SOGEA VDSL without using a filter

[Colin_London]

I am being (force) migrated to SOGEA Digital Voice over VDSL next week.

My broadband is just on the cusp of full 80/20, but the line capability sometimes drops down to 77Mbps (2.5 to 2dB margin) causing gradual attrition of the profile and eventually an inevitable resynch. If I can get an extra dB or so overhead on the line I think these resynchs can be avoided.

I am thinking that the move to digital voice presents the opportunity to dispose of the faceplate filter altogether and maybe gain a dB or two from removal of the filter insertion loss. I have even bought a standard faceplate to replace the v4 filtered faceplate. I see that Openreach support use of the standard faceplate to pass 'everything', but there is no cable specified.

I understand that there is no issue connecting the SmartHub 2 to the SOGEA line without a filter in place. However, which cable to use? Most of the 'BT plug to RJ11' cables on sale on Amazon are for old VF modem connections. These may be crossover cables. I have also seen 2 and 4 wire cable versions on sale. No idea if these would work for this connection.

Does anyone know of a short (1m) BT to RJ11 cable with the correct pinouts at each end to connect a SmartHub 2 directly to a standard 5c Master Socket faceplate? I don't want to damage the SH2 modem by using a cable with incorrect pinouts.

[licquorice]

You won't damage anything.

[Colin_London]

Thanks - but which cable to buy?

A two wire crossover?

A two wire non-crossover?

A four wire crossover?

What is a 'rollover' spec? Or will literally any old fax/modem cable work??

[pippincp]

Any old Fax/Modem cable. The only reason for the filter was to be able to use the voice service without interference from the broadband signal.

[licquorice]

Broadband (and telephony for that matter) only uses 2 wires and isn't polarity conscious.

[Colin_London]

Excellent. I have found a supplier who I have used before that makes high quality CAT5 cables including an option for BT to RJ11. So hopefully all set now for when the dial tone disappears.

I know that the ‘replacement’ SmartHub 2s are sent out without cables to reduce waste and I guess that the approach of just using the existing filtered faceplates is to reduce confusion for digital voice switchover. But keeping all those old filters in-situ is not the best technical solution and is diminishing the benefit of the migration to SOGEA.

[licquorice]

The broadband signal passes straight through the filter, it is the phone signal that is filtered.

[Colin_London]

If you remember your filter theory you will recall that any filter will introduce some loss in the passband - maybe only 0.5dB but it is there. This becomes more significant at the edge of the passband. If you don’t need a filter, get rid of it.

edit: just googled some micro filter specifications. They all have an insertion loss of between 0.5 and 1dB. That will be enough to reduce my resynchs.

[rbz5416 · Accepted solution]

I'm afraid previously replies aren't necessarily correct. While the BT plug always uses pins 2 & 5, some RJ11 sockets are wired 3 & 4. Hence why there is the choice between straight & crossover cables. So "Any old fax/modem cable" only has a 50:50 chance of working. You need a straight two wire cable, although four wire would do as the spare wires would simply be ignored.

But @licquorice is correct in that the filtering is only applied to the BT socket & not the RJ11. So unless you're faceplate is faulty, what you're proposing is a waste of time. If anything you want an old style extension socket that wires directly to the BT socket without a test socket.

[Colin_London]

Of course the intent is to pass the signal straight through but there is an insertion loss - as I quoted between 0.5 and 1dB. Is this not worth removing?

Thanks for the confirmation that a straight cable is required. Thankfully the one I have ordered is straight.