[Asterisk-Users] ECHO discussion & (TLP) Transmission Level points for *

[Rich Adamson]

> What is the equivalent decibel level of the RTP stream being transported
> through *.  This may sound like a funny question but I will try to show what
> I mean by a diagram below:
> 
> PSTN Receive level into FXO card		Center point level after FXO or FXS  Gain
> adjustment
> 
> PSTN		>-----Transmit in -8db				>---Receive  0db
> PSTN		<-----Receive out -5db				<---Transmit 0db
> 
> 
> When you attempt to make a connection from analog line to analog line the
> idea is to have both channels achieve a 0/0db level at what would be termed
> the center of the SWITCH> This way the call is optimized for connectign both
> legs of the call.  A typical tlp (Test Level Point) for PSTN lines would be
> to send -5db into the PSTN from your FXO card and expect the receive level
> to be -8 or -9 db at the input of your FXO card. This is the same concept
> used when dealing with channel banks the digital stream level should be
> 0/0db.
> 
> It stands to reason( at least to me:) ) that if digital levels are not 0/0
> for the codecs used for transport of voice within the digital stream then
> when you convert sip/iax and send it out over an analog line you will have a
> gret potential for reflected power and will experience an echo issue.
> 
> Does any of this make sense to anyone out there?

In theory, it makes sense. Based on about seven months of working with *,
I'm under the impression additional work should be focused on various
transmission levels and not just the pstn interface.

In another week or two I'll have access to a high quality transmission
test set, and am hoping to document some of the very items you've noted
as well as others.

The current levels are obviously usable, however I think the overall
system and its interfaces can be improved to approximate a commercial
pbx.

Rich