Voice API Programming Guide — June 2005 91
Recording and Playback
Note: On Springware boards, voice coders listed here are not available in all situations on all boards, such
as for silence compressed record or speed and volume control. Whenever a restriction exists, it will
be noted. For a comprehensive list of voice coders supported by each board, see the Release Guide
for your system release.
8.6 G.726 Voice Coder
G.726 is an ITU-T recommendation that specifies an adaptive differential pulse code modulation
(ADPCM) technique for recording and playing back audio files. It is useful for applications that
require speech compression, encoding for noise immunity, and uniformity in transmitting voice and
data signals.
The voice library provides support for a G.726 bit exact voice coder that is compliant with the
ITU-T G.726 recommendation.
Audio encoded in the G.726 bit exact format complies with Voice Profile for Internet Messaging
(VPIM), a communications protocol that makes it possible to send and receive messages from
disparate messaging systems over the Internet. G.726 bit exact is the audio encoding and decoding
standard supported by VPIM.
VPIM follows the little endian ordering. The 4-bit code words of the G.726 encoding must be
packed into octets/bytes as follows:
•The first code word (A) is placed in the four least significant bits of the first octet, with the
least significant bit (LSB) of the code word (A0) in the least significant bit of the octet.
•The second code word (B) is placed in the four most significant bits of the first octet, with the
most significant bit (MSB) of the code word (B3) in the most significant bit of the octet.
Table10. Voice Encoding Methods (Springware)
Digitizing Method Sampling Rate
(kHz) Resolution (Bits) Bit Rate (Kbps) File Format
OKI ADPCM 6 4 24 VOX, WAVE
OKI ADPCM 8 4 32 VOX, WAVE
G.711 PCM
A-law and mu-law
6 8 48 VOX, WAVE
G.711 PCM
A-law and mu-law
8 8 64 VOX, WAVE
Linear PCM 8 8 64 VOX, WAVE
Linear PCM 11 8 88 VOX, WAVE
Linear PCM 11 16 176 VOX, WAVE
GSM 6.10 full rate
(Microsoft format)
8 (value ignored) 13 WAVE
GSM 6.10 full rate
(TIPHON format)
8 (value ignored) 13 WAVE
G.726 bit exact 8 4 32 VOX