G.711
G.711 is a guideline of the ITU-T for the digitization of analog audio signals using pulse code modulation (PCM). Areas of this codec are the classic fixed network - telephony and IP telephony in the A-law - or μ-law -Digitalisierungsverfahren (PCMA and PCMU).
With G.711, one sample of the audio signal is generated in time steps of 125 µs, which corresponds to a sampling rate of 8000 Hz . The sample is lossy compressed to 8 bits. The generated data stream has a data transmission rate of 8000 Hz × 8 bit = 64 kbit / s. According to the Nyquist-Shannon sampling theorem, the highest frequency of the analog signal may not exceed 4000 Hz. According to G.711, only the frequency range from 300 to 3400 Hz is encoded during digitization. For the subsequent non-linear coding of the digital signal, two different methods are used for the quantization : In Europe the A -law method , in North America and Japan the µ-law method. Because of the overhead , data transmission rates of 80 kbit / s to 128 kbit / s are required for the IP transmission of G.711 voice channels.
commitment
G.711 is one of many codings in IP telephony (VoIP) and is used in ISDN . The voice quality of a VoIP phone call with G.711 therefore corresponds to that of ISDN. In the classification of the Mean Opinion Score (MOS), G.711 achieved a value of 4.4. The MOS determines the subjective perception of the speech quality of a user. G.711 thus achieves a higher subjective voice quality than most other codecs, such as G.726 and G.729 . These have the advantage that a lower data transmission rate is required due to data compression .
See also
literature
- Nugehally S. Jayant, Peter Noll: Digital Coding of Waveforms . Prentice-Hall, Englewood Cliffs, NJ 1984, ISBN 0-13-211913-7 .
- Peter Vary, Rainer Martin: Digital Speech Transmission. Enhancement, Coding and Error Concealment . Wiley, Chichester 2006, ISBN 0-471-56018-9 .