4. Filters defined in group propagation time

Some applications require a very constant bandwidth group time, attenuation being less important. This is particularly the case for filters placed at the end of the chain of laser disc players, or after the heads of magnetic tape players. The only ones that can be calculated analytically are transfer functions with minimal phase shift, although their attenuation increases very slowly, even for very high n orders. The most commonly used filters are Bessel filters and Gaussian amplitude filters.

Before examining the two transfer functions of these filters, let's recall the relations that link the propagation time τ to the transfer function of a harmonic network:

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