Active video filter

High-performance op amps and specialized software on PCs facilitate the design of wideband active filters, but this does not solve the problems of any specific application. For video filters, specific applications and signal formats bring subtle differences to each circuit design. Below are the two main categories of video applications.

Anti-aliasing filter: This type of device is placed before the analog-to-digital converter (ADC) to attenuate the components in the signal above the Nyquist frequency, that is, the signal components above half the ADC sampling rate. The response of these filters is usually designed to be as steep as possible, with the goal of filtering out as much signal as possible above the cutoff frequency¹. For ITU-601 and other applications, this performance is achieved by combining analog filters, digital filters, and oversampling ADCs. For applications such as PC images, a small amount of filtering is enough.

Reconstruction filter: Also known as (sinx)/x or zero-order holding corrector, this type of device is placed after the digital-to-analog converter (DAC) to eliminate multiple images caused by sampling, not to eliminate DAC clock crosstalk. Reconstruction filters are rarely as selective as anti-aliasing filters because the hold function of the DAC also acts as a filter—an effect that reduces the selectivity requirements but at the cost of response. Existing video signal formats include RGB, component video, composite video and RGB PC images.

All applications and formats hope that the video filter can be "phase linear", which is required by the group delay (delay at different frequencies) characteristics. The required phase linearity depends on the specific application and video format. For example, anti-aliasing filters and component formats have more stringent requirements than reconstruction applications and composite video. Requirements for various applications and formats are specified by NTSC, PAL/DVB, ITU, SMPTE and VESA.

This article will compare different filters to determine the best design for a given application or format. The ratio of GBW to cutoff frequency of two types of filters, Rauch and Sallen-Key, was compared, and predistortion and parametric sensitivity techniques were used in the design to obtain high accuracy. Filters examined include:

• ITU-601 anti-aliasing filter

• 20MHz anti-aliasing and reconstruction filter

• HDTV reconstruction filter