Sometimes displays that have HDMI inputs don't receive a picture from some cable television set-top boxes with HDMI outputs. What causes this problem, and can it be fixed?

Sometimes displays that have HDMI inputs don't receive a picture from some cable television set-top boxes with HDMI outputs. What causes this problem, and can it be fixed?

Sometimes displays that have HDMI inputs don't receive a picture from some cable television set-top boxes with HDMI outputs. What causes this problem, and can it be fixed? There are a few reasons that this could be happening. One of the causes of this problem concerns HDCP, or high band width digital-content protection.

HDCP is a specification developed by Intel for the protection of digital entertainment content that utilizes the DVI interface. HDCP gives encryption for the transmission of digital content between the video source, or transmitter, like a computer, a DVD player, or a set-top box, and the digital display, or the receiver, like a monitor, a television or a projector. HDCP was not designed to prevent the copying or the recording of digital content, but to protect the integrity of the content as it is being transmitted. To implement HDCP, a license is required that is obtainable from the Digital Content Protection, LLC, which will then issue a set of unique secret device keys to all the authorized devices. During the authentication, the receiver will only accept the content once it demonstrates a knowledge of the secret keys. Plus, to prevent any eavesdropping and stealing of the transmitted data, the transmitter and receiver will both generate a shared secret value that is consistently checked through the entire transmission. Once the authentication is established, the transmitter encrypts the data and then sends it to the receiver to be decrypted. In addition to paying the license fees, all licensees agree to limit the capabilities of their products. For example, high-definition digital video content must be restricted to DVD quality on non-HDCP compliant video outputs when requested by the source. DVD-Audio content is restricted to DAT quality on non-HDCP digital audio outputs, analog audio outputs have no quality limits. Licensees can not allow their devices to make any copies of the content, and must design their products to effectively frustrate attempts to defeat the content.

There are three systems in place which were developed to achieve the prevention of the transmission of non-encrypted high definition content.. The first system utilizes the authentication process, which does not allow any non-licensed devices to receive any HD content. The second system utilizes encrypting the actual data that is sent over the DVI or the HDMI interface, which prevents any eavesdropping of information. The third system in place is the key revocation procedures which ensure that devices which are manufactured by any vendors who violate the license agreement could be blocked relatively easily from receiving HD data.

Each device model that has an HDCP capability has a unique set of keys. There are a total of 40 keys, and each key is 56 bits long. These keys are kept strictly confidential, and failure to keep them a secret could be construed as a violation of the license agreement. For each set of keys, a special key called a KSV, or Key Selection Vector, is created. Each KSV has one half the bits, or 20 bits, set to 0 and one half the bits, or 20 bits, set to 1. When the authentication process occurs, both parties will exchange their Key Selection Vectors. Then each device will add, without overflow, its own secret keys together, according to a KSV that it received from another device. If one particular bit in the vector is set to 1, then the corresponding secret key is used in the process of addition, otherwise the bit is ignored. Keys and the KSVs are manufactured in such a way that during this process both devices will get the identical 56 bit number as a result. That specific number is used later in the encryption process. The encryption process is done by a stream cipher. Each pixel, that has been decoded, is encrypted by using an XOR operation with a 24-bit number produced by a generator. The specifications for HDCP ensure the continuous updating of keys after each encoded frame. If a specific model is considered to be "compromised", its Key Selection Vector is put into the revocation lists, which are written on newly produced disks with HD content. Each revocation list is signed with a digital signature using the DSA algorithm. This digital signature is supposed to prevent any malicious users from revoking legitimate devices. During the authentication process, if the receiver's KSV is found in the revocation list by a transmitter, then the transmitter will consider the receiver to be compromised, and will refuse to send HD data to it.

Sometimes the set top box software may not support the HDMI port or the software may not activate. Other times, the set box will not work correctly when used with an A/V receiver, but will usually function correctly if hooked directly to the monitor or television. Some of these devices may reveal that this problem is caused by an error in the device implementation of HDCP. Some of the set-top boxes may not support HDCP repeaters, which are devices that pass along the signal to another device. This may be a flaw in the initial version of the products, and there may be a new firmware that is available to fix this in HDMI. Newer versions may already have the fix. Users should contact their own cable operator and request the newest software available to resolve their issues.