BLS1 - Quick guide

| ## BLS1 - Quick guide
| BLS1 is a digital realisation of the 'Blumlein Shuffler', invented by Alan Blumlen in the early 1930s and analysed in detail by Michael Gerzon in a paper presented at the 1993 AES Convention in San Francisco. The Blumlein Shuffler is used to convert binaural stereo signals into a form suitable for reproduction using a convential stereo speaker pair. Binaural signals are provided by e.g. a dummy head, or a pair of closely spaced omnis with a baffle (e.g. a Jecklin disk) in between them. Such signals have no significant level differences for off-centre signals at low and low-mid frequencies, only a phase difference which depends on the frequency, the distance between the mics and the source direction. The shuffler turns these phase differences into amplitude differences by applying a specific filter to the (L-R) signal. This can be done only as long as the phase difference is not ambiguous, i.e. less than half a cycle, so the shuffler is normally set up to not affect higher frequencies. For these, sufficient channel separation must be provided by the input signals themselves, typically by using some sort of baffle between the mics. The ideal difference channel filter would be of the form 1 + a/s, i.e. unity plus an integrator, or the inverse of a first order highpass. But since this has unbounded gain as frequency goes down it can't be used. An analog implementation would use an LF shelf filter instead. The problem with this is that limiting the gain at LF also means that the phase response returns to zero, while it should ideally stay at 90 degrees. This could be compensated for by using all-pass filters in both the sum and difference channels, but this complicates the circuit (in particular if the shuffler parameters are variable) and of course also affects the overall phase response. Zita-bls1 uses a FIR filter (1024 taps at 48 kHz) that has the amplitude response of the shelf filter but the phase response of the ideal filter, and the sum channel is just a matching delay. The finite length of the FIR means there will be some roll-off at very low frequencies, in this case -3dB at 40 Hz, which is probably not a bad thing. It could be avoided by using a longer FIR. Apart from the basic shuffler algorithm some extras are provided: an input gain balance control, an 18 dB/oct highpass filter, and a variable frequency LF shelf filter.
| The rotary knobs can be used in two ways: * Click on the knob with the left mouse button, keep it pressed and move either left..right or up..down. * Using the mouse wheel. This provides 'sensible' steps, e.g. 1 or 1/2 dB for gains and 1/3 or 1/6 octave for frequencies. Press Shift for smaller steps. From Left to right we have: * Input balance. Not all mic preamps provide perfect matching of channel gains, and the mics themselves may have slightly different sensitivities as well Since the shuffler amplifies the difference between the L and R inputs, even small gain errors could have a significant impact. This control is provided to allow exact matching of the two input signal gains. * Highpass filter. As the shuffler boosts LF signals it's advisable to remove any low frequency rumble. Ideally this should be done by the mic preamps, but not all of them all provide such a filter. Filter slope is 18 dB/oct. When set to 10 Hz it has no significant effect in the audio band. * Shuffler gain. This the maximum gain the difference channel filter will have as frequency goes down. Depending on the combined gain and frequency settings It may not reach this value since there is also a fixed rolloff (-3 dB at 40 Hz). * Shuffler frequency. This is the frequency at which the difference channel filter will have 3dB gain. * LF shelf filter frequency and gain. The LF shelf filter is provided to correct the tonal balance in the low frequency range which may be affected by the action of the shuffler. It is a second order design and has a somewhat steeper slope than most shelf filters.