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Resurrecting analogue video format with FPGAs

Posted: 04 Apr 2013     Print Version  Bookmark and Share

Keywords:high definition video  analogue broadcasting  FPGAs 

Digital video broadcasting, video compression and ever increasing video resolutions such as 4k x 2k dominate the news in electronics magazines. Yet that little yellow RCA connector remains ubiquitous and large numbers of people still rely on NTSC or PAL analogue broadcasting for their viewing pleasure. This article looks at how FPGAs are breathing new life into this presumed dead format.

There are two main components to enable analogue video transmission: the encoder at the transmitter (e.g., the camera) and the decoder at the receiver (e.g., the television). Most major semiconductor manufacturers offer at least one of these components – most offer both. Not surprisingly, these components are usually older designs, some perhaps having their origins as much as 20-30 years ago. After all, the standards have not changed, so why the need to update the ICs. In this article I will consider how the introduction of low-cost, high-functionality FPGAs justifies looking once again at analogue video transmission.

Returning to old friends
Even high definition video sources can benefit from having that RCA connector, even when – for legacy reasons – they aren't obliged to do so (e.g., your Blu-ray player). How many times have you had a blank screen when using HDMI and wished you had another robust output just to check you are not going mad?

Consider an HD security camera, for example, which may offer HD-SDI or analogue component YPbPr outputs. Offering a simultaneous NTSC or PAL output provides an easy method to connect and test the installation, whilst the use of NTSC and PAL allows the transmission over hundreds of meters of existing low cost coaxial cable installations, something neither YPbPr or HD-SDI can do, but of course without the resolution of HD.

Recent low-cost FPGA offerings from, for example, Altera (Cyclone) and Lattice (XP2) offer a way of adding this output at low cost. A broadcast quality NTSC encoder may use approximately 6000 logic elements, much less for a consumer grade encoder, allowing the smallest of FPGAs to be utilised. And, of course, the spare logic of the FPGA may be used for additional functions, such as the camera control interface.

But once we have our own encoder and are not limited by the constraints of a 10-year-old designed ASIC, we can improve its performance. We can change the sampling rate of the encoder to better match the sensor. For example, Sony's Effio image sensors offer 960 pixels/line compared to 720 pixels for a typical NTSC/PAL sensor. This image can be transmitted over conventional 'NTSC' transmission paths, yet gives a much better detailed image for very little cost-up. Our FPGA encoder can easily be modified to transmit this extra information.

Perhaps we wish to transmit some additional data? Again, in a closed system, we can modify our custom encoder to transmit data or digital audio in the vertical blanking interval, similar to how Closed Captioning or Teletext worked.

NTSC and PAL are composite video formats – the chroma and luma occupy the same frequency domain – a restriction we no longer have in closed system. Our own video encoder can separate the luma and chroma, sending them at different frequencies, and therefore avoiding the issues with cross-colour at the video decoder.

Of course, the analogue video decoder has to 'understand' any changes we make to the transmitted signal, but if we implement that too in an FPGA, then we can again easily make the necessary changes.

Combing through the analogue video decoder
Apart from providing the compatibility to the encoder changes we mentioned above, is there any other incentive to moving a well proven IC function to an FPGA? You will not be surprised to find the answer to that is a resounding "Yes!"

The most obvious result of viewing analogue video sources on a large display is that any artifacts are, of course, larger and visually more apparent. For larger displays, the analogue video decoder actually has a more stringent requirement. This problem is compounded because the flat screen displays require additional processing of the analogue source before it can be properly displayed, namely de-interlacing and scaling. The de-interlacer, in particular, can amplify any artifacts left from the video decoder. This is because the de-interlacer is sensitive to motion in the image, and residual artifacts and noise left from the analogue decoder cannot be discriminated from real motion in the image. The result is the de-interlacer may make the wrong mode decision resulting in additional artifacts.

A similar issue arises if the output of the video decoder is to be compressed, since all MPEG compression methods effectively send only the motion of an image. Unable to discriminate between artifacts, video source noise, and 'real' image motion, it can be shown that up 20% of satellite and cable digital broadcast bandwidth is utilised to send unnecessary information. This is extremely useful bandwidth that is especially useful given the high compression ratios used by today's broadcasters, and is the difference between the viewer seeing the highly visible MPEG artifacts – such as blocking – or not.

One large improvement to the video decoder that has been made by some manufacturers is to add a 3D comb filter. Even on the most complex images, near-perfect, artifact-free decoding is the result. However, the memory requirement for this is large enough to require an external device, the cost of which usually precludes this desirable feature being implemented.

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