Reconstructing analog video

After analog video is captured, digitized and transmitted, an even bigger challenge remains: restoring the video image without artifacts or moiré effects to deliver the best possible viewing experience. To do this, designers must examine the process of converting digital video back into analog video (video reconstruction) and the various filtering techniques and implementation schemes that can be used to avoid or minimize moiré effects and picture artifacts.

Television is literally seeing at a distance. Analog light is captured by a camera and typically the video is digitized for convenient transmission. Eventually our analog human eyes require analog light to view the image (Figure 1 ). The process for turning digital video back into analog video is called reconstruction. However due to quantization issues and other issues caused by moving from the digital to analog domains, moiré effects and picture artifacts have to be removed through various filtering techniques to deliver top-quality video.

Figure 1: Television starts as analog light, becomes a digital transmission, and returns to analog light for our eyes.

Digital Video has many advantages, it can be compressed with predictable quality and once digitized it doesn't degrade with storage and transmission. Transmission includes many ways of delivering the image such as DVD, satellite, cable and over the air systems. To view digital video with our analog human eyes it must be reconstructed into an analog video signal and into analog light.

Digital video can be thought of as a jigsaw puzzle made up of picture pieces that may be transmitted out of order and contain spurious artifacts. However, by following a set of rules we can reassemble the picture whose quality corresponds to the original analog video input signal.

At the end of the digital reassembly process both video and the jigsaw puzzle needs some “analog smoothing”. For the jigsaw puzzle that analog smoothing can be squinting our eyes or moving back far enough that the lines between puzzle pieces are not objectionable. For video this analog smoothing is accomplished by an analog low pass filter.

Nyquist, image frequencies and DACs
An analog video signal delivered by a camera or other capture device is digitized in an analog-to-digital converter (ADC) by memorizing the value in an instant of time at each clock edge (Figure 2 ). The arrows at the upper left of the figure show the clock instant that the analog signal data is stored. The analog signal is continuously changing but the digital representation is sampled periodically. After digital processing and transmission the digital signal is converted back to analog video by a digital-to-analog converter (DAC). The DAC's output is shown in the upper right portion of Figure 2 , with the arrows again representing the clock.

Figure 2: Conversion waveforms from analog to digital and back.

At each clock instant the digital value is converted to an analog voltage. That analog voltage is then held until the next clock edge. The output is a series of stair steps compared to the smooth curve of the original analog signal. This is called a “sample and hold” or “boxcar” reconstruction. An analog low-pass reconstruction filter is necessary to smooth the waveform to approximate the original analog video.

Next: Image frequencies, sidebands and edge wiggle artifacts

When you examine the time domain version of Figure 2 , you will see that the little stair steps have unwanted high frequency information but the source of that unwanted information is not obvious.

Figure 3 illustrates the effect of digitizing the signal in the frequency domain. Standard Definition (SD), PAL (European) and NTSC (North America) video has a bandwidth of about 5 MHz, while the High Definition (HD) ATSC 720p and 1080i (U.S.) has a bandwidth of 30MHz. A typical clock frequency for SD is 27 MHz and for HD the clock goes up to 74.25 MHz or above.

Figure 2: Conversion waveforms from analog to digital and back. (repeated for clarity)

The Nyquist frequency indicated is always one-half the clock frequency. Nyquist is important as video components and noise above the Nyquist frequency must be removed before the original analog signal was digitized. If information above Nyquist is present it will be confused with lower frequencies and aliased down to mix with and corrupt the video. Once an alias is created it cannot be removed. Later in this article we will explain why this is important in a home video system.

Image frequencies and sidebands
At the output of the DAC, the video and two image frequency sidebands are present (Figure 3a ). The clock is drawn to clarify the figure, although most modern DACs are well-enough balanced so as to suppress the clock frequency. Mathematically these sidebands are the sum and difference frequencies between the video and clock and are a normal part of every digital video signal.

The upper image sideband has the same characteristic as the video. That is, the low frequency video signals are found just above the clock and the higher video frequencies extend to the right.

Figure 3: The frequency spectrum folding shows the sources of interference caused by poor video filtering.

In the case of SD with a 27 MHz clock, the top of the upper image is 5 plus 27 or 32 MHz. The lower image sideband is inverted so that the low frequency video is just below the clock and the higher video frequencies extend to the left. Therefore the SD lower image extends to 27 minus 5 or 22 MHz. It is important to understand where the system lower image frequency is so that it can be attenuated and thus minimize its visibility. This critical frequency for HD with a 74.25 MHz clock is 74.25 minus 30 or 44.25 MHz.

To reflect the effect of not attenuating the image frequencies, Figures 3b and 3c show the folding the spectrum at the Nyquist and Clock frequencies. These added image frequencies (Figure 3d ) have a random phase compared to the video. Figure 4 shows the picture errors we want to avoid. The “edge wiggles” are the high frequency edge interference that folds back with random and constantly changing phase on top of the video. Moiré is interaction between two frequencies such as the clock and video.

