ADC/DAC

FIR Filter to Reduce the Sampling Frequency

In order to keep the filter specification and the associated implementation effort low, most audio applications use a multiple of the actually required sampling frequency. In order to reduce the computational effort for downstream digital processing caused by the higher sampling frequency, a multi-stage decimation of the sampling frequency is usually carried out with anti-aliasing filters. These are often implemented as FIR (Finite Impulse Response) filters. FIR filters have the advantage that they have a linear phase and by multi-phase splitting, including decimation, the number of filter operations can be reduced by the decimation factor. Before the digital-to-analog conversion, an analog procedure must be followed, i.e. the audio signal is sampled in stages with subsequent anti-aliasing filtering.

The reduction of the filter operations is also achieved by the interpolation factor using the same splitting procedure. Step-up sampling minimizes the design requirements of the downstream analog low-pass filter in the same way as for the analog input filter. Over-sampling therefore helps to reduce the BOM costs in analog filter design to a minimum. And there is another important feature for combining over-sampling and low-pass filtering. If the filter amount are carried out in an sufficiently high word width, the noise power produced by the quantization is reduced by the factor 1/fA. This improves the SNR (Signal to Noise Ratio)and increases the quality of the audio signal.

Word Width Determines the Amount of Quantization Noise

Besides the sampling frequency, the word width is the second important value to consider when selecting converters. It determines the amount of quantization noise in the audio signal. The higher the word width is selected, the lower the proportion of noise power, and the better the SNR.

In order to meet the highest demands in digital audio processing, CODICO offers a wide range of converters with word widths up to 24bits and sampling frequencies of 192 kHz and more. In addition to differential inputs and outputs and integrated amplifiers, converters in multi-channel versions and with digital signal processing are also available.