Vonage is +48VDC?

That is reasonable. It tells you exactly *nothing* about what causes a hum on the line though!

Same as above... :-)

Should be the same on the Verizon terminal too. (The actual voltage may change, and the polarity is not significant.)

Nahhhh. They just happened to design it with a ground.

They are using a floating supply, which is fine too.

That should be the same effect on the Verizon unit too, though if you have a relatively short "loop" there might or might not be much hum. On the other hand, if you run this down the road a mile or two, as would happen with a normal telephone line, it will _guaranteed_ put hum on the line of you do anything that unbalances it (and shorting either side to ground does exactly that, even if the voltages are the same).

The telephone loop is a balanced circuit. It's too complex to try describing all of it in a Usenet article, but you can probably find something in even a small library, or using google on the web, which will describe it in fairly simple terms.

I don't know specifically how either Vonage or Verizon equip their VOIP units, but I can draw a simplified circuit which shows what has to be accomplished. They will do the same thing, but might use other methods. Here is a transformer feeding a telephone set on the right and telephone line equipment on left. All this is doing is adding "loop current" to the line used by the telephone set.

T T )||( )||( +-------+ )||( | | )||( | --- Negative o---)||(---+ ------- | || --- 48V | || ------- "Battery" o---)||(---+ --- )||( | ------- Positive )||( | | )||( +-------+ )||( R R

If you want it grounded, either side of the "battery" can be grounded, and it will have no effect on the AC characteristics of the circuit. But grounding either of the Tip or Ring wires will unbalance the circuit for AC, and that will cause 60 Hz power line energy to be picked up. If the line to the telephone is very short, there might not be much hum. If it is a mile or two long... it makes a wonderful antenna.

In a real circuit there is probably a 2 mfd capacitor across the center tap, and in series with the battery would be a device to detect loop current to determine when the phone is off hook.

If one side of the battery is grounded, here is what that looks like to a voice frequency AC signal:

T T )||( )||( )||( )||( o---)||(---+ | || | | || | o---)||(---+-----+ )||( | )||( ----- )||( --- )||( - R R

Removing the ground make no difference at all, to the AC circuit. That just would make the right side exactly like the left side.

If I read this correct, Then you are expecting to see the same line conditions on a VOIP ATA as you would on a CO line, Is that correct ?

If it is correct then you will be disapointed, Think about what an ATA does and is. Any hum will be more than likly due to a poor quality power supply or mains supply, As to Hiss you will get some with ATAs as it injected in as a comfort tone, Since if there is no sound you will get absolute silence which is unerving to the user.

One other thing is are you using the correct codec, it should be ulaw for the US.

Ian

The Vontage is floating, no connection to ground. I do not know if it is supposed to be floating.

The Green is the Tip side of the line. The ground you see is from the Central Office through many feet of 19 to 14 gauge wire and the C.O. equipment. If you take the phone off-hook and measure you will find both sides of the Verizon line close to -24 vdc. If you place a call the voltage from the C.O. should reverse when the called party answers.

It sounds like induced AC hum, your Vonage wiring acting as an antenna. Unballancing the wiring by grounding either side would increase the antenna-ness of the wiring. I could be wrong. But if the hum goes away when you open all of the AC circuit breakers except for the Vonage (or even that one if the Vonage has battery backup) then you will be on the right track.