Analog video time base correction and processing for nonstandard TV signals
Despite the rapid advancement of digital TV, analog TV will remain dominant for both transmission and display for several years to come. Analog video formats, like digital formats, have precise specifications as to how to format video properly. These specifications include how the luma, chroma, and synchronization information is packaged to provide the line, field, and frames of video information that recreate the images observed on TV screens. To display the video information accurately, each of these components must be extracted correctly from the video signal. Similarly, the timing and phase information must be maintained or recreated as it was when the video signal was first encoded at the source.
This article outlines the challenges encountered when decoding and restoring a correct time base to nonstandard input video sources. Examples of where restoring a correct time base is critical to maintaining good image quality include analog tuners (NTSC, PAL, and SECAM) used on most TVs today and the venerable VCR. Since most people today continue to rely on analog tuners and VCRs, it is important that advanced digital video decoders utilize the latest clock reconstruction techniques to produce outstanding image quality in DTVs.
Noise-induced time-base inaccuracies
NTSC, PAL, and SECAM video consist of lines of video information packaged in fields and frames. The lines, fields, and frames are identifiable by embedded synchronization. The correct extraction of this information enables the receiving device--TV, VCR, or projector--to reconstruct the images and provide a visual display.
Video signals consist of various components, each of which can be altered or corrupted within the transmission path, resulting in distortion of some video package aspects. For RF transmitted signals, the synchronization information is normally present on the recovered signal, but its detection and extraction can be difficult or impossible because of excessive noise. It is important to note that even when recovering the synchronization is possible, its detection can be offset due to noise, which in turn introduces jitter on the recovered synchronization information.
Figure 1: Line Synchronization Stream without Noise at Source
Figure 1 shows a representation of a typical stream of line synchronization information. All line lengths are the same and conform to nominal specification requirements. Receiving and extracting the correct synchronization results in a proper and stable display. Figure 2 shows that slicing the synchronization information from this stream results in a stable display.
Figure 2: Stable Synchronization Extraction with No Vertical Jitter on Displayed Image
NEXT: Noisy sync problems