HDMI cables are used to transmit sound and image between sources and displays. When two devices are connected via HDMI, they negotiate between them which formats one accepts and can send the other. Thus, if all goes well, the connection is fast, reliable and secure. Even "safe" for rights owners, as the protocol includes some anti-piracy measures. Let's see where this standard comes from, what types exist and what we must take into account to get the most out of it.
Origin of HDMI
HDMI connections arrived with the new century to make our lives easier . Not so many years ago, connecting audio and video equipment required many cables. Besides many cables, also a good scheme, because the truth is that it got to be complicated. Just to transmit video, you needed between one (composite video) and three (component) cables. For sound, one cable per channel: two for stereo, but five (or more) for home theater. That for each source, and then there were a few: DVD player, console (s), VHS video, satellite tuner, tape player… etcetera!
The concept: one cable for everything
The HDMI fixed all that because one cable with each source is enough to transmit all the information: audio and video. As it also works in digital format, it is relatively immune to interference or signal degradation. The acronym HDMI comes from the English, High-Definition Multimedia Interface. Its origin dates back to 2003 due to the need to connect high resolution digital video sources to compatible displays or projectors.
The key word is "digital" because until then the other transmission methods (domestic) were analog. This meant using quality cables and connectors , especially on long runs. And certain limits for those runs. Also, the connections were not secured: there could be a lack of compatibility. And the signal could also be used to make copies without problem.
Types: from HDMI 1.0 to 2.1
The first (1.0) appeared in December 2002 as a single cable capable of carrying digital video and audio. The video part used the DVI protocol, up to 1,920 x 1,200 and 60 Hz. The audio part was compatible with digital signals (SPDIF) and up to 8 channels of PCM sound at 24 bit and 192 kHz maximum. HDMI 1.1 arrived in 2004 and only added DVD-Audio compatibility with its unique anti-piracy system.
In 2005 came HDMI 1.2 with more options to transmit audio: SACD up to 8 channels. Variant 1.2a added CEC support for transmitting commands a few months later (see below). A more notable change came in 2006 with HDMI 1.3 (and 1.3a with details on SACD audio). This increased the bandwidth, the number of possible color bits (from 10 to 16) and the color space. Also with him came the HD sound formats (Dolby Digital and DTS).
HDMI 1.4 brought support for 3D video and 4K UHD resolution in 2009 . That is, it jumped up to 4,096 x 2,160 possible pixels at 24 Hz (digital cinema). And 3,840 x 2,160 @ 30Hz (Ultra HD) for televisions or home projection. After two small modifications in 2013 came the HDMI 2.0 or "HDMI UHD". Even more bandwidth (more data capacity) to carry 4K video with another color space. And more audio channels, up to 32, thinking in Dolby Atmos and other extended surround sound formats. To that, HDMI 2.0a added HDR in 2015 and HDMI 2.0b HDR10 and HLG in 2016. HDMI 2.1 (2017) goes up to 10K video and Dynamic HDR among other features to come (Game mode and eARC).
Communication in addition to video and audio
The HDMI cable can not only carry video and audio signals, it also serves as a communication cable between devices. There are several protocols that have come up with different types. From the beginning there are those that affect the transmission of video and audio. They ensure communication between devices (that are understood) and the synchronization of signals.
We care more about three others: CEC (Consumer Electronics Control), ARC (Audio Return Channel) and HEC (HDMI Ethernet Channel). The first is from type 1.0 and is used to send control commands between devices. That is, to use a remote control only, and that the devices transmit the commands between them. As it is not mandatory, not all devices or manufacturers use it. The ARC is used so that a “receiver” (television) can transmit audio to its “transmitter” (an AV amplifier for example). It was introduced with HDMI 1.4 and saves us connecting another cable from the TV . With that also came Ethernet to HDMI, which in its latest version supports communications at 100Mbit / s. Another cable less between devices, if we use it.
There are five types of HDMI connector, but the most used are three. They call them types A, C and D, or "normal", "mini" and "micro" . The normal one is what we find in televisions and household appliances. Also in many computers if they are not very compact laptops. The D or micro is the one that is usually found in cameras or some portable devices and looks like micro-USB. It is not the same or compatible, although there is a USB-C operating mode (not micro-USB) compatible with HDMI. You have to use an adapter.
It is not a unique case. When HDMI was developed, there were already digital video connections: DVI (specifically DVI-D or DVI-I). Since part of the same communication protocol was used, DVI cables can be HDMI-compatible . An HDMI-DVI adapter does not contain electronics, it just redirects “pins”. It can therefore be used from source to emitter or vice versa. There are some limitations depending on the modes that the two devices accept, and also because DVI does not include anti-piracy protection.
Does cable quality matter?
Of course it matters. The HDMI cable contains four pairs of twisted and shielded cables and seven independent conductors. In total there are 19 fine cables inside the case that we see outside. Ethernet-compatible HDMI cables (which can carry a network signal) use three of the seven independent (twisted) conductors.
So many cables, so thin, have a certain limit to the length that is used even though the signals are digital. From about 12 meters, it is necessary to use cables of a higher quality than the standard. If not, or the connection is not established securely between devices, or interference appears. These are usually "sparks" in the image for example. Or directly absence of image. Therefore and although there are those who affirm that being digital cables they do not need to be "good", they do.
Starting with HDMI 1.3, two categories (1 and 2) were defined based on this. Cat.1 cables support up to 1080i60 video with security, and Cat.2 up to 2160p30. With HDMI 1.4, five certified cable categories were achieved (from "standard" to "high speed with Ethernet"). In 2015 another class arrived, with HDMI 2.0, for cables that support transmission at 18 Gbit / s. And in 2017 another one has arrived, with HDMI 2.1, which supports 48 Gbit / s and video resolutions of up to 10K. All are compatible with each other, within their operating limits. And some lower category cables can, over short distances, work well in higher modes.