whats the differance between G.729 codec variations

what is the difference between G.729 G.729A G.729AB

and if I have codecs set up for G729 in asterisk does this mean that G729A and G.729AB will work?

thanks

G.729a is a compatible extension of G.729 that needs less CPU because it has lower speech quality. It is compatible with G729.

Some of its features are:

• Sampling frequency 8 kHz/16-bit (80 samples for 10 ms frames)
• Fixed bit rate (8 kbit/s 10 ms frames)
• Fixed frame size (10 bytes for 10 ms frame)
• Algorithmic delay is 15 ms per frame, with 5 ms look-ahead delay

G.729b:

• Not compatible with G.729 or G.729a
• Has a silence compression method that enables the voice activity detection module (VAD). It is used to detect voice activity in the signal.
• It includes a discontinuous transmission (DTX) module which decides on updating the background noise parameters for non speech (noisy frames).
• Uses 2-byte Silence Insertion Descriptor (SID) frames transmitted to initiate comfort noise generation (CNG). If transmission is stopped, and the link goes quiet because of no speech, the receiving side might assume that the link has been cut. By inserting comfort noise, analog hiss is simulated digitally during silence to assure the receiver that the link is active and operational.

Read more at its wikipedia article