We are using the Advanced Cable Tester v2 (ACTv2) with the HDMI module for testing cables. While our HDMI cables passed the tests, we noticed the graphical representations and reported dB values differed from what we anticipated. To better understand this, we ran the same tests using a Total Phase demo HDMI module. In this run, the results shown were more aligned with our expected values.
What factors might contribute to the differences in measurements here? Here are are excerpts of the measurements we are comparing.
Results from our HDMI module:
Results from the demo HDMI module:
Response from Technical Support:
Thank you for your question! The ACTv2 does not measure dB loss directly. To present the widest eye opening, the dB shown is the de-emphasis that was automatically found by the system. The de-emphasis can be customized on the ACTv2 test profile.
De-emphasis and Cable Test Measurements
De-emphasis indicates that after the initial clock period of a given cycle, the transmitter reduced the Tx (transmission) power by "X" dB until the polarity switches again. This feature de-emphasizes the low-frequency component of the transmission, which is less lossy than the higher frequency component.
- When set optimally, the receiver sees a more consistent pattern.
- When set incorrectly, a PRBS pattern with a quick succession of two transitions can result in a very weak signal. Long runs of no transitions will have higher amplitude, which may cause problems with the receiver’s gain control.
This measurement is very important with long cables. The ACTv2 automatically selects the best de-emphasis value on each run. This feature can be disabled in the profile options.
Adjusting the De-emphasis Values
The automatically selected de-emphasis value may not always match the value that you specified. To use the ACTv2 for evaluation within your standards, you can do the following:
- Clone the profile that is closest to your specifications
- Set the de-emphasis channel for each channel.
We hope this answers your questions. Additional resources that you may find helpful include the following:
If you want more information, feel free to contact us with your questions, or request a demo that applies to your application.