NTM Crossovers
(Neville Thiele Method)

NTM crossovers are exact summing, steep roll off crossovers. As the name suggests they were invented by Neville Thiele. They are now the intellectual property of Immersion Technology International Plc and are protected by patents.

They have the rare advantage of providing steep roll off with low group delay and they have a well behaved phase response.

Advantages of NTM

• NTM crossovers substantially reduce the breadth of interference region between bands to the extent that interference becomes negligible.
• NTM crossovers have significantly reduced group delay variation compared to conventional crossovers of equivalent steepness.
• In analogue implementations NTM crossovers have substantially lower component counts than conventional systems of equivalent steepness. This is because low order NTM crossovers give steep roll offs, whereas with conventional systems a high order crossover is required to get a steep roll off.

NTM crossovers may be implemented as analogue or digital systems. NTM crossovers may be implemented as active or passive implementations. Simple passive NTM crossovers may use the well known Sallen Key topologies.

Why steep roll off is desirable

• To prevent spikes and anomalies from outside the band affecting frequency response
• To eliminate superposition of time and phase mismatched signals from outside the band.
• To eliminate interference effects

Why low group delay variation is desirable
Group delay variations cause audible distortion in the response of a filter. The lower the variation in group delay the cleaner the sound.

The NTM technology
NTM crossovers are low order steep filters. The steep slope is achieved by the introduction of transmission zeros in the filters. Other crossover systems have attempted this, but without the means of selecting values for flat response. The main feature of NTM is that it provides correct component values for exact summing to flat response at the same time as ensuring that the drivers stay in phase with each other.

In the transfer functions for an NTM crossover the denominators of the high pass and low pass functions are made identical and the numerators comprise terms that in each individual filtering function are either all of even order or all of odd order, so that a constant phase difference is maintained between the high pass and low pass outputs.

Also the sum of the two numerators is made to have the same squared magnitude as their common denominator in order to make the response of the combined outputs sum to unity at all frequencies.