- Audio & Video
Cheap or expensive digital TV HDMI 1080p cable - What should I buy?
Which digital cable should I buy?
I am going to argue that you should buy the cheapest cable that works. This is because I am disgusted by several companies that prey on people's lack of knowledge about a complex subject. It's not fair that these companies sell a cable for 600 times what it cost to manufacture, and six times what is a reasonable retail price.
The argument starts by defining what is meant by the term 'digital'.
A signal is the transmission of information. This could be in the form of speech, sign language, facial expression, written word, or smoke signals and more. In some sense, all information is digital because all matter, and energy is quantized. This means there is a fundamental unit of energy and that is the most basic method of conveying information. You can either present a quanta of energy at a given time, or not. This is digital signal transmission at very tiny scales. At normal scales however, there are systems that pack in so many of these tiny discrete elements they appear like a continuous flow. This is what we call analogue signal transmission.
Let's look at a few examples.
The second hand of a clock that sweeps smoothly around in the clock face is considered analogue. The more jumpy type that tick off one second at a time is considered digital because of the inability to depict part of a second, but you might hear people describe those too as analogue because the dial is like the old style clocks.
The alarm clock that displays hours, minutes, seconds in the form HH:MM:SS is definitely digital. The swing of a pendulum is analogue.
In television technology, the old system used analogue signals to convey information through the air, and through cables and on to the TV Cathode-ray screen. These signals are continuous so any information induced onto the desired signal becomes part of the signal, and it's not possible to remove it. Therefore, an analogue signal requires high quality connection, good shielding, and excellent quality overall. You can imagine that an analogue signal transmits the number 7 but might receive 7.34 or 6.87 and if that is part of your viewing or listening experience, that's what you get.
Digital signals are very different. In a digital signal, we transmit a blip at one end and detect a blip at the other. If we get the blip, then it's a 1, if not then it's total failure, not 0.9 or 1.12. Alternatively, we represent a zero by either the absence of a blip or a dip or something other than what we detect as a one. This means that there are only four cases to consider:
- We send 1, and get 1
- We send 0 and get 0
- We send 0 and get 1
- We send 1 and get 0
Clearly, it's undesirable to operate in the latter two cases. So there are two main techniques used to guarantee error-free transmission.
The first is, we impose a threshold for the boundary between 1 and zero. This means we can receive a very degraded 1 and still treat is as if it was perfect. We can be very close to that threshold and still get a perfect interpretation of the signals.
The second technique is to send a little more information than we would normally need under ideal conditions. This is called information redundancy. By sending a little more information than required, we can apply some mathematical algorithms called 'error detection and correction' to notice cases three and four above, then turn those mistaken transmissions into perfect signals.
For the most common cases, digital cables either work absolutely perfectly or not at all.
The way a signal is formed at the source, is usually a square wave. For reasons that I wont go into here, but you can look up as 'Fourier analysis', this square wave contains many frequencies above the fundamental transmission rate. These higher frequencies are required to create a sharp leading edge for the transmitted signal. The cable introduces three unwanted features:
Resistance sucks out some of the amplitude, the other two store and release energy which messes with the timing of the signal.
Manufacturers control these parameters by choosing materials and construction techniques that are adequate for the specified digital transmission. It is very easy to control the parameters for short cables, and becomes difficult for long cables.
How long is long?
About 7 meters is "long". At this length, resistance might be about 3 ohms which means a 5v signal at the source could be degraded significantly (depending on the source and sink impedances). Capacitance and inductance also accumulates which messes with the higher signals.
However, providing that the cable is within specification, in most cases, you will either get a perfect signal or nothing.
There are some odd anomalies with long cables where people experience 'sparkles' on the cable. I am not sure what causes this problem but suspect it is interference caused by bad shielding.
If you buy, say, an HDMI cable rated for 1080p digital TV, and it works, then it should work perfectly. A $10, 1m cable is going to perform exactly like a $400 so called 'high end digital cable'.
Don't waste your money. As long as the physical robustness of the cable is up to par, then the cheapest you can find will be just as good at the most expensive one. Some strange people get sucked into purchasing a $600 dollar cable, and it's just not worth the money.