G.726 is an ITU-T ADPCM speech codec standard covering the transmission of voice at rates of 16, 24, 32, and 40 kbit/s. It was introduced to supersede both G.721, which covered ADPCM at 32 kbit/s, and G.723, which described ADPCM for 24 and 40 kbit/s. G.726 also introduced a new 16 kbit/s rate. The four bit rates associated with G.726 are often referred to by the bit size of a sample, which are 2, 3, 4, and 5-bits respectively.
The most commonly used mode is 32 kbit/s, which doubles the usable network capacity by using half the rate of G.711. It is primarily used on international trunks in the phone network and is the standard codec used in DECT wireless phone systems. The principal application of 24 and 16 kbit/s channels is for overload channels carrying voice in digital circuit multiplication equipment (DCME). The principal application of 40 kbit/s channels is to carry data modem signals in DCME, especially for modems operating at greater than 4800 bit/s.
G.721 was introduced in 1984, while G.723 was introduced in 1988. They were folded into G.726 in 1990.
G.727 was introduced at the same time as G.726, and includes the same bit rates, but is optimized for packet circuit multiplex equipment (PCME) environment. This is achieved by embedding 2-bit quantizer to 3-bit quantizer and same for the higher modes. This allows dropping of the least significant bit from the bit stream without adverse effects on speech signal.
- Sampling frequency 8 kHz
- 16 kbit/s, 24 kbit/s, 32 kbit/s, 40 kbit/s bit rates available
- Generates a bitstream, therefore frame length is determined by packetization time (typically 80 samples for 10 ms frame size)
- Typical algorithmic delay is 0.125 ms, with no look-ahead delay
- G.726 is a waveform speech coder which uses Adaptive Differential Pulse Code Modulation (ADPCM)
- PSQM testing under ideal conditions yields Mean Opinion Scores of 4.30 for G.726 (32 kbit/s), compared to 4.45 for G.711 (µ-law)
- PSQM testing under network stress yields Mean Opinion Scores of 3.79 for G.726 (32 kbit/s), compared to 4.13 for G.711 (µ-law)
- 40 kbit/s G.726 can carry 12000 bit/s and slower modem signals, while 32 kbit/s G.726 can carry 2400 bit/s and slower modem signals well and 4800 bit/s with some more degradation than clear channel codecs.