Annex C+ combines several distinct components of the G.729 family into a single, cohesive floating-point implementation. While the main G.729 Recommendation and its annexes provide a bit-exact, fixed-point specification, Annex C+ defines the floating-point alternative.

• G.729 Main Body: The core 8 kbit/s CS-ACELP speech coding algorithm.
• Annex B (DTX): A silence compression scheme that utilizes Discontinuous Transmission (DTX). It incorporates Voice Activity Detection (VAD), Silence Description (SID) coding, and Comfort Noise Generation (CNG).
• Annex D (6.4 kbit/s): A lower bit-rate speech coding algorithm based on CS-ACELP.
• Annex E (11.8 kbit/s): A higher bit-rate speech coding algorithm, which provides enhanced quality for signals such as music.

2.1. Integration with Annex D (6.4 kbit/s)

The integration of Annex B’s DTX functionality with Annex D is described as “straightforward.” The existing VAD, SID, and CNG modules from Annex B are reused without modification. The primary consideration is to ensure proper updating of the postfilter parameters during periods of discontinued transmission.

2.2. Integration with Annex E (11.8 kbit/s)

The integration of DTX with Annex E is significantly “more involved.” This is due to the differing design goals of the two annexes: Annex E was developed with strict quality requirements for music signals, whereas Annex B’s DTX functionality had no such requirements.

To resolve this, a correctional module for music detection was introduced. The key challenges and solutions are:

• The VAD function runs after the forward adaptive LPC analysis but before the backward adaptive LPC analysis used in Annex E. If the VAD detects “non-speech,” the backward adaptive LPC analysis is skipped.
• To guarantee quality for music during DTX operation, the music detection function forces the VAD’s decision to “speech” when music segments are present.
• This ensures music is always coded using the full 11.8 kbit/s of Annex E, rather than being handled by the SID/CNG mechanisms of Annex B.