Wright's Aerials

The Lesser Spotted Decibel

An illuminating exposition covering its habitat, diet, and mating rituals
by Woody

Unfortunately we use one word, ‘decibel’ for several units that have different uses.

The decibel is a ratio – nothing more, nothing less. In practice it is usually accepted as a ratio of powers measured logarithmically.

As it is a ratio it must have a reference, but this is where usage falls down. Sound is measured in dB relative to 'the threshold of hearing' which I believe actually has been defined as an absolute sound pressure level. It has become practice to measure sound levels in dB relative to that threshold but just to give the reading in 'dB' without nominating the reference.

In electronics you will come across dBm, dBW, dBuV, and dBV.

dBm is dB relative to 1mW, which by normally accepted usage will be 1mW into a 600 ohm resistive load in audio, or 1mW into a 50 ohm resistive load if used in radio (RF.) Similarly dBW is 1W but this is more often used only in radio so is relative to 50 Ohms (50R); dBuV and dBV are relative to 1uV and 1V respectively, and as these tend to be used for signal strengths in the broadcast industry they are usually at 600R if audio (dBV), or 75R if RF signal (dBuV.)

The formula for dB is 10 x log(power out/power in) so if the signal measured drops, the dB reading will be negative, log being to the base 10.

As power is related to current and voltage also by the formulae

P=V2/R where V2 is V squared then dB in current or voltage is dB = 20 x log(Vout/Vin) or 20 x log(Iout/Iin) as R will be a constant.

Common dB levels
3dB = twice power, –3dB = half power
10dB = 10 x power

6dB = twice voltage or current
–6dB = half voltage or current

In usage practice, 3dB is about the smallest audio power level that human hearing can detect – although the 'golden-eared' brigade will doubtless dispute that – and 10dB actually only sounds twice as loud.

To make any significant increase in a usable (RF) signal level on, say, a tuner, a 6dB change or doubling of the signal will be necessary - that will equate to a quadrupling of the transmitter power!

DTTV is a different prospect. The difference between a working and a non-working DTTV signal can be as little as 1dB - but it can be argued that if you are working at such a fine tolerance you have, in effect, no signal. The actual signal can vary as much as 6dB or more across the year with propagation changes and the weather (depending how far you are from the transmitter) so at 1dB above limit threshold you will likely often get pixelation. As to what 60% on your DTTV box signal indicator means is anyone’s guess. You could have '70%' and still have problems due to trees, etc, in your line of sight to the transmitter, whilst your neighbour could get '40%' and have perfect reception.

Signal to noise ratios are usually a voltage reading in terms of audio (as in an amp.) However in radio it is more common practice in the industry nowadays to use SINAD or (I think) Signal Including Noise And Distortion – technically s/n does not take account of distortion effects.

I will probably get 'corrected' and/or flamed for this script and told I am wildly wrong in the minutest detail – I am just quoting from over 30 years as a radio comms engineer. The principle is correct even if there could be (deliberate?) misinterpretation in the detail!

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