VoIP Protocols

Hybrids and analysis by synthesis (ABS) speech coders (VoIP Protocols)

2.6 2.6.1 Principle In previous sections we have studied two types of coders: • Waveform coders that remove the inter-sample correlation by using linear prediction. The differential coding scheme used with adaptive quantizers gives good performances with a bitrate between 32 kbit/s and 24 kbit/s. • Linear predictive coders (or vocoders) use a simple model […]

Codetopic-excited linear predictive (CELP) coders (VoIP Protocols)

2.7 CELP coders are in essence linear predictive coders equipped with an ABS search procedure. They were invented in the 1980s by Bell Labs (under the supervision of B.S. Atal and M.R. Schroeder). As we have already seen, once the short-term correlation in the signal has been removed by the LPC filter and the long-term […]

Conclusion on speech-coding techniques and their near future (VoIP Protocols)

2.9 2.9.1 The race for low-bitrate coders Many coding schemes have not been described in this topic: • The MELP (mixed excitation LPC) coder, retained in the new 2,400-bit/s US Federal standard.. • The VSELP (vector sum excited LP) coder, used in the half-rate 5.6-kbit/s GSM system. • The multi-rate Q-CELP (Qualcomm CELP) at 1, […]

Quality of speech coders (VoIP Protocols)

2.8 Most speech coders have been designed to achieve the best possible level of speech reproduction quality, within the constraints of a given source-encoding bitrate. For narrowband coders, the reference is ‘toll quality’, or the quality of speech encoded by the G.711 coder. For wide-band coders (transmitting the full 50-7,000-Hz band), the reference is the […]

Annexes (VoIP Protocols)

2.11 2.11.1 Main characteristics of ITU-T standardized speech coders 0-48 kbit/s2 0 (24 kbit/s) -(6.6 kbit/s) Noise ‘quality’ + + +’ +’ + 0 – +2 +/0 + Capability to 0 0 0> 0> 0 – - + 64 kbit/s2 + -10 encode music 0 56 kbit/s -48 kbit/s Robustness to 0/0 +/0 +/01 +I01 […]

Voice Quality (VoIP Protocols)

3.1 Introduction A common joke among IP telephony engineers is to say that if they had proposed to carry voice over IP a couple years ago, they would have been fired. This remains a private joke until you make your first IP phone call to someone on an old PC without a headset, to find […]

Reference VoIP media path

3.2 The media path of IP telephony calls can be modeled as shown in Figure 3.1, when there is no POTS or ISDN terminal involved. The situation gets a little more complex when interworking with an ISDN phone through a gateway (Figure 3.2): When the gateway interfaces with an analog network, the user-network interface is […]

Echo in a telephone network (VoIP Protocols)

3.3 3.3.1 Talker echo, listener echo The most important echo is talker echo, the perception by the talker of his own voice but delayed. It can be caused by electric (hybrid) echo or acoustic echo picked up at the listener side. If talker echo is reflected twice it can also affect the listener. In this […]

Acceptability of a phone call with echo and delay (VoIP Protocols)

3.5 3.5.1 The G.131 curve The degree of annoyance of talker echo depends on the amount of delay between the original signal and the echo, and on the attenuation of the echo signal compared with the original. This attenuation is characterized by the ‘talker echo loudness rating (TELR) as described in G.122 and annex A/G.111. […]

Delay (VoIP Protocols)

3.4 3.4.1 Influence of the operating system Most IP phone applications are just regular programs running on top of an operating system, such as Windows. They access sound peripherals through an API (e.g., the Wave API for Windows), and they access the network through the socket API. As you speak the sound card samples the […]

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