Speech Coding

The coding used from the beginning of the project was G711 which is ISDN-quality e.g. 64 kbit/s. G711 need  about 80 kbit/s bandwidth over the Internet.
G711 uses the A-law companding/expanding algorithm which reduces 14 bit to 8 bit, This coding is still available Quality (0) which can be used if you have limited bandwidth. Practical test have shown that G711 is not the optimal coding if the signal to noise ratio is low which it common on the shortwave bands. Of that reason we have implemented a number of different coding which have different needs of bandwidth. Quality 8 gives a very good audio quality even if you listen to music on FM. 16 kHz sample rate and 16 bits deep gives a very good dynamic range and a large SNR and good audio bandwidth. This coding needs about 380 kbit/s bandwidth over the Internet ( no problem with 2 Mbit/s ADSL). Quality 1 or 2 is recommended for shortwave and BC-listening. They are good compromises between quality and need for bandwidth. Later we have also introduced a few ADPCM based codecs which can be used when low bandwidth links like 64 kbit/s are used, they give lower audio quality of course,  but maybe better than no audio. The other is more of academic interest. The Quality settings is done via the WEB interface, Sample rate must be same in both ends, Coding can be different. A good speaker is always a good investment also.

Remember that you can use different settings in Control and Radio-RRC as long as you chose two codecs with the same sample rate. This could be useful if the Internet connection is assymetrical like ADSL and many mobile systems.

More info about transport parameters is available under Common configs.

Detailed information about the relationships between the parameters is available in this document =>

The following quality settings is available:

0
1
2
3
4
5
6
7
8
9
10
11
12
13

8 kHz
8 kHz
8 kHz
12 kHz
12 kHz
12 kHz
16 kHz
16 kHz
16 kHz
24 kHz
24 kHz
24 kHz
8 kHz
16 kHz

G711 aLaw
Lin 12
Lin16
G711 aLaw
Lin 12
Lin16
G711 aLaw
Lin 12
Lin16
G711 aLaw
Lin 12
Lin16
IMA ADPCM 4 bits 
IMA ADPCM 4 bits