HDMI has become indispensable in a home cinema. The flat plug is the universalConnection for almost all home cinema devices, regardless of whether Beamer, Blu-Ray-Player or AV-Receiver.

But precisely because there are many manufacturers with HDMI products on the market, problems arise in your own four walls. A suddenly missing Signal or a disturbed image are among the most common cases. In addition, the HDMI standard is constantly evolving, backwards compatible DVI should stay and also with one Copy protection is charged. This report should therefore help you to identify and correct these problems!

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The "handshake"

The handshake describes the greeting process between the devices connected via HDMI. This is quite complicated and therefore the most critical situation when connecting both devices; once this process is complete, not much should go wrong.

The greeting process consists of

• the synchronization of the frequency to be transmitted

• the exchange of HDCP-Certification and transmission of a key created at that moment for decoding

• querying the EDID information of the connected screen (Resolution & frame rate)

• of querying the audio capabilities of the target device in matters PCM-Channels, bit stream- decoding etc

• the check of CEC-Channel for mutual control.

If the greeting process is successfully completed, the source starts transmitting the data – the lines are set to “mute” beforehand.

The copy protection "HDCP"

A prerequisite for the transmission of high-resolution material is the copy protection HDCP (High-Bandwidth-Digital-Content-Protection). However, this is not an integrated component of HDMI, but is placed on top of it, so to speak.

HDCP consists of an encryption of the video signal, which is generated by the broadcaster. The copy protection is decoded using a code that was created during the welcome process between the two devices. If the source, when regularly checking the availability of the transmitter, finds that it is no longer connected, the transmission is interrupted.

HDCP must then be switched on if the content to be transmitted requires it. According to the recommendations of the HDMI licensors, it makes sense to leave HDCP switched on continuously, otherwise, for example, a satellite receiver carry out a new welcome process every time you change a program and, among other things, also create a new code.

But how do you identify problems with HDCP? If you see no picture or only the famous "snow", there is a problem with HDCP. If you are sure that both sides are HDCP compliant, trouble-free operation can only be achieved by a new welcome process. For you this means: Switch off devices or unplug cables.

The cable

Unlike analogue transmission, digital HDMI offers some great advantages, because the signal remains stable even with long cable lengths; Only when the cable is too long does it start to show image errors, until the signal finally breaks off completely. This process is referred to as a "cliff" in signaling terms. So should you read somewhere that with Kabel X or Kabel Y Colors and image sharpness are better, this is pure nonsense or pure imagination, because, as already mentioned, the quality deteriorates just before the "cliff".

But when is a cable too long? It is important to know in advance that the "cliff" shifts depending on the signal to be transmitted. According to neutral measurements at a length of one meter, a high-quality cable delivers a maximum bandwidth of 30 Gbit/s, i.e. three times the maximum provided for HDMI 1.3. With a length of six meters, it is still 13 GBit/s, while the critical limit for 1080p etc is reached at twelve meters - here the bandwidth is only 7 GBit/s. At 20 meters it is still 4 GBit/s under optimal circumstances.

If, despite an acceptable cable length, you only want 576p instead of 1080 As you can see, the problem lies with the Bit Error Rate (BER), which describes the bit errors within a certain time. However, the signal quality is often acceptable until you switch on an AV receiver. The fault in most cases is the one built into the cable amplifier does not harmonize with the AV receiver, but only with the Blu-Ray player. In the normal range, a cable operates at a rate of one fault per day. Even values ​​of one error per second are unproblematic, but this is where the whole problem begins: If the cable were only one meter longer, the rate would be 5000 errors, with another meter more the signal would drop out.

Cables fall into two categories: standard and high-speed. With the standard cables, 1080i, 24p or 720p (approx. 2,3 GBit/s) securely transmitted over 15 meters. Conversely, this means that higher bandwidths can be achieved with shorter cables and consequently also 1080p with 60 Hertz is made possible. High-speed cables, on the other hand, guarantee the maximum rate of 7,5 GBit/s even over 10,2 meters.

