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Peter Cochrane's Hard Drive 2000 Bandwidth and brandwidth THIS column recently provoked a flurry of email inquiries on the topic of bandwidth. While in principle it is simple enough to understand, the detail can be problematic, and the more so since it has been promoted as an improper noun in general conversation. For sure, we can all detect when there is insufficient bandwidth as we struggle to identify the telephone voice of a loved one, or watch a football jerk across the TV screen when the camera pans to record a goal from 30m out. A lack of bandwidth is evident when we look at a grainy or blocky photograph. Although Claude Shannon did a brilliant job of defining and quantifying bandwidth in his seminal papers of 1946-47, there is still much confusion on the topic. An increasingly used word, it rolls off the tongue like honey and is in danger of becoming meaningless. So let's see if we can shed some light on the topic. Bandwidth is essentially about information flow - bits (binary digits - ones and zeros) - mostly in the context of speech, text, pictures, movies, animations and data. Our analogue past sees it quoted in terms of a wave quantity - cycles per second (Hertz), although we think of information flow in bits per second (bit/s), and while the two are inextricably related, there is no singular ratio. If you need a number to hang on to, then in general one Hertz conveys at least one bit/s, and commonly two bit/s or more. Here we skip this added complexity to get down to the kernel of the idea. Suppose I whistle a continuous note at a fixed intensity: it contains almost zero information and needs zero bandwidth. But if I impress a tune on this resonant flow of air, it demands bandwidth, and the more complex the tune, the more change per second, the more bandwidth required. However, no matter what I do my use of bandwidth will be insignificant compared with a 70-piece orchestra during the finale of the 1812 overture. So bandwidth is about change, about energy being converted or conveyed from one point to another, from a photograph to your eye and into your brain, or from mouth to ear via a telephone, or vibrating air molecules across a crowded room. A moving colour TV image requires about 34 million bit/s, about 500 times that for a fixed line telephone call at 64,000 bit/s, and 2,000 times that for some mobile phones. The data from some panorama of forests, mountains, rivers and fields can be around 70 billion bit/s, which after pre-processing by our sensory systems enters our brains at around 1 billion bit/s. Hence, we have to stand and take it all in. The text you are now reading contains a nominal 600 words, each an average of five characters, all defined by eight bits. So the total number of bits in the column (and I am taking some liberties here so please don't complain) = 600x5x8 = 24,000 bits. If you read the whole text in a leisurely four minutes, the number of bits per second = 24,000/(4x60) = 100 bit/s. Typing at 100 words a minute equates to 100x5x8/60 = 67 bit/s. At a recent conference a delegate made two key errors in confusing silicone with silicon, and then saying "brandwidth" instead of "bandwidth". I liked this new concept of brandwidth, and thought the definition should be Number of Customers x Loyalty Duration x Average Spend Rate. Peter Cochrane holds the Collier Chair for the Public Understanding of Science & Technology at the University of Bristol. His home page is: |
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