Introduction

HDMI is a digital interface using differential signaling that was designed for the transmission of digital video and audio. It is widely used throughout the audio-visual market. HDMI 2.1 is an extension of HDMI 2.0, and it supports speeds that are twice as fast as HDMI 2.0.


■Enhance picture quality


Background

HDMI 2.1 has a maximum bandwidth that is about 2.7x that of HDMI 2.0, and its maximum signal transmission speed per lane is twice that of HDMI 2.0.
Whereas HDMI 2.0 consists of three data lanes and a clock lane, HDMI 2.1 does not have a clock lane, and it consists of four data lanes.
HDMI2.0
(conventional standard)
HDMI2.1
(new standard)
Maximum data rate 18 Gbps
(6Gbps/lane x3)
48 Gbps
(12Gbps/lane x4)
number of lanes data 3 lane
clock 1 lane
data 4 lane
clock 0 lane
connector, cable specified by the
conventional standard
new (category 3)
upward compatibility
receiver equalizer must must

In this way, HDMI 2.1 transmits signals faster than before, and so care must be taken to ensure that the signal quality is not harmed when implementing noise suppression measures.

Two noise issues are conceivable when HDMI communication is performed.

Radiation noise①: Noise originating from HDMI interferes with the Wi-Fi antenna inside the equipment, reducing the Wi-Fi communication sensitivity (Intrasystem EMC)
Radiation noise②: Noise is radiated from the HDMI equipment or cable (Emissions)

This study focuses on the intrasystem EMC of radiation noise ① and examines corrective actions for it.




Noise Suppression Measures for HDMI 2.1

If normal noise filters are used for high-speed differential signaling lines like HDMI, the signal quality will deteriorate, and the signals will not be transmitted properly. For this reason, common mode choke coils (CMCC) are a suitable solution. Because HDMI signals are high speed, we should select a CMCC that is best suited for high-speed signals.
CMCCs are installed near the output pins of the IC at the HDMI source side and near the connectors at the sink side. The source-side CMCC installed near the IC can suppress the common mode noise generated from the transmission ICs at the source side by preventing the propagation of common mode noise at the sink side.
The sink-side CMCC can suppress the common mode noise generated from the HDMI signal wiring of the sink equipment by preventing common mode noise from entering the Wi-Fi antenna.


Recommended CMCCs for HDMI 2.1 Noise Suppression Measures





Noise Suppression Measures for Intrasystem EMC

First, we verified how much HDMI communication affects Wi-Fi. We confirmed that, when HDMI 2.1 communication was started, the Wi-Fi reception sensitivity dropped by 6 to 10 dB in the 2.4 GHz and 5 GHz bands. When noise actually entering the Wi-Fi was confirmed, we found that it overlapped with the 2.4 GHz and 5 GHz bands.

■Wi-Fi Receiver sensitivity


Evaluation of Intra-EMC

Mounting of CMCCs (DLM0NSB120HY2) showed an improvement of 3 to 4 dB.

■Wi-Fi Receiver sensitivity



Noise Suppression Measures for Radiation Noise

Next, we examined noise suppression measures for radiation noise.
First, we checked for any differences in the noise spectra for HDMI 2.0 and HDMI 2.1.
  • Radiation noise from conventional HDMI 2.0
    → Broad noise occurs in the 1 GHz to 3 GHz frequency band
  • Radiation noise from HDMI 2.1
    → Noise occurs in the 1 GHz to 6 GHz frequency band
This showed us that higher frequency noise occurred in HDMI 2.1.




Results for Radiation Noise Suppression by CMCCs

Mounting a CMCC (DLM0NSB120HY2) to the source side enables removal of HDMI 2.1 noise.
Radiation noise is still left after using a CMCC, but most of this noise was radiation noise of the DRAM installed separately on the board of the source equipment.



Check of Effect on Signal Quality

The eyemask test for HDMI 2.1 showed that the signal waveform was identical even after using CMCCs.



The AC common mode noise was reduced by 47% by using CMCCs.




Conclusion

  • Measures for preventing drops in Wi-Fi reception sensitivity
    An issue arose where the Wi-Fi reception sensitivity dropped during HDMI 2.1 communication, but this was improved by using CMCCs.
  • Noise suppression measures for radiation noise (Measures for preventing emissions)
    In HDMI 2.1, radiation noise occurred in the range from 1 GHz to 6 GHz, but this was improved by using CMCCs.
  • Effect on signal waveform
    Inserting CMCCs had no effect on the eyemask test.
    Use of CMCCs reduced the AC common mode voltage.

CMCCs Mentioned in This Study

Recommended CMCCs for HDMI 2.1 Noise Suppression Measures