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
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 ×3)||48 Gbps (12Gbps/lane ×4)|
|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|
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.
Noise originating from HDMI interferes with the Wi-Fi antenna inside the equipment, reducing the Wi-Fi communication sensitivity (Intrasystem EMC)
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.
2. 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
3. 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.
・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.