What is Signal De-Emphasis and When is it Used?

Signal de-emphasis is a signal-enhancing technique often used to improve the quality of electrical signals transmitting at gigabit rates over devices including PCBs and long cables. Signal degradation can be due to a variety of reasons, including long transmission lines and jitter, so this technique helps negate these issues by decreasing the low frequency data to minimize signal loss. Learn how de-emphasis works and how it is used to improve signal integrity issues.

Signal Loss Within Cables

Signal loss within cables is a naturally occurring effect, particularly within cables using copper wires. This signal attenuation, or reduction in signal strength throughout the cable, can be due to a number of factors, including resistance or impedance in the wire, and can be especially prominent with higher frequency signals. The high-frequency loss is also greater with longer cables and can lead to loss of bit data.

Within a receiver there is circuity that performs automatic gain control. This automatic gain can compensate for some of the signal loss as it is designed to keep a constant output signal despite any variations within the signal at the input of the amplifier or system. In other words, it essentially “turns up the volume” to make a weak signal louder, or reduces the amplification when it is too strong.

When the high-frequency loss is significantly different from the low-frequency loss, that gain will get set according to the stronger frequency and the weaker frequency will be unrecoverable. This poses a problem with outputting a quality signal.

How Does Signal De-Emphasis Work?

There are a few techniques that are used to compensate for such signal degradation issues, including pre-emphasis and de-emphasis. Both pre- and de-emphasis occur on the transmitter side. Assuming loss in the cable, the transmitter will boost the high frequency content (pre-emphasis) or decrease the low frequency content (de-emphasis).

Pre-emphasis works by boosting the high-frequency portion of the signal. This compensates for the high-frequency loss in the cable.

De-emphasis works by cutting the low-frequency portion of the signal. This may be coupled with an increased transmit voltage.

Pre-emphasis and de-emphasis provide essentially the same function, which is to provide a flat frequency curve on the receiver side. In actual implementation, de-emphasis can be technically simpler, so it is more often seen between the two.

While pre/de-emphasis helps to create a more stable signal, it can also create issues if the system applies too much of either. For instance, if you surpass the optimal amount, you can end up with too little low-frequency and too much high-frequency.

De-Emphasis in USB, HDMI, and DisplayPort Cables

Where do we often see the use of de-emphasis within the cables? De-emphasis is regularly used in high-speed systems, which are often classified as being 1 Gbps or more. Signal de-emphasis can be found commonly in HDMI and DisplayPort cables. HDMI cables are known to use signal de-emphasis the most aggressively, as they are frequently fabricated to be longer. Because SuperSpeed USB is a higher-speed interface than its USB 2.0 counterpart, it must consider transmission-line effects and use equalization to ensure there are fewer reductions in the signal integrity. Therefore, de-emphasis is also commonly used, but is used less compared to HDMI or DisplayPort.

Using De-Emphasis with the Advanced Cable Tester v2

Advanced Cable Tester v2 Front Panel

As explained previously, de-emphasis is commonly used to compensate for loss occurring along the transmission path within cables. This same concept is used within the Advanced Cable Tester v2 to provide insight into a cable’s signal performance.

The Advanced Cable Tester v2 is a comprehensive cable testing tool that tests USB, HDMI, DisplayPort, and Apple Lightning cables for quality and safety measures including pin continuity, DC resistance, E-marker accuracy, and signal integrity. The signal integrity test measures the quality of signal by essentially outputting a known-good signal and then measuring the signal received at the other end. The Advanced Cable Tester v2 signal integrity test provides a visualization of the signal in the form of an eye-diagram. It also includes masks calculated from the insertion loss values per cable specifications. Wider eye openings represent a better signal while a smaller opening indicates a lower quality signal. Any portion of the eye-diagram touching the mask will automatically fail the signal integrity test.

One of the features of the signal integrity includes applying de-emphasis. When enabled, this is applied on the data pairs when testing HDMI, DisplayPort, and SuperSpeed USB cables. The de-emphasis values for both DisplayPort and HDMI have been taken from the respective specification. The value, however, is adaptive and the Advanced Cable Tester v2 will choose the value that is ideal while running the tests. Values available include: 0 to -15dB. When multiple values are available, the Advanced Cable Tester v2 will start with the least de-emphasis value (closest to 0) and continually repeat testing until the best signal integrity is found. For USB, a fixed value of about -3.5dB is used.

By default, the Advanced Cable Tester v2 uses signal de-emphasis for the higher speed tests, but it can be disabled. By enabling this feature, it allows users to test worse-quality cables that otherwise would result in a “no lock” outcome. This is because applying de-emphasis allows the signal transmitted through the cable to successfully be locked at the other end. For this reason, cables with more loss are difficult to test without de-emphasis, even if the cable is within the relevant specifications. On the other hand, while disabling de-emphasis may result in a more closed eye, the results are more easily compared across multiple cable samples.

Below are examples of various passing signal integrity tests using de-emphasis values:

USB Type-C to USB Type-C Signal Integrity Test Signal integrity test for USB Type-C to USB Type-C cable

 

HDMI to HDMI Signal Integrity Test Signal integrity test for HDMI to HDMI cable

 

DisplayPort to DisplayPort Signal Integrity Test Signal integrity test for DisplayPort to DisplayPort cable

Summary

Because cables naturally experience signal loss, de-emphasis is a helpful technique in preserving the integrity of the signal throughout the cable. De-emphasis does this by decreasing the low frequency portion of the signal, which in turn allows the receiver to output a stable signal. The Advanced Cable Tester v2 offers the ability to apply de-emphasis values in order to test worse-quality cables, allowing users to thoroughly test USB, HDMI, and DisplayPort’s signal quality.