Critical Transition Points

Another critical point in a home theater is the routing, switching, or amplification of signals. This happens not infrequently, especially in AV receivers, since these devices have to deal with a whole range of complicated tasks. On the one hand, the target is the sound format, ie the receiver must tell the source which formats they can process. At the same time, they forward the image to the TV or the projector, but they have to call up the information from there about which standards are required here.

In order to split the signal and process the image, it must of course first be decoded, i.e. freed from HDCP copy protection. But before the signal can be sent out again, it has to be re-encrypted. In this multitude of exchange processes and protocols, there are occasionally minor misunderstandings between the devices involved. This is because specific time intervals are provided for all of these processes, in which the procedures must run. If one component reacts too quickly, too slowly or not at all, the other device does not understand the message to be transmitted and switches off to be on the safe side.

Repeaters are not quite as complicated, but not free from problems. With these signal-amplifying devices, however, there is the problem that not all devices support repeaters. The error here is mainly due to the required voltage, which is at a Pin of the socket must be provided. Therefore, repeaters and switches should always be active devices, i.e. have their own power supply. Another advantage of active repeaters is that the cable lengths no longer add up because the original level is restored.

What is the best way to proceed if the above problems apply to your devices? Once you've followed every piece of advice and checked that the software is up to date, it's best to keep swapping devices around until you have a combination that works perfectly together. Because for you it is almost impossible to find out where the error is. Only specialists who are able to read the signals can do this.

The image errors

As previously explained, image errors occur just before the so-called "cliff". Signs of this are small bright stripes, mostly running horizontally through the image; In rarer cases, these stripes can of course also run diagonally through the image. If your picture errors look different, you can assume that it is a problem caused by radio or magnetic impulses. These errors often occur when you use a cable that is too long. To fix the error, you can try a different cable routing, for example.

The best way to check if the interference is coming from outside is to place the devices right next to each other and connect them with a very short HDMI cable.

Occasionally there is also a small phenomenon: the picture quality is worse than with one, even with short HDMI cables YUV-Connection. The cause then lies in a poor implementation of the HDMI specifications, which authorizes you to return the device immediately. However, it can almost be ruled out that HDMI delivers a worse signal than YUV. Therefore, you cannot initially rule out a defect in the device.

HDMI versions

With the announcement of the new HDMI 1.3 standard, the impression was initially created that certain device properties were defined by this standard of the HDMI variant, such as e.g. B. DeepColor or the ability for bitstream playback. As a result, many potential buyers were very unsettled and didn't know which devices they could buy with peace of mind.

However, it is often forgotten that HDMI 1.3 is nothing more than a large catalog of definitions, which is newly added to the already existing, larger catalog of definitions. HDMI 1.3 says absolutely nothing about the device itself.

Assuming a source supports DeepColor, i.e. the ability to transmit colors with twelve instead of eight bits, the signal is initially sent with twelve bits. If the receiver does not respond positively because it does not support DeepColor, eight bits are transmitted. In other words, there is clear downward compatibility here.

Disruptive influences in the CD-Reproduction

When playing a CD, the HDMI output can be a DVD-Player can be disturbed or even switched off when a television is switched on.

This phenomenon is apparently a protocol error, which looks like this: The television reports to the HDMI via HDMI DVD-Player but does not get a "handshake" because the DVD player only plays CDs only, so it reports back the missing release, which causes the DVD player to switch off the digital output, as is prescribed for screens without appropriate copy protection.

The solution to the problem here can be that you lay an extra cable for the sound connection (coaxial or optical) and in the HDMI menu the Ton off.
However, this procedure only works if the DVD player should not also play HD sound.

If this does not solve the problem, only a separate CD player remedy in the home cinema combo. If the problem occurs even when the televisions are switched off, you can almost assume that you need to look around for a new screen.

The cause of disturbances in CD playback is usually that copy protection does not have to be activated here. And in the discussion about it, DVD players, AV receivers and televisions get tangled up, so that it ultimately ends with the digital signal being switched off via HDMI.