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The quality of VoIP calls depends on the codec used for the transmission and on the bandwidth of Internet connection. I want to use best optimized codec for my Internet connection.

I have to select codecs from the following list:

  • G.722
  • G.711 u law
  • G.711 a law
  • G.726
  • G.729

Which one to choose and why?

I want to have best high quality voice on both sides.

I have ADSL2+ Internet connection with bit rates:

  • download 6 Mbit/s
  • upload 0.5 Mbit/s
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up vote 2 down vote accepted

According to a few studies I've read, G.711 seems to provide the best tradeoff between required bandwidth, compression delay and subjective audio quality.

Cisco published an article in 2006, Understanding Codecs: Complexity, Hardware Support, MOS, and Negotiation, where they also evaluated the subjective Mean Opinion Score (MOS) over a couple of codecs, which mostly include those you mention. MOS range from 1–5, with 5 being the best quality, as averaged over a number of listeners (usually more than 12–15).

The results were as follows:

+---------------------+------------------+------------+-------------------------+
| Compression Method  |  Bit Rate (kbps) |  MOS Score |  Compression Delay (ms) |
+---------------------+------------------+------------+-------------------------+
| G.711 PCM           |  64              |  4.1       |  0.75                   |
| G.726 ADPCM         |  32              |  3.85      |  1                      |
| G.728 LD-CELP       |  16              |  3.61      |  3 to 5                 |
| G.729 CS-ACELP      |  8               |  3.92      |  10                     |
| G.729 x 2 Encodings |  8               |  3.27      |  10                     |
| G.729 x 3 Encodings |  8               |  2.68      |  10                     |
| G.729a CS-ACELP     |  8               |  3.7       |  10                     |
| G.723.1 MP-MLQ      |  6.3             |  3.9       |  30                     |
| G.723.1 ACELP       |  5.3             |  3.65      |  30                     |
+---------------------+------------------+------------+-------------------------+

As you can see, G.711 still requires more bandwidth than the other codecs, which were developed for ultra-low bandwidth applications, but in your case, with ADSL, this is no issue. What you get is a very low delay with good MOS values.

A more recent 2009 study by Karapantazis et al gives an even better overview (click to enlarge):

As you can see, there are certain broadband codecs which you could also take into account, Speex being very popular.

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