Setting the SNR is done via an extension to the router's address in the Web address field, which then takes you to a sidelined part of the 7800 user interface where all you need do is enter a decimal value that corresponds to a required SNR value, eg. entering 50 is reckoned to get you a target line SNR of 3dB, ie. 3dB below BT's default SNR. Various websites, and including within one or two forums on this one, have published a table of these values for entering.
However, none of the tables indicate the values you need when you want to go positive in SNR. Instead, the tables are designed primarily for those users wanting to increase their downstream sync by lowering their SNR. Despite this, some months ago I was passed some values by one or two helpful souls for increasing the SNR. These didn't seem to work properly but after many attempts I managed to obtain the sort of SNR I needed pretty much by trial and error.
Since then my line's SNR has dropped a small amount and occasionally now I'm losing sync, particularly in streaming applications. So I've tried bumping up the SNR by another 3dB (to around 15dB total). The point is that the enterable values for increasing the SNR that I was given don't actually fit with the results. I've just made around a dozen attempts to home in on 15dB but none of the values I tried - which included 150, 200, 250 - would seem to get near 15dB. I got 18dB. I got even 21dB. And at the other end of the scale I got 6dB and 3dB, all with sensible corresponding sync speeds. But I just couldn't go from 12dB to 15dB. I'd have thought that entering the value 150 would have done the trick but it certainly didn't.
For the positive SNR values above 6dB there seemed to be no consistency. For example, I'd have thought that entering 100 would have produced a 0dB change, 150 a 3dB positive change, 200 a 6dB positive change, and 250 a 9dB positive change, but this was not what I found. Instead, the results swung from one extreme to the other, and seemed almost arbitrary. One would think that the target SNR values here would be relative to 6dB but since that's not how it worked out, I thought that perhaps instead the target value would be with respect to the last SNR value. But no, that didn't fit either.
So, has anyone managed to fathom a set of values that actually work for going positive in SNR, rather than negative? And if so, are the values always with respect to 6dB or instead with respect to the last SNR line value achieved?
As matters stand at present I'm running at an SNR of 15.7dB, giving me around 4.6M bps sync, but I only got that after a dozen attempts with all sorts of values, as the recommended ones just didn't seem to work properly. I'll now leave it at that, but if I ever need to tweak the SNR again it'd be nice to know which values will actually render the required SNRs.
Edited by meditator (Sun 23-Dec-12 12:13:28)