Is digital… digital?

Er, well, in the wider sense, yes. But in the world of HDMI, no, not really. Yes, when it comes to factors such as picture quality and device performance, digital is digital, it is a lossless format; technically there are no degrees or grades of difference, that is impossible. But with HDMI the key consideration is the amount of digital data being sent down the cable and whether or not the cable cope with all those 0’s and 1’s? If we imagine the cable as a pipe and the digital data as water, is the pipe large enough to let all the water flow easily and comfortably through it?

Unlike the other common digital cables out there in the AV world that we are familiar with today, audio cables such as TOSlink and coax optical cables for instance, HDMI is a little more complex… a lot more complex in fact. It's important to understand just why, with HDMI, it is difficult to over simplify things and just say "digital is digital, it either works or it doesn't." As we referenced in detail in our article 'HDMI - It's All in the Bitrate,’

Its certainly not as snappy, but in the case of HDMI it should read “HDMI is a massively data intensive digital signal, so as long your HDMI cable length can accommodate the data bandwidth needed for the signal, it will work, if it doesn’t have enough bandwidth support over that distance it won’t”. And because not all HDMI cables are made equal with exactly the same bandwidth capabilities, in reality you cannot really make the generalized statement that a HDMI cable “either works or it doesn’t”. Different HDMI cables, and different HDMI cable lengths will have differing data bandwidth capabilities, just like HD signals have different data intensities, 1080i being three times lower than 1080p 12-bit, this is why some HDMI cables will work and others wont.

Confused? Lets see if I can explain things a little better.

The amount of data (bandwidth) being sent over HDMI has progressed and increased, evolved if you like, up from a simple 720p/1080i 8-bit per channel signal to the current 1080p 12-bit (or more) per channel signal it is today. Crucially, the amount of bandwidth is being sent over the cable depends on the HD source device and the capability of the display, but the potential has vastly increased since HDMI was first released in 2003. As these signals have got higher in resolution and bit depth, the amount of bits per second traveling through the HDMI cable has also increased consequentially. When that happened and most recently culminating in the release of the HDMI 1.3 specification, the tolerance requirements for HDMI cables changed as well. The version 1.3 spec being essentially an increased capability requirement to cope with the increased data involved when used with high-resolution full HD deep colour material.

If we take another digital cable as example to use for comparison purposes, a digital audio coax cable requires maximum bitrate of just over 3 Mega bit/s. For a S/PDIF connection, be it TOSlink or digital coax, this is an extremely low amount of data when compared to a HDMI cable. HDMI version 1.3 can, per the spec, handle up to 3.4 Giga bit/s per channel, a massive 1000 times more bits per second!

Unlike HDMI, the specification for that digital connection hasn't changed since 1997 and the demands made on the cable are far less than the cable's potential capabilities, in contrast HDMI version specs have changes over 5 times, increasing the capability requirement each time! This means that there is a great potential that some HDMI cables will not carry a full HD 1080p deep colour signal, but can quite comfortably carry a 1080i signal.

In conclusion, the statement that "digital is digital, it either works or it doesn't." is correct in the context of most digital applications, a digitally produced video image cannot suffer from varying grades of picture quality, we have talked about this in other articles. The important point about HDMI however, is that due to the extreme data intensity and huge amount of variables in involved with HDMI, it just really isn’t that simple. From a consumer’s perspective, you need to ensure that your HDMI cable can support the highest-resolution bandwidths so you’re never let down.