The British Library has made available to the public recordings of some of the first experiments in stereo sound made by Alan Dower Blumlein to coincide with the anniversary of the engineer’s birth 109 years ago today.
According to the library, the 22 recordings, which can be played at the British Library’s Sounds website, represent some of the earliest known experiments in two-channel recording. The family of the late Angus McKenzie, audio engineer and founder of Olympic Studios, donated the recordings, captured on a set of single-sided vinyl discs, to the library in 2005.
The recordings, mostly made in late 1933, demonstrate the use of what Blumlein called a “shuffling network” – now known as mid-side processing – that makes it possible to alter the perceived width of a stereo audio signal electronically. The shuffling technique works by combining the left and right channels and converting them to sum and difference signals.
The processing allows the amplitude of the difference, or ‘side’, signal to be altered relative to that of the summed ‘mid’ channel. When converted back to a left-and-right stereo pair sounds that are not centred will appear to the listener to move further to the sides as the level of the difference signal increases.
On the recently released British Library recordings, Blumlein and colleagues working at EMI Studios at Hayes, Middlesex demonstrate the effect of increasing the relative strength of the side signal by changing the resistance of an electrical circuit and speaking from different positions within the auditorium.
In his memo explaining the technique to EMI director of R&D Isaac Schoenberg, Blumlein argued that the technique would make it easier for listeners to distinguish between sound sources. One of the recordings demonstrates this by playing two different pieces of music from different sides of the studio.
The last set of recordings in the series, recorded in early 1934, demonstrates the technique on a full orchestra. The vinyl discs capture sections of a rehearsal of Mozart’s Jupiter Symphony by the London Philharmonic Orchestra at EMI’s Abbey Road Studios in London.