VoIP codecs Day Five: Better than PSTN?

Can VoIP outperform the PSTN? Adam Turner concludes his feature with some surprising insights!

PREVIOUSLY: Does VoIP sound bad?

If you want to hear how good VoIP can sound, even running over the open internet, ISPhone's Spenceley recommends playing around with Skype.

"Many people just don't appreciate that VoIP can be better than PSTN. PSTN is terrible because it's copper and you have trouble when it rains, you get echos and all sorts of problems. Skype is great quality wise because it uses a wideband 16 KHz codec, I think it might be a variant of G.722. I'd say it sounds better than PSTN," Spenceley says.

"G.722 is one of our favourite codecs internally and we're trying to get more people to run it. Since it's wideband you can hear more tones in your voice. It's a pretty high bandwidth codec but it's variable, it works at 48, 56 or 64 kbps. I suspect Skype probably uses 48 but it's adaptive so it can use a little bit more if available."

Switching to SkypeOut, the Skype service that lets you call PSTN numbers rather than other Skype users, results in a significant drop in call quality.

"When you're using SkypeOut, one leg of the call is clearly using a different codec. I think that's because when you're talking to another Skype user it's not using Skype's bandwidth, but when you're using SkypeOut you're connecting to Skype. If everyone was using something like 64 Kbps G.722 for SkypeOut then Skype would face a massive bandwidth bill. SkypeOut sounds like it's 8 to 10 kbps and the call quality is very disappointing after using Skype."

So is any more than 64 kbps overkill for a voice connection? Spenceley believes as bandwidth becomes more abundant we might see one more generation of codecs.

"I think we're right at that edge, there's probably one more round of codecs that will come out when bandwidth becomes so abundant that people are happy to spare 128 Kbps for a call. There's probably scope for a wideband version of G.711, sampling at 16 KHz, but really when you call on G.711 or an ISDN line such a Telstra OnRamp 30 you're not looking for much better quality."


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