Digital-to-Analog Conversion (DAC)
Figure 1 illustrates the process of converting digital signal into analog signal. It is a simple process that can be accomplished with the help of a few off-the-shelf electronic parts, yet the quality of the output signal can vary a lot with different system designs. In order to distinguish a good system design from a generic one, a basic understanding of the system building blocks becomes essential.
The Input Receiver is the digital interface with will receive a digital signal from a CD Transport; it provides the functions of data extraction, control and user bits extraction, incoming frequency measurement, error detection and correction, and low jitter clock generation and regeneration.
The digital filter is used to reduce phase shifts, and to improve signal to noise ratio. It is used at the input of the D/A converter to suppress the frequency spectrum immediately above the audio band. In this particular application, digital filter is preferred because it does not produce the sharp cutoff output normally found with analog filters.
The D/A converter are used to convert the digital signal to an analog current signal. In general, system with the higher number of bits has lower noise, better linearity, improved dynamic range and lower distortion.
Because the output signal of D/A converter is in the form of a current, an I/V converter (usually an integrated function of the OP-AMP) is used to convert the current based signal into a voltage waveform.
Low pass filter
Used at the final stage of the D/A conversion process, a low-pass filter reconstructs the analog signal from the original digital sample and smoothens the output waveform for further processing or amplification.