G.726
Encyclopedia
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-bits, 3-bits, 4-bits, 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
. 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 kbit/s.
It also is the standard codec used in DECT wireless phone systems and is used on some Canon cameras.
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.
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
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
G.723
G.723 is a ITU-T standard speech codec using extensions of G.721 providing voice quality covering 300 Hz to 3400 Hz using Adaptive Differential Pulse Code Modulation to 24 and 40 kbit/s for digital circuit multiplication equipment applications...
, 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
Sampling (signal processing)
In signal processing, sampling is the reduction of a continuous signal to a discrete signal. A common example is the conversion of a sound wave to a sequence of samples ....
, which are 2-bits, 3-bits, 4-bits, 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
G.711
G.711 is an ITU-T standard for audio companding. It is primarily used in telephony. The standard was released for usage in 1972. Its formal name is Pulse code modulation of voice frequencies. It is required standard in many technologies, for example in H.320 and H.323 specifications. It can also...
. It is primarily used on international trunks
Trunking
In modern communications, trunking is a concept by which a communications system can provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually. This is analogous to the structure of a tree with one trunk and many branches. Examples of...
in the phone network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
. 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 kbit/s.
It also is the standard codec used in DECT wireless phone systems and is used on some Canon cameras.
History
G.721 was introduced in 1984, while G.723G.723
G.723 is a ITU-T standard speech codec using extensions of G.721 providing voice quality covering 300 Hz to 3400 Hz using Adaptive Differential Pulse Code Modulation to 24 and 40 kbit/s for digital circuit multiplication equipment applications...
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
Quantization (signal processing)
Quantization, in mathematics and digital signal processing, is the process of mapping a large set of input values to a smaller set – such as rounding values to some unit of precision. A device or algorithmic function that performs quantization is called a quantizer. The error introduced by...
to 3-bit quantizer and same for the higher modes. This allows dropping of the least significant bit
Least significant bit
In computing, the least significant bit is the bit position in a binary integer giving the units value, that is, determining whether the number is even or odd. The lsb is sometimes referred to as the right-most bit, due to the convention in positional notation of writing less significant digits...
from the bit stream without adverse effects on speech signal.
Features
- Sampling frequency 8 kHz
- 16 kbit/s, 24 kbit/s, 32 kbit/s, 40 kbit/s bit rates available
- Generates a bitstreamBitstreamA bitstream or bit stream is a time series of bits.A bytestream is a series of bytes, typically of 8 bits each, and can be regarded as a special case of a bitstream....
, therefore frame length is determined by packetization (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)
- PSQMPSQMPSQM is a computational and modeling algorithm defined in ITU Recommendation ITU-T P.861 that objectively evaluates and quantifies voice quality of voice-band speech codecs....
testing under ideal conditions yields Mean Opinion ScoreMean Opinion ScoreThe Mean Opinion Score test has been used for decades in telephony networks to obtain the human user's view of the quality of the network. In multimedia especially when codecs are used to compress the bandwidth requirement , the mean opinion score ...
s of 4.30 for G.726 (32 kbit/s), compared to 4.45 for G.711G.711G.711 is an ITU-T standard for audio companding. It is primarily used in telephony. The standard was released for usage in 1972. Its formal name is Pulse code modulation of voice frequencies. It is required standard in many technologies, for example in H.320 and H.323 specifications. It can also...
(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.