This was the first of the three techniques to be invented. As at the time (1931), only omnidirectional microphones were available to Blumlein, he found an elegantly simple but clever way to get real stereo from omnis. BDT uses two forward facing microphones spaced apart by the same width as human ears. Blumlein put a baffle in-between the mics to simulate the effect the human head has on our hearing, although it has to be said that the technique also works very well without a baffle. The effect of the baffle is to sharpen the imaging. This gives intensity stereo down to around 700Hz. Below that, sound waves are large enough to get round the baffle and reach both microphones with the same amplitude at the same time, regardless of where the sound originated from, thus giving equal amplitude microphone outputs which are perceived as mono at lower frequencies when listening via loudspeakers.
Blumlein's stroke of genius was to create a simple but very clever circuit that converted phase differences between the microphones into left / right channel intensity differences at these frequencies. He called this process shuffling. The output of the two microphones is fed into the shuffler (which only works below approx. 700Hz) which restores stereo at lower frequencies, thus giving intensity stereo throughout the entire frequency range. Unfortunately, later that year the directional microphone arrived in the form of the ribbon microphone and this technique was deemed to be obsolete. As someone said, "Invented in 1931, forgotten in 1932".
However, in recent years, this technique has been rediscovered by a small band of engineers and it has been found to have unique qualities. Since there are some engineers who much prefer the sound of omnis to directionals (and they do have certain advantages), this technique gives real stereo from omnis. It can also be used with directional microphones and PZM's (see below). We have also used it with forward facing figure of 8's with great success as well as with cardioids. The shuffler has the effect of increasing the directivity of directional microphones and this can be really useful in certain circumstances like highly reverberant acoustics where it would be desirable to cut down the amount of reverberation reaching the microphones. BDT excels in small and/or very dry acoustic spaces, giving an often astonishing re-creation of the original acoustic. It's also rather good at playing binaural recordings over loudspeakers.
If you would like to try BDT for yourself, you can build your own shuffler. Download the circuit diagram. This shuffler assumes a spacing of 20cm and will give best results with this spacing. Remember that, if you use a baffle, it has the effect of spacing the mics much further than their physical spacing. In the pictures below, you will probably need to reduce the spacing between the capsules. I have found that making the mics virtually parallel to each other works best when used with this kind of baffle.