Alan Blumlein - close-up of face

Blumlein stereo legacy still resounds on 70th anniversary of death

A technique devised by the British inventor and electronics engineer Alan Dower Blumlein is making a comeback as mixing engineers try to push the boundaries of what is possible with a pair of audio channels, 70 years after his untimely death.

As part of his patent on stereo recording and processing, Blumlein developed the concept of mid-side recording in which the two channels are not represented as left and right channels but are transformed mathematically into the mid and side channels. The former contains audio information that is common to both; the latter contains the difference between them.

The mid-side concept makes it possible to capture stereo signals with two closely spaced microphones rather than, a technique that can be used by portable audio recorders. Devices such as the Zoom H4n, launched in 2009, use two microphones oriented 90° to each other to capture stereo audio.

Blumlein’s breakthrough was the realisation that phase plays a vital role in how the brain perceives the sound that reaches each ear. A number of stereo ‘widening’ effects – which process the audio in such a way that it seems to be coming from two sides instead of centred between two speakers – have been developed that take advantage of this. The most famous is one devised in 1949 by Helmut Haas, which uses the phase cancellations between a source signal, that is sent to one speaker and one that is delayed by up to 30ms, which is sent to the other.

The mid-side representation makes it possible to alter the perceived stereo ‘width’ after a recording is made. In the transition to computer-based recording, a number of companies have developed software that will use mid-side techniques to manipulate the stereo image. Dirk Ulrich, lead developer at Netherlands-based Brainworx, says the technique has become in recent years “one of the secret weapons that professional audio engineers and producers use”.

To increase the perceived loudness and presence of recordings, engineers now routinely use mid-side processing to alter the balance between instruments in a full mix. Typically, they will boost the bass levels in the mono signal part of the signal, and enhance the side signal for higher frequencies to make the mix sound ‘wider’ to the listener. The trend to remaster old recordings for digital formats in recent years means that many recordings have received treatment using mid-side processing, even though they were recorded using a variety of microphone setups – and only rarely using Blumlein pairs.

As well as innovations in audio, Alan Blumlein also made a major contribution to radar development during the Second World War, but was tragically killed in an air crash while engaged on tests on the H2S radar prototype on June 7, 1942.

More information:

http://www.theiet.org/resources/library/archives/biographies/blumlein.cfm

http://www.theiet.org/resources/books/history/21864.cfm

http://eandt.theiet.org/explore/reports/audio/index.cfm

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