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The HDMI® - Technology

HDMI system architecture is defined as consisting of ‘Sources’ and ‘Sinks’.

Fig 1.1 shows a typical AV system consisting of a Blu-ray player, AV Receiver and a TV.

HDMI flow diagram - Click to enlarge

Fig 1.2 – HDMI® Block diagram

HDMI block diagram- Click to enlarge

As shown in figure 1.2, the HDMI cable carry four different pairs that make up the TMDS data and clock channels. In addition, HDMI carries a shielded pair and drain within a bundle and further four insulated conductors.

 Signals travelling inside the HDMI cable (v1.4)

 Transition Minimized
 Differential Signaling
 (4 pairs of wires)

 3 digital video signals and 1 clock signal
 plus multiplexed digital audio signals
 into the digital video signals.
 HEAC   HDMI Ethernet and
 Audio Return Channel
 Ethernet compatible data networking and
 Audio Return Channel in the opposite
 direction from TMDS.
 DDC   Display Data Channel
 (SDA & SCL)
 Serial Data & Clock (i2C) signals
 CEC   Consumer Electronics Control  Data line distributes remote control
 signals for one touch system controls.
 HPD  Hot Plug Detection  Allows the source equipment to detect
 a connected display in real time.
 +5 V   DC +5V (50mA)  Power line supports remote circuits for

Inside a HDMI® cable (WireWorld Symmetricon™ design)

Cross section - Click to enlarge

HDMI® employs Transition Minimized Differential Signaling (TMDS) transmitted over 4 pairs of wires to carry video, audio and auxiliary data via one of three modes, called the Video Data Period, the Data Island Period and the Control Period. During the Video Data Period, the pixels of an active video line are transmitted. During the Data Island period (which occurs during the horizontal and vertical blanking intervals), audio and auxiliary data are transmitted within a series of packets. The Control Period occurs between Video and Data Island periods.

TMDS packets - Click to enlarge

Error Reduction/Correction
During the data island period for each channel, every group of four information bits is coded into 10 bits using the TMDS Error Reduction Coding (TERC4). In order to attain the higher reliability required of audio and control data, this data is protected with BCH error correction code and is encoded using a special error reduction coding to produce the 10-bit word that is transmitted.

'According to the HDMI Licensing, LLC technical application document, the handling and processing of errors by the sink (TV, projector or AV Receiver) is at the discretion of the hardware manufacturers. There are various techniques in use and the quality of these systems varies from manufacturer to manufacturer.'

Error correction techniques
All digital equipment employs some sort of error correction/reduction system. There is a wide range of error correction techniques in use today. However, error correction can generally be realized in two different ways:

  • Automatic Repeat Request (ARQ):
    This is a technique whereby an error detection scheme is combined with requests for retransmission of erroneous data. Every block of data received is checked using the error detection code employed, and if the check fails, retransmission of the data is requested – this is repeatedly done, until the data can be verified.

Computers (Internet) and other communication devices predominantly use Automatic Repeat Request (ARQ) error control and/or Hybrid ARC (a combination of ARQ and FEC).


  • Forward Error Correction (FEC):
    This transmitting system encodes the data using an error-correcting code (ECC) prior to transmission. The additional information (redundancy) added by the code is used by the receiver to recover the original data. In general, the reconstructed data is what is deemed the "most likely" original data.

Digital Audio & Video transmission interfaces, e.g. HDMI interface & USB Digital Audio, employ Forward Error Correction (FEC) techniques.


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“HDMI, the HDMI Logo, and High-Definition Multimedia Interface are trademarks or registered trademarks of HDMI Licensing, LLC in the United States and other countries."

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