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Voice Quality Enhancement (VQE) is required if the quality of the voice signal is degraded in a telecommunication system. Acoustic echo, background noise, and reverberation are some of the possible causes for the degradation of the voice signal. VOCAL's Voice Quality Enhancement System (VVQES) is designed specifically for PSTN, TDM and VoIP communication networks and can significantly improve the quality of conversations by removing echo and background noise as well as by adjusting speech levels to achieve a consistent and comfortable listening experience. The enhancement of the voice quality is implemented by integrating the right combination of our industry-leading voice improvement modules.
Acoustic echo cancellers operate on the digitally sampled audio signals of the communication device. The transfer function of the acoustic environment from the loudspeaker to the microphone on the device is estimated to cancel the received echoes from the microphone signal. Acoustic echo cancellation is an essential part of providing voice quality enhancement to any form of voice communication.
Line echo cancellers are voiced operated devices placed in the 4-wire portion of a circuit (which may be an individual circuit path or a path carrying a multiplexed signal) and are used for reducing the echo. This solution is fully G.168 compliant for echo tail lengths up to 32ms.
Noise reduction can adaptively reduce background noise and effectively increase the signal to noise ratio of the speech signal. In the acoustic environment there are many sources of noise consisting of different spectrum characteristics, either time-invariant or time-varying. Noise reduction method should be robust to all types of noise.
Voice Activity Detectors (VAD) are used to detect the whether voice is active in the communication. This detection is important for efficient use of bandwidth during periods of silence for vocoders. Also, can be used as an control parameter for estimating statistics in speech processing algorithms such as, acoustic echo cancellation, noise reduction, and automatic gain control.
Comfort noise generation produces an artificial background noise in order to improve perceptual quality of the communication. A low level noise provides a perceptual clue to the listener that the line of communication is still active. Also, CNG can be used in conjunction with VAD to reduce the bit rate during periods of silence.
Packet Loss Concealment (PLC) algorithms, also known as frame erasure concealment algorithms, can help hide transmission losses in a packetized network. PLC can occur at both the transmitter and receiver ends of the communication. For transmitter-receiver based techniques requires support and standardization in order for the receiver to properly support the forward error correction applied at the transmitter end. For receiver-only based techniques, PLC tries to interpolated missing packets from the information of surround received packets.
Dynamic range compression is the final processing step of voice quality enhancement for the Tx channel. The compressor limits the level of a signal over a preset threshold. DRC is performed to prevent quality impairments of a loudspeaker caused by over-driving of the loudspeaker. In addition, to help prevent saturation of the echo path.
Automatic gain control is the final processing step of voice quality enhancement software for the Rx channel. AGC is part of VQE, especially with systems limit volume level controls because voice levels which are too low requires additional effort of listener. Hence, the perceptual voice quality is lowered. AGC automatically adjusts the output signal level to an appropriate signal level for ease of the listeners.
Blind signal separation, also known as blind source separation, is the separation of a set of speech signals from a set of mixed signals, without the aid of information (or with very little information) about the source signals or the mixing process.
De-reverberation aims to remove the reverberation generated by room acoustic environment. It generally attempts to model the impulse response of the acoustic environment and filter the reverberation from the received microphone signal.
With microphone array, ABF is able to filter out speech signals generating from different locations in space, and to provide noise reduction and interference suppression in the acquired signal with minimal signal distortion of the desired signal.
The key to integrating a VQE solution is to put the various algorithms together in such a way that maximizes speech quality enhancement. The figure below shows one possible configuration for enhancing the quality of the speech signal sent from the near end to the far end.
