Re: [Linphone-users] Softphones as fax carrier

[geo cherchetout]

Le 25/06/2013 11:55, *Darren Longhorn* wrote:

> That may well be your problem. Fax reception can be very sensitive to
> jitter.

I think I have found to-day what is the cause of the problem. (But not its 
solution, unfortunately, since I am not a developer.)

Using Linphone on A side, I have called my own pstn number on B side. While 
playing a test audio signal and capturing data with wireshark on A side, I 
recorded received audio on B side. My test signal is a 2000 Hz frequency 
modulated in amplitude with low frequency in such way that any time shift 
would be easily visible in a sound editor.

As I thought (because I had already made tests), there is not any disorder 
in the received signal, meaning paquets are all received and in the same 
order they are sent, thanks to my ISP and my VoIP provider's gateway, but 
*there are holes*. Every 6 to 8 seconds or about, there is a 25 to 31 ms 
hole. :-( Most of these holes are not audible by human ear but it seems 
obvious fax-modem's one don't like them. (Yes, fax-modems have ears, didn't 
you know this? ;-))

In the wireshark capture I see that only 2990 paquets instead of 3000 are 
sent in one minute. As the gateway can't send data it don't receive, it is 
not surprising there are frequent holes in the received audio signal!

For a comparison, about 3.5 minutes are needed to cause one 20 ms lateness 
when Ekiga is used.

Adaptive rate control is disabled. Am I wrong when I think the problem is 
not related to this setting? My upstream capacity is about 1000 kbit/s and 
video is disabled.

How to speed up Linphone? Is a bug report needed?