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Analog video signal requirements: Similarities and differences (Part 4)

Posted: 10 Aug 2007 ?? ?Print Version ?Bookmark and Share

Keywords:analog video? signal requirements? video standards?

By Randy Stephens
Texas Instruments

Analog video has been around for decades and is still in use today. The original and most common video standards include the National Television System Committee (NTSC) and Phase Alternation Line (PAL). Other modern consumer analog video transmission systems include S-Video, Component Video, professional G'B'R' video and computer R'G'B' systems.

This article will explore some of these analog video signal requirements and how they are similar, yet different, from each other and how to simplify the analog I/O design in these video systems.

Power supply voltages and power dissipation
Most video systems use single-supply data converters with a 3.3V supply. If this supply could be used for the video filter/amplifier, it would make the system easier to use and potentially could eliminate a power supply or tworesulting in lower costs. The THS73x3 parts exploit this capability by operating from 2.7V to 5V single-supply. The BiCom-3 process was designed to operate at these voltages and has no performance disadvantage over the entire operating range. In fact, some specifications such as differential gain and phase improve with lower power supply voltages.

Figure 8 shows a typical configuration for the THS7303 amplifier buffering a DAC and accepting an external input, while utilizing a 3.3V power supply with SAG correction on the output. This figure should be a reference for the remainder of this paper.

Another factor to consider is power consumption. It is not uncommon to find 5V single supply parts similar to the THS73x3 products. But many consume over 500mW of power with some as high as 1.2W. This can result in very high die temperatures and can easily impact long-term reliability. The THS73x3 consumes only 55mW of power while running on 3.3V supply. This virtually eliminates thermal and reliability concerns.

To conserve power, each channel can be shutdown individually. If all channels are shutdown, the current consumption is less than 1?A. Together, these parts are applicable for power sensitive systems such as portable or USB powered systems.

Signal coupling
One concern about running from a single-supply down to 2.7V is whether or not the video signal will have clipping. This is where proper DC-biasing is very important in the design. With so many different types of video systems and designs, having the flexibility to properly bias the THS73x3 is critical.

Belkin's USB Hub and dongle

Figure 8: Typical system configuration for the THS7303 in a 3.3V single-supply, DAC DC+Shift and AC-STC and AC-bias coupled inputs, along with SAG corrected output line driving.
(Click to view image.)

If the THS7303 or THS7313 6dB gain amplifier is designed into a system which is driven from a ground-referenced DAC or encoder, the DC input mode is ideal. The question is how low is the voltage created by the DAC? If the sync signal (which is typically the lowest voltage of a video signal) is below 50mV, then the output of the 6dB amplifier needs to generate voltage less than 100mV. This can be very difficult for any amplifier due to transistor saturation limitations, which exists in both CMOS and bipolar amplifiers.

To remove this limitation, all THS73x3 products have a DC + Shift mode that adds an internal DC voltage offset to the video input signal. This offset is internal only and will not impact the applied signal. This offset will ensure that even if 0V is applied to the input of the THS73x3, the output will not saturate and clip.

If the DAC output voltage only goes down to 100mV, then the DC input mode is best. This mode does not add an offset into the system as it is not required. Keep in mind that there are offsets in any amplifier and this is true for the THS73x3. This offset voltage is typically small, but does have part-to-part variations.

If the DAC is referenced from a power supply such as 3.3V or 1.8V, or is an external input, then utilizing AC coupling is the best mode. AC coupling allows the THS73x3 to ignore the source DC-bias point and will re-establish its own DC-bias point. The AC coupling options include AC-bias or AC-sync tip clamp.

AC-biasing is very simple. The THS73x3 has two resistors that create a voltage divider between the power supply and ground. The input impedance of this mode is about 20 kiloohms. As such, the capacitor used should be large enough to ensure any tilt or droop issues are minimized. In general, a 4.7?F to 10?F capacitor will minimize any tilt problems. This mode is best used with Chroma or color difference signals. It can also be utilized for the Luma signal, G'B'R' signals, or computer R'G'B' signals. Since the signal is AC-coupled and the DC-bias point varies with the average signal level, it is best to utilize the AC-Bias mode with signals that have sync information with 5V supply to ensure no clipping will exist.

The patent-pending AC-sync tip clamp (STC) mode (Figure 9) is best for signals with syncs at the lowest point of the video signal. This implies the Luma (Y'), G'B'R' with sync, or computer R'G'B' with sync signals are best utilized with AC-STC mode. The sync-tip clamp system in the THS73x3 has an internal current-sink to discharge the coupling capacitor, a filter to minimize interaction with high-frequency interference signals that may be present, an amplifier that monitors the difference between the voltage on the input and the reference voltage, and finally a transistor to charge the capacitor when the signal goes below the reference. As such, this is a dynamic system that does not rely upon timing in any way. This type of system is also generally called a DC-restore system rather than a diode clamp system. The problem with diode clamp systems is they react to any high-frequency signal or overshoot. This can result in an undesirable excessive DC-bias point shift and can clip the signal.

Belkin's USB Hub and dongle

Figure 9: The patent-pending AC-sync tip clamp mode is best for signals with syncs at the lowest point of the video signal.
(Click to view image.)

The flexibility of the THS73x3 family allows the user to tweak some of the AC-STC functions. This includes the STC filter between 500kHz, 2.5MHz and 5MHz. This is important as the horizontal sync width varies with the signal standard applied (see Table 1). If the 500kHz filter was utilized with a 720p Luma signal, the STC circuit would never engage and the system would float. But, if a CVBS signal is noisy or has considerable ringing, the 500kHz filter would be best to minimize any DC-bias shift internal to the THS73x3.

The AC-STC allows a discharge current to be selected. If the voltage appearing at the input to the THS73x3 drops below the reference voltage, the system can charge as much as 2mA of current to increase the voltage. What happens if the voltage is considerably higher than the reference voltage? The discharge current will reduce the voltage on the capacitor at a rate equal to I/C = dV/dT. This current is selectable from about 2?A, 6?A and 8?A. Having a high discharge current allows the system to capture a signal quickly or increase hum rejection (when 50Hz or 60Hz line signals couple into the system). Other systems require a very low discharge rate to improve line-tilt or droop. This is when a video signal is held constant over the entire line. Because of the AC-coupling and discharge current, the DC signal will tilt downwards. It is generally acceptable to have less than 1IRE of tilt over a line. This selectability allows the system to connect to essentially any outside source without requiring the need to change input capacitors manually.

Figure 8 also showed the 2:1 input MUX feature. This, coupled with the user configurable input coupling that is completely independent of the other channels, allows the THS73x3 to be utilized in many different systems.

For Part 1, please click here.

For Part 2, please click here.

For Part 3, please click here.

About the author
Randy Stephens
is a member group technical staff at Texas Instruments Inc.




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