What is SDI pathological?
November ,18 ,2025
In this series, we're going to discuss the Serial Digital Interface, or SDI. Today, we'll focus on a video pattern used to test SDI retimers called the Pathological Pattern. First, we'll review the origin and negative effects of the Pathological Pattern, namely baseline wander, and then show how a retimer can counteract these effects. Lastly, we will discuss a practical test using this pattern that demonstrates the effect of increased capacitance on the input to a retimer.
In broadcast video applications with SDI, cameras output video data through long, single-ended 75-ohm coaxial cables to a line card. Retimers on the line card then equalize and send a clean version of the camera data to an FPGA for processing and overlay. For example, consider a live football game: during the game, many cameras simultaneously send video data over long coaxial cables to the control room for further processing. Due to the length of the cables, a retimer is often needed to ensure that the video data is free from intersymbol interference and jitter, whether the sources are equalizable or non-equalizable. Retimers ensure that critical events, like a touchdown, are reliably captured.
The Society of Motion Picture and Television Engineers (SMPTE) defines a standard for the SDI protocol that includes a scrambler to encode the data and maintain DC balance. However, there is a flaw: when a magenta and gray static image is applied to this scrambler, it results in a DC imbalance pattern consisting of one “1” and 19 “0”s repeated over several microseconds. A complementary pattern of 19 “1”s and one “0” is equally likely to occur. These DC imbalance patterns, called pathological patterns, can last several microseconds and pose significant problems in SDI systems.
Consider a system with a transmitter, a cable, and an AC coupling capacitor connecting to the receiver chip, as shown in a typical SDI block diagram. When the data pattern is DC-balanced or PRBS-like, the eye diagram at the equalizer output looks clean. However, when pathological patterns arrive, the AC coupling capacitor blocks low-frequency information. This results in baseline wander, where the waveform shifts up and down, leading to increased jitter and reduced eye opening—affecting system margins if left uncorrected.
When pathological patterns arrive, the AC coupling capacitor blocks DC content. For example, when one “0” and 19 “1”s occur, the DC content of the transmitted waveform is high. Since the capacitor blocks DC, the voltage on the other side shifts down, as shown in the figure labeled CTLE-OUT. Passing this signal through a limiting amplifier creates a quantized replica of the transmitted waveform (LA-OUT). However, this replica is imperfect because baseline wander introduces significant jitter. Filtering the LA-OUT signal with a low-pass filter matching the cutoff frequency of the AC coupling capacitor recovers the low-frequency content (BLWC-OUT). Adding BLWC-OUT to the CTLE-OUT output cancels baseline wander, resulting in a clean eye at the linear and limiting outputs. This architecture is commonly called quantized feedback.
The AC coupling capacitor (labeled CC) is crucial to baseline wander. In the following slides, we discuss the effects of changing its value. The test setup is as follows:
1. Generate the pathological pattern using a video pattern generator.
2. Route the signal through a one-meter 1694A coaxial cable to an LMH1297 cable driver.
3. The LMH1297 output has adjustable capacitance. The input AC coupling capacitor of the device under test (DUT) is effectively the combination of these capacitors, adjustable to different values.
4. The signal is either retimed by the DUT and sent through another one-meter coaxial cable to a video analyzer to check for bit errors, or sent directly to an oscilloscope to observe baseline wander at the DUT input.
The pathological pattern can be generated using a video pattern generator
An adjustable capacitance on the output of an LMH1297 cable driver is used to demonstrate the effect of input capacitance on baseline wander
Baseline wander is proportional to the equation shown in the slide (bottom right), where:
• Constant one relates to the average pathological voltage.
• Constant two relates to the width of the pathological signal.
• The time constant is proportional to the total input capacitance, including the AC coupling capacitance.
Lower input capacitance increases baseline wander. At high baseline wander values, bit errors are more likely on the video analyzer.
Effect of input capacitance on BLW
VOSCOM’s 12G-SDI Fiber extender and 3G-SDI fiber extender reliably pass Pathological Pattern tests, confirming their stable performance even under challenging signal conditions.
