Topic: Hearable difference in sound quality for Asterisk install ?

[WillKemp]

Well, i've built the 2.6.16.22 kernel with low latency preemption configured and installed it in my SME server. I'll let you know if i notice any difference!

[arne]

But these factors have ben hold constant without any change:

hardware, configuration files, network quality, ITSP quality, PSTN quality, etc...

First Astlinux has been installed to PC A, and Centos 4.3 / Asterisk on PC B.

PC A with Astlinux have had clearly bether sound.

Then the whole setup have been reversed. PC B have got Astlinux. PC A have got Centos 4.3 / Asterisk.

PC B With Astlinux has then got clealy bether sound.

Sound quality has been checked with connection to 5 different external sip service providers (termination). Astlinux has given superior sound quality for all tested connections, and for all tested hardware.

Number of running processes for the Cenos 4.3 installations have been reduced to a minumum, anf there have (of cource) been no x-win.

It could be a lot of coinsidents, and still I find this in some way "unbelievable", so sooner or later I will make some more testing.

[arne]

I did not see your last post .. this test result would be very interesting

[WillKemp]

Astlinux has given superior sound quality for all tested connections, and for all tested hardware.

Ok, for your final test, now copy the kernel image and the ramdisk image (.img) from the Astlinux sytem's /boot to /boot in the Centos system, then copy the /lib/modules/<version> tree from Astlinux to Centos, then modify /etc/grub.conf on the Centos machine and try booting the Astlinux kernel in the Centos system.

I don't think i've left anything out, that ought to work.

[arne]

This sounds like a very interesting experiment .. unfortunately I'm on holiedays in Odessa, Ukraina just now, so I can't test anything before I'm home. Quite sure I will try when I'm home.

Have you tested ? How did it work ? Any hearable difference ? Did the Astlinux kernel work well for web server, fie server, etc.

I'm rather courious about what would be the result of such a test ....

(Actually it would not surprice me if the Astlinux kernel worked well on a sme server and if there were an hearable difference in the sound quality for the Asterisk server (?????))

Will I have to go home ?

[WillKemp]

Have you tested ?

No. I may check it out though - when i get round to downloading Astlinux.

Will I have to go home ?

No. It will still be there when you get back!

[WillKemp]

Here's a couple of interesting articles on stuff related to what we've been discussing. The second one specifically mentions about the realtime requirements of VoIP.

http://www.linuxdevices.com/articles/AT4503827066.html

http://www.linuxdevices.com/articles/AT9837719278.html

[arne]

Thanks for interesting info again. I have just moved from Ukraina to the republic of Moldova, so it's not so easy to test things out, here either, but its still interesting to work on "the theory".

I think that the following can be summed up:

1. Signal prossessing and data transport for Asterisk is a realtime application.

2. It has been recognized for a while that a standard Linux kernel do not perform "true realtime prosessing".

3. In environments that is working with or using ip telephony it has been quite usual to concider that IP telephony gives slightly porer sound quality than ordinary analogue telephony.

The interesting question is then: When the real time signal processing is performed by the same kernel that also handle quite other processeses like web servers, dastabase servers, etc, will this reduce the capabilyty to perform true realtime processing for the IP telephony function ?

I think that in some way it is likely to believe that the answer is "yes" and that the properties of the kernel, how the Asterisk program is processed by the kernel and the amount of load on the kernel by other processes will have influence on the real time capability, and therebay also the audio sound quality.

If this should be true, one basic design rule for well functioning Asterisk servers (and other iptelephony servers) should be: Let the ip telephony server allways have its own dedicated processor, and never try to mix it with general server processes and general processing platforms like the SME server.

It would be very interesting if also someone testet out for instance Astlinux as the dedicated Asterisk server versus Asterisk on sme or Centos as a more general distro, to come up with a conclusion - yes, there is actually a difference in sound quality, or the oposit - no, there is no hearable difference at all.

Actually I do not trust myself, to much but, I believe that there is a hearable difference that can be confirmed.

From this interesting discussion I will say the well known words - I'm still confused (and I have a bit daubt about it all), but on a higher level.

Anybody else that has tested and compared sound quality ?