In the past, numerous competitive interfaces for transmitting video and sound between AV devices made life quite difficult to ordinary people and professionals alike. Technological development brought great challenges, as manufacturers of computers and audio-video equipment and TV broadcasting companies realized the potential hidden in high definition images. It was not the creation or transfer of HD images, but their reception and display on the end user’s screen that was the most troublesome part. The signal transmission between the receiver, video recorder, TV, screen, display, and other individual devices was the main problem, as the standards used before (composite image or VGA standard, and even DVI) turned out to be insufficient.

Although many solutions competed on the market at the beginning of the 21st century, the multimedia standard originating in Japan, developed together by companies such as Hitachi, Panasonic, Sony and Toshiba, among others, proved to be the undisputable winner of that race. Its known as HDMI.

What is HDMI?

HDMI is an interface, which means that along with the transmission protocols and their electrical properties, it also defines the mechanical specifications of the connectors. HDMI connectors are available in a few versions: HDMI standard, HDMI mini, and HDMI micro. Plugs and sockets of all sizes have similar capabilities, but miniaturized versions are a very practical solution for small devices. For instance, HDMI mini is used in various models of Canon and Nikon's LR (DSLR) cameras, whereas the Raspberry Pi 4B board features two HDMI micro sockets instead of one, which extends its scope of functionalities.

HDMI was developed as a multimedia standard meant for transmission of video, audio and other data. Because of that, even a very general description of HDMI will discuss a number of features of this interface.
Akyga adapter
Akyga adapter to connect a standard HDMI cable to an HDMI mini socket

What audio-video formats does the HDMI interface use?

Respecting the minimum specifications of the CEA-861 standards, metadata and audio-video data are transmitted in any format. Successive revisions of this data structure are used, depending on the version of the interface (HDMI 1.0, HDMI 1.1 etc.). The HDMI standard develops as the display technology evolves, so it is up-to-date and ready to meet the needs of modern products, but at the same time remains backward compatible to support older designs. Uncompressed video data is usually transmitted using the sRGB colour space (8 bits per channel), but the image can be represented in even deeper colour spaces (xvYCC 4:4:4, Y′CBCR 4:4:4 etc.). As image resolutions adapted by manufacturers of TVs, monitors and related displays have grown, from FullHD (i.e. 1920x1080) to 4K and even 8K and 10K, new versions of the HDMI interface have managed to keep up with them.

The sound is transmitted via HDMI cable as uncompressed PCM samples. This means that it is transmitted to the target device using bits that head directly to the DAC (D/A) converter. The lossless signal can be pre-processed in an analogue or digital process, in a similar way as compact discs are played over a digital connection. HDMI supports even 8 audio channels sampled at up to 192 kHz, with a resolution of up to 24 bits (compare with the 16-bit accuracy and 44.1kHz in the case of music recorded on a CD).

How does HDMI transmission work?

We need to stress here that the data are not compressed by the HDMI interface. This means that video and sound are transmitted without any loss, but also that the possibilities of HDMI are proportional to the data transmission speed. And this is where the challenge really begins. In successive HDMI versions, adapted to support the growing resolutions of video and sound, as well as more and more extensive metadata packets, the method of transmission has not changed, but its throughput has been improved. The first generation reached a speed of approx. 5Gb/s, and the latest version (HDMI 2.1) operates at up to 48Gb/s. We need to remember here that all these developments are taking place in the consumer segment of the market, where connectors and cables are used in conditions that might be far from favourable. The HDMI standard still remains valid and backwards compatible thanks to the transmission technology used, namely TMDS.

TMDS – what is it and how does it work?

TMDS stands for transition-minimized differential signalling. Its use in the HDMI standard allows us to limit the impact of electromagnetic radiation on the data transmitted. Every byte requires two 10-bit frames to be sent using this method. Three channels are used for communication, each consisting of two lines according to the principles of differential signalling (which itself ensures some signal protection). Each byte of data is processed, bit by bit, by the Exclusive-OR or Exclusive-NOR function (XOR or XNOR) to ensure that there are no more transitions between logical states than needed. The lowered frequency of switching further decreases interference between the conductors and reduces the effect of external factors. As the byte is being retransmitted in the second phase, the data are logically negated if the average current needs to be retained. Apart from the transmitted byte, every 10-bit data frame comprises two bits that indicate which logical operations were performed.