[Asterisk-Users] InfoWeek Article on VOIP

[Bruce Ferrell]

trixter http://www.0xdecafbad.com wrote:
> On Sat, 2005-07-16 at 17:05 -0500, Michael Graves wrote:
> 
>>I agree with others who have chimed in that IP-to-IP calls can sound
>>better than PSTN calls. I have a co-worker who has a SipGate account in
>>the UK. Calls to him via SipGate go out through my FreeWorldDialup
>>account. They sound great. So good that in silent moments we often
>>think that we've been severed, even with no silence suppression on the
>>line. 
> 
> 
> One thing that many PSTN providers are doing for calls when they went
> digital is to insert small quantities of noise into the line.  That way
> people do not think they are disconnected.  There is a bunch of
> documentation on this, and even some that applies to VoIP
> providers/equipment doing the same (its basically a faint bit of white
> noise so you hear *something*).

It's sometimes called comfort noise... As far as I'm aware, it's only 
done in VoIP.

I spent 15 years working with digital switches/T1 channel banks.  I 
guess it might have been built in and I just didn't know about it, but 
we were very concerned about excess noise and quantization noise as it 
was.  We used to inject a 1004 test tone and then use a notch filter to 
measure the amount of quantization noise at the reciever.

Just as a by the by, G.711ulaw is the codec used in channel banks.