Broadband signal to noise ratio

  Al94 12:41 01 Mar 2004

I posted in Consumerwatch this morning click here re my initial delight with Netgear help.

Query here is on BT line test (Nildram is ISP) carried out this morning. They measured the SNR twice in the past week as modem is not synchronising at times (Netgear DG824M) and find it to be 57 which they say should be OK. With my modem router connected it measures 64 and they say this is a "massive" 7db increase in SNR and probably a faulty modem. This is getting a bit too technical for me, does it sound right?

I have been loosing synch most evenings from around 5.00 pm

  Stowit 12:51 01 Mar 2004

Don't know, but Im sure db scale is exponential - an increase of 2 db is a doulbling over original or something like that

  Al94 14:10 01 Mar 2004


  Microdot 14:18 01 Mar 2004

The higher the signal to noise ratio, in dbs, the better?

Cheers de Art.

  [email protected]@m 14:54 01 Mar 2004

is correct. Signal minus Noise = 64 is good.

Stowit, correct, dB figures are exponential. 2Db is double 1dB, 5dB is double 4dB.

  Al94 15:28 01 Mar 2004

now I'm really confused - higher is better - yet BT tell me higher is not good, needs to be 57 or lower - Help!!

  Valvegrid 15:56 01 Mar 2004

It depends on what you are looking for. For line communications you are dealing with noise that is already there in the wire or fibre optic line also all the amplifiers that contribute to the noise. Generally the longer the line the greater the noise, this can be overcome with low noise amplifiers, this is backgound noise, the same hissing sound when you listen on a radio without a station tuned in.

If the background noise is for instance 7db then a signal has to be greater than this to overcome the background noise. With data, the signal has to be considerably greater than the background noise to make it efficient, if you have a noisy modem it will have difficulty in hearing the signal in the first place, this of course happens in both directions if your noisy modem is trying to communicate with it's chum at the other end, especially if the line noise is high to start with, this is probably the problem with you becomming unsycronised.

Sorry to go on a bit, but I hope that's explained it without getting to technical.


  Gongoozler 16:17 01 Mar 2004

Signal to noise is a power ratio. In decibels it is 10 times the logarithm of the ratio. The logarithm of 2 is approximately 0.3, 10 times that is 3. The reason we use decibels is that it helps to deal in whole numbers rather than decimals. Therefore every doubling is 3dB. Doubling a voltage gives 4 x power (power is proportional to the square of the voltage), or 6dB. This is why although 64 dB doesn't sound a lot more than 57 dB, it in fact means that either the signal power is 5x greater, or the noise is 5x less.

  Al94 22:15 01 Mar 2004

Thanks all, Paul in particular

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