Broadcast video applications
In broadcast video applications with SDI, cameras output video data through long, single-ended 75-ohm coaxial cables to a line card. Retimers on the line card then equalize and send a clean version of the camera data to an FPGA for processing and overlay. For example, consider a live football game: during the game, many cameras simultaneously send video data over long coaxial cables to the control room for further processing. Due to the length of the cables, a retimer is often needed to ensure that the video data is free from intersymbol interference and jitter, whether the sources are equalizable or non-equalizable. Retimers ensure that critical events, like a touchdown, are reliably captured.
A retimer blocks residual ISI and jitter from equalizable and non-equalizable sources and re-transmits a clean data.
The pathological pattern
The Society of Motion Picture and Television Engineers (SMPTE) defines a standard for the SDI protocol that includes a scrambler to encode the data and maintain DC balance. However, there is a flaw: when a magenta and gray static image is applied to this scrambler, it results in a DC imbalance pattern consisting of one “1” and 19 “0”s repeated over several microseconds. A complementary pattern of 19 “1”s and one “0” is equally likely to occur. These DC imbalance patterns, called pathological patterns, can last several microseconds and pose significant problems in SDI systems.
Broadcast video (serial digital interface, SDI) patterns, defined by SMPTE, may consist of long durations (several us at a time) of dc-imbalanced patterns called "pathologicals".
Issue of baseline wander (BLW)
Consider a system with a transmitter, a cable, and an AC coupling capacitor connecting to the receiver chip, as shown in a typical SDI block diagram. When the data pattern is DC-balanced or PRBS-like, the eye diagram at the equalizer output looks clean. However, when pathological patterns arrive, the AC coupling capacitor blocks low-frequency information. This results in baseline wander, where the waveform shifts up and down, leading to increased jitter and reduced eye opening—affecting system margins if left uncorrected.
Baseline Wander Correction
When pathological patterns arrive, the AC coupling capacitor blocks DC content. For example, when one “0” and 19 “1”s occur, the DC content of the transmitted waveform is high. Since the capacitor blocks DC, the voltage on the other side shifts down, as shown in the figure labeled CTLE-OUT. Passing this signal through a limiting amplifier creates a quantized replica of the transmitted waveform (LA-OUT). However, this replica is imperfect because baseline wander introduces significant jitter. Filtering the LA-OUT signal with a low-pass filter matching the cutoff frequency of the AC coupling capacitor recovers the low-frequency content (BLWC-OUT). Adding BLWC-OUT to the CTLE-OUT output cancels baseline wander, resulting in a clean eye at the linear and limiting outputs. This architecture is commonly called quantized feedback.
Increasing Baseline Wander
The AC coupling capacitor (labeled CC) is crucial to baseline wander. In the following slides, we discuss the effects of changing its value. The test setup is as follows:
1. Generate the pathological pattern using a video pattern generator.
2. Route the signal through a one-meter 1694A coaxial cable to an LMH1297 cable driver.
3. The LMH1297 output has adjustable capacitance. The input AC coupling capacitor of the device under test (DUT) is effectively the combination of these capacitors, adjustable to different values.
4. The signal is either retimed by the DUT and sent through another one-meter coaxial cable to a video analyzer to check for bit errors, or sent directly to an oscilloscope to observe baseline wander at the DUT input.
The pathological pattern can be generated using a video pattern generator
An adjustable capacitance on the output of an LMH1297 cable driver is used to demonstrate the effect of input capacitance on baseline wander
Baseline wander is proportional to the equation shown in the slide (bottom right), where:
• Constant one relates to the average pathological voltage.
• Constant two relates to the width of the pathological signal.
• The time constant is proportional to the total input capacitance, including the AC coupling capacitance.
Lower input capacitance increases baseline wander. At high baseline wander values, bit errors are more likely on the video analyzer.
Effect of input capacitance on BLW
VOSCOM’s 12G-SDI Fiber extender and 3G-SDI fiber extender reliably pass Pathological Pattern tests, confirming their stable performance even under challenging signal conditions.