Understanding what bandwidth means in HDMI connectivity is crucial as we move into the era of ultra-high definition. Kordz managing director David Meyer explains.

HDMI has been around for 10 years, and for most of this time custom AV professionals’ main expectation has been for it to support 1080p.

Why? Well, for the most part nobody has needed more than that. However, consistency in HDMI interoperability cannot be counted on.

For example, do you know why an HDMI cable or extender is able to support 1080p – and if it doesn’t then why not? Or why one product works in some jobs (or at least seems to) but not in others? There are many and varied reasons, but understanding bandwidth is a major part of the puzzle.

WHAT IS BANDWIDTH?
Bandwidth is fundamentally an analogue term, referring to frequency range. Frequency means repetitions in a given timeframe, standardised at one second.

Analogue motion video is time based, and the more information that must be pushed through in a one-second timeframe, by way of higher resolution and/or frame rate, the shorter (in time) each unit of information will be.

More information then translates into higher frequency (more units per second), which means greater bandwidth.

HOW DOES THIS RELATE TO DIGITAL HDMI?
One of the ‘high-speed’ channels in HDMI is the ‘clock’ channel. Right away it indicates a time base, which is perhaps the most analogue-like part of HDMI.

This channel is essentially a metronome, in itself containing no data, rather just providing the precise timing for the three main data channels (TMDS – primarily video and audio data).

The digital bits in TMDS are packeted in 10-bit words (through a process called 8b/10b encoding), most commonly being one video pixel per 10-bit word. Each 1Hz unit of clock defines the start and stop times for each of these words (see Figure 1).

The standard reference of the one second timeframe still applies, so the 10 bits we are talking about would be expressed as 10 bits per second (bps). In HDMI the clock invariably operates in the MHz range. For the record, mega = million; giga = billion (1,000 million). So if 1Hz of clock equals 10bps of TMDS, then 1MHz equals 10Mbps of TMDS data per channel. There are three channels, therefore 30Mbps aggregate data.

Table 1 shows the common clock frequencies employed in HDMI, resulting TMDS data rates and the video formats supported.

Additionally, you can add 25% bandwidth to any of the HD formats to step up to 10-bit deep colour, or 50% for 12-bit colour (e.g. 1080p60 goes from 4.455Gbps at 8-bit colour to 6.68Gbps at 12-bit). The bandwidth for 2160p, otherwise known as 4K/UltraHD, is double that of 1080p60. So this new generation of HD video that is emerging will require a big step up in capability for the HDMI connection you choose.

Genuine high-speed HDMI cables should already support this, as they are tested to 340MHz or 10.2Gbps. That is, we will start to use the upper limits of high-speed HDMI for the first time. Furthermore, the announcement of the next iteration of HDMI is imminent and is expected to double again the bandwidth to enable 4K high frame rate, being 48-60fps, possibly including 3D variants.

So we’ll add another row to the accompanying table, for a whopping 594MHz clock, and an aggregate data rate nearing 18Gbps. At that level you can say goodbye to the passive HDMI cable, even in short lengths.

CAT X CABLE VERSUS HDMI
Category X cable has become highly popular for the connectivity extension of HDMI, even for installations in which a native HDMI cable could reach. The relatively low cost and ease of site termination of Cat X cable certainly enhances its appeal. But at the heart of HDMI is an ever-increasing hunger for bandwidth, and that’s where the two cable types differ vastly.

On the surface you will see each cable type referenced with what seems to be a similar set of figures – e.g. Cat 6 at 250MHz, and high-speed HDMI at 340MHz. However, these figures reference completely different aspects and cannot be directly compared.

For Cat 6, the 250MHz represents the transmission frequency of aggregate data. As discussed, the MHz figure referenced in HDMI is for the clock, which doesn’t contain any data, just the timing for what’s passing through the three separate data lanes.

To truly compare HDMI and Cat X cable, we need to calculate the transmission frequency of TMDS channels in HDMI.

Step 1 is to convert digital bits (bps) to transmission Hertz (Hz). Texas Instruments summed it up best in its 2009 application report Comparing Bus Solutions: “The relation of Hertz to bits per second is that a single bit is typically considered to be a 180 degree unit – a 360 degree cycle is two bits. Therefore, 1Hz is equal to two bits per second.” Remember there are 10 bits per word in the TMDS, so each word must equal 5Hz. An easier way to look at it is that every 1Hz of clock is equal to 5Hz of TMDS data transmission frequency.

One crucial consideration is that these figures are all per channel, meaning a single twisted pair. Cat X cable quotes transmission frequencies for the whole cable, being an aggregate of four pairs (in practice depending on operating mode).

Cat 5e cable is specified to 100MHz, Cat 6 is 250MHz, Cat 6a 500MHz, Cat 7 600MHz, and Cat 7a whopping 1GHz. Again these are all aggregates over four pairs.

Even at common 1080p60, HDMI pushes 742.5MHz over just one pair. With 4K video, HDMI transmits over a single pair 50% higher bandwidth than Cat 7a is expected to achieve over four pairs.

So why does HDMI work over Cat X cable, and at greater lengths? The short answer is – it doesn’t. It relies entirely on electronic ‘smarts’ at both ends of the cable run (including HDBaseT), and this inevitably adds another layer of complexity to the system.

If these same electronics were connected on each end of native HDMI cable, the results would be even better. Here’s a challenge for you: try terminating HDMI connectors directly to a mere 1m of Cat 6 cable (needs two runs of Cat 6 to get the wire count up) to make a passive HDMI cable. In brief, it won’t work, not even at SD resolutions.

CONCLUSION
We have been using HDMI for so many years now, with the ‘golden standard’ application being 1080p60 to date. That is 148.5MHz clock, amounting to 4.455Gbps aggregate data rate. Put another way, the data is transmitted at 742.5MHz over a single pair in the cable – greater than Cat 7 is specified to do over four pairs. Now that 4K UltraHD is happening, these figures double. Within a few years they will double again.

Additional and evolving active technologies will come about to make interoperability success more predictable for the custom AV professional. It will be ever more important for you to understand the capabilities and limits of the connectivity solutions you choose, and their degree of scalability as we evolve into the 4K era. Get to know more about HDMI bandwidth, make it your friend, and hang on for the ride.

For more information, visit Kordz.