G.722

from Wikipedia, the free encyclopedia

The guideline G.722 7 kHz audio-coding within 64 kbit / s of the ITU-T describes the coding of audio signals for transmission over a digital transmission path with 64  kbit / s, for example a B-channel of the ISDN . G.722 was approved in November 1988. In the meantime, the patents relating to the process have expired. G.722 is the de facto standard for HD telephony with VoIP hardware telephones . B. used in VoIP connections from Deutsche Telekom. The narrowband codec based on the same technology is G.726 .

The procedure

G.722 covers the frequency range from 50  Hz to 7000 Hz with a sampling rate of 16 kHz and quantizes with 14 bits. The codec thus covers a bandwidth of approximately 7 kHz. An encoder reduces the data transmission rate to 64 kbit / s (mode 1) , 56 kbit / s (mode 2) or 48 kbit / s (mode 3) . The technology is based on subband ADPCM .

RFC 3551

In RFC 3551 "RTP Profile for Audio and Video Conferences with Minimal Control" the modalities for the use of G.722 with the Internet-typical transmission protocol RTP are regulated.

Endianness

"The G.722 encoder produces a stream of octets, each of which SHALL be octet-aligned in an RTP packet. The first bit transmitted in the G.722 octet, which is the most significant bit of the higher sub-band sample, SHALL correspond to the most significant bit of the octet in the RTP packet. "

- IETF : RFC 3551, Section 4.5.2 
  8  7  6  5  4  3  2  1
┌─────┬─────────────────┐
│ high│    low-band     │
│ A  A│ A  A  A  A  A  A│
│o2 o1│32 16 o8 o4 o2 o1│ 1
├─────┼─────────────────┤
│ high│    low-band     │
│ B  B│ B  B  B  B  B  B│
│o2 o1│32 16 o8 o4 o2 o1│ 2
├─────┼─────────────────┤
│ high│    low-band     │
│ C  C│ C  C  C  C  C  C│
│o2 o1│32 16 o8 o4 o2 o1│ 3
└─────┴─────────────────┘
                  G722-64

sampling rate

"Even though the actual sampling rate for G.722 audio is 16,000 Hz, the RTP clock rate for the G722 payload format is 8,000 Hz because that value was erroneously assigned in RFC 1890 and must remain unchanged for backward compatibility. The octet rate or sample-pair rate is 8,000 Hz. "

- IETF : RFC 3551, Section 4.5.2

a=rtpmap:9 G722/8000

application areas

G.722 is currently used in particular for VoIP . Also CAT-iq , which on the DECT -building standard new transmission method for cordless phones, sees this codec before for wideband voice transmission between the base and handset.

G.722.2

In contrast to G.722, G.722.2 uses a compression process and is therefore not simply a technical extension of G.722, but technically fundamentally different. The 3rd Generation Partnership Project has extended the specifications of the G.722 with the guideline G.722.2 Wideband coding of speech at around 16 kbit / s using Adaptive Multi-Rate Wideband (AMR-WB) to newer generation of cellular networks .

Web links