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Explanation of decibels in the context of ADSL

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xreyuk:
Any update on this guys? :)

GunJack:
one thing to bear in mind is that, in the examples quoted, all the numbers are theoretical maximums, and therefore some allowance needs to be made for variances in everything. If every line was perfect, then you could work out a definite formula for bits/tone, sync speed for a given SNRM, etc. Unfortunately, as we all know only too well, there's no such thing as a "perfect" line :(   

..and it's not only the line that has variances. Take any amplifier circuit - whilst the theoretical frequency response curve will be absolutely perfectly symmetrical about the centre frequency, in practical terms this will pretty much never happen. So, every router, DSLAM, etc involved in the overall circuit will degrade the signal to some degree, another reason why an absolute speed/line attenuation/snrm equation can only ever be an estimate :)

roseway:
In addition to that, some available bits will be deliberately be left unused, in order to leave some space for bitswapping. Without this breathing space, the connection wouldn't be able to cope with bursts of interference.

Weaver:
In ezzer's table back there, the ratios on the right are in Watts where it says 2:1 it means double the power in Watts which is approx 3dB from the table. By definition, 10dB means "ten times the power (W)".

In comms, we often don't want to talk in Watts, but rather about the voltage level of the signal on the line, in Volts (V). So watch out for this. Saying something such as merely "double", means "double what", Watts or Volts? You have to say which units you're talking in.

Now, power W = V2/R which is just Ohm's law combined with the definitions of Volts and Watts, so for example 100 x power in W = 102 equates to 10 x Voltage, because  power is proportional to voltage squared. Another example: 2 x V equates to 22 x W = 4 x W.

In ezzer's table, if you want to convert dB to V, you need to take the square root of the power ratio given. Talking in voltage terms not W, to convert voltage to dB, double voltage means 6 dB roughly, look down the rh col for the ratio you want, then double the number in the dB column. (why, the doubling of dB comes from the square in the previous para). Remember double V = 6dB approx.

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