Best Practice: Understanding Bandwidth

By Peter Aylett, Archimedia Middle East. Many a decade ago on my first physics A-Level lesson, my teacher said a very strange thing to the class: Most things I’m going to tell you over the next few years are not true. If I told you the full true story, we would be here for 10 years.’ Our topic of conversation, bandwidth, is much the same. What I’m about to discuss is a distillation of the truth, but hopefully enough that when you hear that HDMI 2.0 has a Bandwidth’ of 18GHz, you know what that means. Digital is, in fact, all analogue anyway! The word bandwidth in itself is misunderstood. When discussing a cable, bandwidth is expressed as the analogue signal bandwidth and is measured in Hz. For example, the bandwidth of Cat-6 is 250MHz and the bandwidth of broadcast quality RG6 is 4.5GHz (That’s 14 times greater than Cat-6). The reason that the bandwidth of a cable is expressed in Hz is because all signals sent over a copper cable are in the form of an analogue waveform with the bandwidth being the highest frequency of signal the cable is able to carry. All signals over copper are expressed as changes in voltage over time. This is also the case with digital signals, which use very high frequency square waves to represent digital data. The higher the frequency of analogue square wave the cable can carry, the more bits of digital data can be encoded. [caption id="attachment_9649" align="aligncenter" width="685"] There are many different ways of using analogue signals to represent digital data. An example of this is Manchester Coding. As the frequency of the square wave increases, so does the encoded binary bit rate.[/caption] There are many different ways of using analogue signals to represent digital data. An example of this is Manchester Coding. As the frequency of the square wave increases, so does the encoded binary bit rate. You can see from the picture that key to this encoding is having very fast rises in voltage to give rise to the near vertical walls’ of the square wave. As a cable’s bandwidth increases, so does its capability to preserve these fast changes in voltage over its length. For twisted pair cable, the construction of the cable, the pair twists and the relationship between pairs are all critical in maintaining this bandwidth. If the cable is kinked, crushed, stretched or bent excessively, it is then unable to transport these fast voltage changes and will potentially fail. The bandwidth of a digital system is measured in bits per second. For example, 10GBaseT is 10 Giga (Billion) bits per second. Remember, b’ is a bit, B’ is a Byte (8 bits). Transmission speeds are quoted in bits, storage is quoted as Bytes. HDMI Bandwidth As an example of this, let’s look at HDMI. HDMI uses three TMDS (Transition Minimised Differential Signalling) channels over which video, audio and auxiliary data are transmitted. The HDMI 1.3 spec (this was carried through to v1.4) has a clock rate of 340MHz per TMDS channel which translates to a maximum throughput of 3.4Gb/s. Adding the three channels up gives us 10.2GB/s. TMDS uses a coding called 8b/10b, which itself has some overhead that sucks up throughput. Once we allow for this, we are left with 8.16Gb/s to play with. So, how does this relate to the picture you see on the screen? Here’s a calculation taking into account my opening paragraph of this article. The calculation puts us in the ballpark, but is by no means comprehensive as it does not take chroma subsampling or the bandwidth necessary for audio into account. On a Blu-Ray disk, we have video at 1920 x 1080 resolution at 24 frames per second in 8-bit colour. Let’s look at this in terms of bits per second: 1920 x 1080 is around 2 million pixels Each of those 2 million pixels is made up of three colours: Red, Green and Blue. Each of those is encoded with 8 bits of binary data (The largest decimal number we can make with 8 bits is 255. Add 0 and that gives us 256 possible values per colour. 255 x 255 x 255 = Around 16 million possible different colours once you mix red, green and blue). So 8 bits + 8 bits + 8 bits = 24 bits. Multiplied by 2 million (amount of pixels) gives us around 48Mbits per frame. There are 24 frames per second so - 48Mbits x 24FPS = Around 1.15Gb/s. That’s over one Billion binary bits per second. It’s amazing it works at all! In 4K this would be around 4.6Gb/s, and at 60 frames per second at 4K this would be around 11.5Gb/s. If we then had 10 bit (deep) colour … I hope you get the idea by now. The above is the reason that we now have HDMI 2.0. HDMI 1.4 simply does not have enough bandwidth for our increasingly 4K future. Going back to square waves, this is what they look like on HDMI: [caption id="attachment_9650" align="aligncenter" width="1024"] A poor eye pattern.[/caption]   [caption id="attachment_9651" align="aligncenter" width="1024"] A good eye pattern.[/caption] On the poor eye pattern, you can see how much shallower the lines are representing a slower voltage rise. This can be caused by under specified electronics, poor quality cables or length issues. And don’t forget, it only takes a single choke point’ in the signal chain such as a kinked or crushed cable to cause a problem. Remember, ultimately digital is sent as an analogue waveform and so needs treating with just as much respect. Currently, the highest testing standard for HDMI cables is THX 4K certification. Currently only cables manufactured by Kordz are listed on the THX website. Conclusion We often think of digital as being perfect’ and simple to transmit because it’s just ones and zeros. Nothing could be further from the truth with today’s high bandwidth digital signals pushing the bandwidth capability of cables to the maximum. Getting 4K distribution working is far more than just selecting the right active electronics. Ensure that the cables are of the highest possible quality and ensure that they are treated with the respect that you would give the most expensive analogue interconnect. Peter Aylett is a world-renowned speaker and lecturer in residential technology, and the Technical Director at Archimedia, a multinational high-end residential integrator in The Middle East. He is also currently Chair of CEDIA’s International Technology Council Applied Content Action Team, and a regular contributor to HiddenWires. You are welcome to comment on this article. See below.

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