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  • G.711


Contents

BandNarowband
Sampling frequency8kHz (ISDN quality)
Bitrate64 kbit/s
MOS (Codec) 4,4


G.711 is the oldest codec. It was approved by the ITU as early as 1965. It requires only a low computing power and generates a data stream of 64 kBit/s.

G.711 is an ITU-T guideline for digitizing analog audio signals using pulse code modulation (PCM). Areas of application for this codec are classic fixed-line telephony and IP telephony using the A-law or μ law digitization method (PCMA or PCMU).

With G.711, one sample of the audio signal is generated in time steps of 125 µs, corresponding to a sampling rate of 8000 Hz. The sample is lossy compressed to 8 bit. The generated data stream has a data transfer 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 coded during digitization. For the subsequent non-linear coding of the digital signal, two different methods are used for quantization: In Europe the A-law procedure, in North America and Japan the µ-law procedure. Because of the overhead, data transfer rates of 80 kbit/s to 128 kbit/s are required for IP transmission of G.711 voice channels.

In the Mean Opinion Score (MOS) , G.711 reaches a value of 4.4. The MOS determines the subjective perception of a user's voice quality. This gives G.711 a higher subjective voice quality than most other codecs, such as G.726 and G.729 , which have the advantage that data compression requires a lower data transfer rate.

IP telephony providers often use G.711, which is the same procedure as ISDN. The advantage lies in the simple transmission of voice data from the fixed network to the IP network and vice versa. A recoding of the language data is not necessary.

However, this codec is not suitable for narrowband Internet access or network connections.


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