HDMI Australia - TMDS Bitstream

At the heart of HDMI technology is the bitstream itself. The HDMI connector and cables have been developed as a way of getting the bitstream from point A to point B intact, but all of the real action is in the digital contents. Before we get too involved, you may simply ask "What's a bitstream?"...

A Bitstream is a collection of data bits which are transmitted in serial form. Huh what?... plain English please! Okay, a digital computer file, be it a program, picture, video or audio, is made up of millions of "bits" of information. A digital bit is either a "I" or a "0". Put millions (or rather billions) of bits in a continuous line, and they start to make up a unique packet of information. This is called a bitstream, quite literally because it's a stream of data bits. It's then up the digital devices being used to decipher what the digital bitstream all means, and to know what to do with it.

Problems can occur when the digital bitstream is interrupted by such things as external interference, voltage fluctuations and internal cable restrictions. Many of the "I"s and "0"s can change order or drop out entirely. This then compromises the digital bitstream, and in the case of video may mainfest as colour or resolution errors, 'sparklies' (snowy interference or lines) or it may not even work at all. The higher the bandwidth of the bitstream, the more this becomes an issue. Silicon Image, LLC figured out a way to get very high bandwidth video through a relatively small cable. They call it "TMDS".

"Transmission Minimized Differential Signalling", or TMDS for short, was developed as a means of transmitting high speed serial data. We first saw it introduced in the DVI format - Digital Visual Interface, but it was later introduced as the heart of HDMI (hence the backwards compatibility).

How does it work? Keeping the explantion as simple as possible, TMDS varies the DC voltage level of the signal, or selectively inverts the signal in the bitstream to ensure that the data stream stays around a pre-defined centre threshold. This significantly reduces the potential for errors in a way which is loosely similar to the method used in a balanced analogue audio connection.

There are five major components to the complete HDMI bitstream; they are the three high speedTMDS channels (0, 1 & 2), the TMDS clock channel and the CEC/DDC channels. There are two other terminals in a HDMI connection that are not related to the HDMI bitstream; the "Hot Plug Detect" and "+5V DC" power line, of which are covered in our "Connectors" page. All other pins relate to the bitstream itself.

With DVI, the TMDS bitstream was used exclusively for the transmission of uncompressed digital video. In HDMI it has evolved to include full support for multi-channel digital audio. This includes PCM, Dolby Digital and DTS soundtracks. The new HDMI 1.3 format even supports the latest Dolby Digital True HD and DTS Master audio formats. Digital audio is sent through in 'packets', pulsed in between the digital video bitstream in the same TMDS channels, and does NOT use separate conductor channels, as it often believed.

The final component of the HDMI bitstream is the CEC and DDC channels. CEC is Consumer Electronic Control. This feature gives the potential for devices to communicate with each other through the HDMI cable for features such as auto power and input selection, amongst other things. CEC is a facility which is already in place in the HDMI specification, and is then up to manufacturers to optionally implement by way of firmware to actually start using it.

DDC stands for Dispaly Data Channel, a facility which enables a source device to communicate with a display to verify its screen ratio and audio capabilities. It can then output the appropriately formatted picture and sound to best fit the capabilities of the display. This feature also is dependent on the firmware in both source and display device offering the capability.