[asterisk-users] How can I improve call quality?

[Tim Panton]

On 23 Apr 2007, at 10:56, Gordon Henderson wrote:

> On Mon, 23 Apr 2007, Adrian Marsh wrote:
>
>> So which is the best quality?
>>
>> Gradwells www site lists g711u and g729a, but we currently use  
>> ulaw/alaw
>> with them too..
>
> ulaw is g711u ...
>
> g711 (u or a), or ulaw or alaw which are the same things will give  
> you the best audio quality, but it's not better than a perfect  
> copper-wires telephone line because that's what BT (& others) use  
> internally inside their digital networks.
>
> So if you have the bandwidth, you'll never get better audio quality  
> than g711.
>
> But if you're bandwidth starved then you may have no choice to use  
> a compressing codec in which case, it's a trade-off between CPU  
> power required to encode/decode the audio data vs. speech quality.  
> You can get some fantastic compressions, but then you end up  
> sounding like a dalek...
>
> The difficulty is when you have packet loss - in this case, which  
> codec gives the best results when it loses a packet or 2...
>
> I wish I knew the answer to that last one... I've used g711 over a  
> lossy wifi connection and it's been acceptable to me, but maybe not  
> to others!

This is a complex area, but :-
	If you are dropping packets due to  network congestion, then using a  
high compression codec
like gsm or 729 or amr  over IAX will reduce your packet size (but  
not packet count) and so hopefully
improve the chances of all your packets getting through.
	If you are dropping packets due to a lossy channel (eg wifi), then  
the size of the packet
probably won't matter, so you need a codec with good packet loss  
concealment. AFAIK gsm is poor at this because it keeps state across  
packets. Some codecs have mechanisms to cope with
lost packets, but I don't think asterisk supports this. So in this  
case you are better off with a simple
codec like {au}law.



Tim Panton

www.mexuar.net
www.westhawk.co.uk/