By: Preethi Ramkumar
Meaning of CNG - "Comfort Noise Generator", is used to reduce the transmission rate during silence periods of speech. Systems allowing discontinuous transmission are based on a Voice Activity Detection (VAD) algorithm and a Comfort Noise Generator (CNG) algorithm that allows the insertion of an artificial noise during silent intervals of speech.
This feature is necessary to avoid noise modulation introduced when the transmission is switched off. If the background acoustic noise that was present during active periods abruptly disappears, this very unpleasant noise modulation may even reduce the intelligibility of the speech. The purpose of the VAD is to reliably detect the presence or absence of speech and to convey this information to the CNG algorithm. The purpose of the CNG algorithm is to create a noise that matches the actual background noise with a global transmission cost as low as possible.
“…To use bandwidth more efficiently, VoIP networks employ functionality known as silence suppression or voice activity detection. A VAD is a component of a voice gateway or terminal that suppresses the packetization of voice signals between individual speech utterances, such as during the silent periods in a voice conversation. VADs generally operate on the send side of a gateway and can often adapt to varying levels of noise versus voice. Thus, similar to adaptive jitter buffers and echo cancellers, VADs can converge on appropriate thresholds to optimize their performance for a given voice conversation. As human conversations are essentially half-duplex in the long term, the use of a VAD can realize approximately 50 percent reduction in bandwidth requirements over an aggregation of channels.” [International Engineering Consortium]
“…There are generally two reasons for using VAD (also known as Voice Operating Switch or VOX) and Comfort Noise Generator (CNG), bandwidth saving, and prolonging battery life. VAD-CNG are therefore applicable to many devices including cellular phones, telephony devices, mobile radio communications, bone-conducting microphones, headsets, digital recorders and other voice related applications which are battery operated.
” [Tecteon]
“…The magnitude of the noise level is packed into the least significant
bits of the noise-level byte with the most significant bit unused and
always set to 0 as shown below in Figure 1. The least significant
bit of the noise level magnitude is packed into the least significant
bit of the byte.”
[Lucent Technologies]
“…Adaptive Digital's VAD/CNG software performs voice activity detection and comfort noise generation. It is used in systems that used reduced bandwidth channels during silent or non-voiced portions of a conversation. The VAD algorithm is used at the transmit side of a communication link. The VAD determines whether or not speech is present in the outbound signal. If speech is not present, the VAD algorithm determines the level of the background noise. The background noise level is transmitted to the opposite end of the link in place of the speech signal. The noise level requires little channel bandwidth and only needs to be transmitted when the background noise level changes. The CNG algorithm runs at the receive side of the link. When speech is not present, the CNG algorithm generates a noise signal at the level sent from the transmit side.” [ADAPTIVE DIGITAL TECHNOLOGIES]
“…Kane Computing Ltd (KCL) has recently signed a distribution agreement with Adaptive Digital Technologies, Inc. (ADT) to represent them in the UK and Ireland, to promote and sell their portfolio of DSP algorithms, chips, and solutions, such as voice compression, conferencing, telephony signalling, Voice over IP, noise reduction and echo cancellation for use with Texas Instruments' TMS320™ family of DSPs.
” [KCL]
