Silence suppression
Encyclopedia
The term silence suppression is used in telephony
to describe the process of not transmitting information over the network
when one of the parties involved in a telephone call
is not speaking, thereby reducing bandwidth
usage.
Voice is carried over a digital telephone network by converting the analog signal to a digital signal which is then packetized
and sent electronically over the network. The analogue signal is re-created at the receiving end of the network. When one of the parties does not speak, background noise is picked up and sent over the network. This is inefficient as this signal carries no useful information and thus, bandwidth is wasted.
Given that typically only one party in a conversation speaks at any one time, silence suppression can achieve overall bandwidth savings in the order of 50% over the duration of a telephone call
. (While both parties may sometimes speak at the same time, there are times when both parties are silent.)
Silence suppression is achieved by recognizing the lack of speech through a speech processing
mechanism called voice activity detection
(VAD) which dynamically monitors background noise and sets a corresponding speech detection threshold. This technique is also known as speech activity detection (SAD).
A similar principle is used for Discontinuous Reception
and discontinuous transmission
in GSM mobile telephone systems.
For further bandwidth gains, silence suppression is normally done after echo cancellation
.
When silence suppression is active, the line appears to have gone dead at the other (egress) end of the call. For this reason, so-called comfort noise
needs to be generated to compensate for the lack of background noise. The ingress end must therefore signal the egress end that silence suppression is in effect. For best results, the level of comfort noise being generated on egress should match that of the background noise at the ingress end.
Speech activity detection must occur very quickly, otherwise clipping might occur.
Speech activity detection does not work well on non-speech calls (fax or modem communication, for example).
Thus, silence suppression is generally an optional feature on telephony devices. In some cases, it is automatically turned on based on the characteristics of a call.
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....
to describe the process of not transmitting information over the network
Telecommunications network
A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...
when one of the parties involved in a telephone call
Telephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...
is not speaking, thereby reducing bandwidth
Bandwidth (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
usage.
Voice is carried over a digital telephone network by converting the analog signal to a digital signal which is then packetized
Packet switching
Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...
and sent electronically over the network. The analogue signal is re-created at the receiving end of the network. When one of the parties does not speak, background noise is picked up and sent over the network. This is inefficient as this signal carries no useful information and thus, bandwidth is wasted.
Given that typically only one party in a conversation speaks at any one time, silence suppression can achieve overall bandwidth savings in the order of 50% over the duration of a telephone call
Telephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...
. (While both parties may sometimes speak at the same time, there are times when both parties are silent.)
Silence suppression is achieved by recognizing the lack of speech through a speech processing
Speech processing
Speech processing is the study of speech signals and the processing methods of these signals.The signals are usually processed in a digital representation, so speech processing can be regarded as a special case of digital signal processing, applied to speech signal.It is also closely tied to...
mechanism called voice activity detection
Voice activity detection
Voice activity detection , also known as speech activity detection or speech detection, is a technique used in speech processing in which the presence or absence of human speech is detected.. The main uses of VAD are in speech coding and speech recognition...
(VAD) which dynamically monitors background noise and sets a corresponding speech detection threshold. This technique is also known as speech activity detection (SAD).
A similar principle is used for Discontinuous Reception
Discontinuous Reception
Discontinuous Reception is a method used in mobile communication to conserve the battery of the mobile device.The mobile device and the network negotiate phases in which data transfer occurs...
and discontinuous transmission
Discontinuous Transmission
Discontinuous transmission is a means by which a mobile telephone is temporarily shut off or muted while the phone lacks a voice input.-Misconception:...
in GSM mobile telephone systems.
For further bandwidth gains, silence suppression is normally done after echo cancellation
Echo cancellation
'The term echo cancellation is used in telephony to describe the process of removing echo from a voice communication in order to improve voice quality on a telephone call...
.
Drawbacks
Background noise detection may be difficult in some circumstances (relatively low speech level, or relatively high background noise level, for example).When silence suppression is active, the line appears to have gone dead at the other (egress) end of the call. For this reason, so-called comfort noise
Comfort noise
Comfort noise is synthetic background noise used in radio and wireless communications to fill the artificial silence in a transmission resulting from voice activity detection or from the audio clarity of modern digital lines....
needs to be generated to compensate for the lack of background noise. The ingress end must therefore signal the egress end that silence suppression is in effect. For best results, the level of comfort noise being generated on egress should match that of the background noise at the ingress end.
Speech activity detection must occur very quickly, otherwise clipping might occur.
Speech activity detection does not work well on non-speech calls (fax or modem communication, for example).
Thus, silence suppression is generally an optional feature on telephony devices. In some cases, it is automatically turned on based on the characteristics of a call.