As discussed previously, early methods of output reproduction with multi-bit DACs include ?brickwall? filters. These filters had very sharp cutoff characteristics and held the signal gain close to unity until cutoff (early CD players used elliptical filters). This was necessitated as the data was at a frequency such that aliasing and noise artifacts existed immediately above the audible band. The inherent problem with such a filter design was that they had tremendous phase non-linearities at high frequencies, and high frequency group delay; change in phase shift with respect to frequency. The second method of output reconstruction deals with an oversampling digital filter prior to the DAC and a gentle analog filter. By gentle, it is meant that the cutoff slope of 12 dB/octave and a -3 dB point of 30-40KHz can be used. Its design then is non-critical and low-order, which guarantees excellent phase linearity. In fact, for most practical reconstruction filters, phase distortion can be held at plus or minus 0.5 degrees over the entire audio band.
To combat the problems of multi-bit converters, two competing converters called low-bit converters were introduced in CD audio reproduction technology. These converters converted the data in a serial fashion rather than in a parallel stream of 16 bits. These converters that are used in modern CD players for high fidelity sound reproduction are 1-bit converters. Principally, these converters convert a stream of PCM modulated bits into a pulse width modulation technique (PWD). As PWD can only take two values of amplitude, high or low (some reference value and some maximum value and no intermediate values), but of varying width, this type of encoding can be carried out with a single bit; thus the name. Figure 1 shows the concept of PWM.