Figure 4: Picture inteference resulting from poor video filtering.

Next: Reconstruction filters minimize artifacts, designing with Maxim's MAX7443

Reconstruction filter design
Figure 5 demonstrates the results of a proper reconstruction filter. Some literary license has been taken to make the frequency folding easy to understand. Normally the frequency domain is plotted with both frequency and amplitude having logarithmic scales. The reconstruction filter would then be a smooth curve. However to illustrate the sideband position of the folding, the frequency and amplitude are plotted on linear scales. To show the attenuation of the low pass filter on the same scale the filter curve has been bent to show that there is little attenuation of the wanted video and greater attenuation of the image frequencies.

Again, Figure 5b and 5c show the folding effect. But note that the image frequencies in Figure 5d are greatly reduced as compared to Figure 3d .

Figure 5: Proper reconstruction filtering reduces the visibility of the folded interference by smoothing the video waveform.

Designing with Maxim's MAX7443
Consumer products are very cost sensitive, of course, but thankfully there are only a few analog and digital conversions. A typical home has one DAC and one ADC for example in a Set Top Box feeding, a LCD TV set (Figure 6 ). Here having an attenuation of 20 dB (to less than 10 percent of the signal) is acceptable. A Set Top Box and a LCD TV typically have at least 12 dB lower image attenuation in each unit. Two filters with 12 dB attenuation add to create 24 dB of rejection. This provides a comfortable manufacturing tolerance over the needed 20 dB of attenuation.

The block diagram of Figure 6 details a Set Top Box or DVD player on the left side. The System On a Chip (SOC) contains the DAC's whose outputs are routed to low-pass filters such as the Maxim MAX7443, which reconstruct the video. The filter ICs also contain 75 ohms coaxial cable drivers to deliver the signals through the cables in the center of the diagram. The right side of Figure 6 receives the signals which are anti-alias filtered before going into the TV set's ADCs in the SOC.

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Figure 6: A block diagram of a consumer's home television system showing proper video reconstruction and anti-alias filters.

Next: The “last piece added syndrome”

The “last piece added syndrome”
Suppose a consumer has an operating multi-unit system and adds a new piece of equipment. If the picture degrades it is always the fault of the “just added piece”. However, this might not always be true.

For example, let's say the consumer replaces a DVD player with a new unit. His existing LCD TV has poor input filter (with 2dB attenuation). The old DVD player happened to have a very good output filter (with 18dB attenuation), but the new DVD only has a poor or non-existent filter (with 6dB or less attenuation).

Remember that Nyquist is important because video components and noise above the Nyquist frequency must be removed before LCD TV re-digitizes the DVD analog signal. If information above Nyquist is present it will be confused with lower frequencies and aliased down to mix with and corrupt the video.

Therefore the old DVD and LCD TV set had 20dB attenuation and the new combination has 8dB attenuation. As a result, the consumer returns the new DVD player even though the LCD TV is more responsible for the mysterious errors.

What can a manufacturer of Set Top Boxes, DVD players and TV sets do to protect themselves from the “last piece added syndrome”?

First, provide filters that function as designed. Some manufacturers may use filters made of discrete inductors and capacitors on their PC Boards. However these discreet filters are subject to manufacturing mistakes. The wrong part value can be stuffed on the board. In the heat of mass production one might want a 270pF capacitor but pick up the reel marked “270” which is really 27pF. The resulting filter would pass unwanted high frequencies creating picture artifacts in the next piece of equipment. Being cost sensitive, the final production test doesn't test every parameter and typically filter bandwidth is not tested.

Second, use filters that provide more than the bare minimum 10dB attenuation. The additional attenuation will protect the manufacturer from consumer returns and keep the customers happy.

The Maxim filter family solves both of the above “last piece added syndrome” issues. Maxim's integrated filters are 100 percent bandwidth tested by the Maxim's Automatic Test Equipment (ATE) before they are mounted to the PC Board by the manufacturer. Maxim's filters provide more than the typical industry attenuation. For example, the MAX7443 for Standard Definition TV provides more than 30dB image and more than 40dB, 27MHz clock attenuation. The MAX9500 for High Definition TV provides more than 38dB image and more than 38dB 74.25MHz clock attenuation.

About the author
Bill Laumeister is an engineer with the Advanced Video Strategic Applications Group at Maxim Integrated Products and works with companies selling consumer products. He has thirty-eight years experience and holds several patents in the video field. He is the inventor of a video communications method called VEIL (Video Encoded Invisible Light). It is being considered by the U.S. Congress in the “Digital Transition Content Security Act” as a possible patch for the “analog hole”. He can be reached at .

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