Board-level DVRs ease design for innovative video apps

If your application requires digital video recording, you can design a homegrown solution or, rather than starting from scratch, begin with a board-level DVR. This decision may be complex, depending on what your objectives are and how you set out to accomplish them.

First you must determine what your specific application requires. For example, you may be starting with some form of an analog video signal. Simply recording your analog signal onto tape for archiving is now widely considered an outdated concept, and with the continued decline of memory costs, the economic justification of remaining tied to a tape-based storage process is also considered outdated.

Begin with the input signal. In the analog context, your input signal is probably S-Video, composite, or component. So the first decision that will need to be made is whether or not you will change the signal itself by replacing the camera, or if converting that signal to a digital format would be suitable. Given the capabilities, costs, and image quality available today with most commercial analog-to-digital converters (ADCs), there are few instances where the input itself would need to be digital as opposed to analog. An ADC is an electronic circuit that converts continuous signals into discrete digital numbers.

The type of ADC you select will greatly influence the quality of the image recorded and subsequently viewed. In order to ensure the integrity of the images you are recording, use ADCs with YUV digital color space, 60 fields/second NTSC, 50 fields/second PAL, and pixel resolution of 720 x 486 (525/60) and 720 x 576 (625/50). Otherwise, it will be impossible to deliver high-quality recorded images.

Note that the conversion of analog signals to digital does require a bit more effort than simply selecting an ADC. It's easy to underestimate the task of decoding analog video into digital components, and then manipulating the output of a video encoder into a high-quality analog signal.

Adapt reference designs with care
Careful handling of the analog signal, especially the A to D conversion step, is fundamental to high picture quality. Losses at this stage are the single biggest culprit in degrading the image. While the danger is loss of resolution and signal distortion, a more insidious issue is that high-frequency noise can easily be injected, which the downstream compression encoder sees as high-frequency picture information. This effect exacts an extremely high cost in reduced compression efficiency, increasing the data rate by twenty percent or more, and subsequently exacerbating compression artifacts. In bandwidth-limited applications, the side effect is increased quantization and further reduction of high frequencies to keep the data rate on target. A picture compressed from a noisy input is visibly degraded when compared to a picture compressed to the same size from a clean input.

Monolithic analog video decoders and encoders are available from several well-known vendors, along with reference designs that neatly specify values for supporting discrete components. Regrettably, this cookbook approach is deceptively simple and often provides poor results. For better or for worse, the legacy composite video signal is packed with idiosyncrasies. Frankly, intimate knowledge of the interdependencies of subcarrier, sync, luminance, and chroma belongs to a tiny number of individuals who have invested the time to learn the details, spending countless hours in real-world video situations playing witness to common problems and workarounds.

When applying a reference design to new hardware with the goal of maximum picture fidelity, the designer must navigate the pitfalls of selecting high-performance components and judiciously enhancing or sometimes discarding discrete filtering. During the all-important layout phase, one must pay fanatical attention to such details as component placement, power supply filtering, and proper use of analog and digital planes in close proximity to one another. At this point, the real work begins. After power-up, the engineer's familiarity with video is put to the test. The first issue is adjusting component values and layout. The second is judging the effects of adapting the silicon's bewildering register settings for optimum results in the context of a unique circuit board.

At this point you have taken your analog signal and converted it to digital. Next you need to determine how much data you will need to store. This is where video compression decisions become extremely important. By its very nature, digital video requires high data rates, and the higher the quality of the image, the more data that is required. Video compression is a process that enables a digital video signal to use much less data. There are many forms of compression available today, each with their own advantages and disadvantages. Though we do not have the time to get into all the differences between compression methods in this discussion, it is worth noting that the terminology used to describe some of these compression methods can be a bit misleading.

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