakers - all the important narrowing of the stereo image at high frequencies. It supported what I and so many hi-fi fans knew to be the case, that vinyl really does sound better than CD - especially in LP's presentation of a realistic soundstage.
The excitement for me was discovering that high frequency crosstalk emulated the effect of the 'shuffler' circuit and improved the stereo image without incurring tonal penalties from sum and difference processing. The result of my work was a patent application and the development of a frequency dependent crosstalk injection network named 'Francinstien'. I reasoned that this circuit could be fitted onto the output of any CD player or DAC and be used to enhance the image quality from digital. Ten years on and sixty years late, to my knowledge Francinstien still represents the first commercially (and sonically) successful implementation of the last piece in Blumlein's stereo.
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