Echo Cancellation Chapter 4

Echo : Echo is the repetition of a signal back to the transmitter; either due to a coupling between the loudspeaker and microphone or due to a reflection of the transmitted signal from the points or surfaces where the characteristics of the medium through which the signal propagates changes significantly so as to impede the propagation of the signal in the original direction such that some of the signal energy is reflected back to the source.

Chapter 4 The echo phenomenon is usefully employed for detection, exploration and navigation purposes in electronic surveillance and radar imaging instruments such as in sonar, ultrasonic imaging, infrared imaging and radar and also by some animals such as bats and dolphins. Echo can severely affect the quality and intelligibility of voice conversation in telephone, teleconference or cabin communication systems.

Chapter Types of ECHO : 1) Acoustic Echo : Acoustic echo due to acoustic coupling between the speaker and the microphone in hands-free phones, mobile phones and teleconference systems. 2)Electrical line echo : Electrical line echo due to mismatch at the hybrid circuit connecting a two-wire subscriber line to a four-wire trunk line in the public switched telephone network.

Chapter 4 Echo Return Time : The delay, also known as latency, of an echo is the round-trip time taken for the signal to arrive back at the source. In voice communication, end-to-end (i.e. one way) propagation delay is the sum of delays required for the voice of the speaker to be compressed, packetized, transmitted and propagated through different network devices and network link to reach the listener. In general the end-to-end delay may be expressed as

Chapter 4 Delay= coder delay + packetization delay + packet transmission (serialization) delay + propagation delay+ network (gateway) delay+ decoder delay. Round-trip delay of an echo is the propagation time from the transmitter to the receiver and then back to the transmitter. Generally, voice communication operators regard an echo delay of up to 150 ms acceptable and a delay of above 400 ms seriously degrading to voice communication. Between 150 to 400 ms the echo may be tolerated( accepted ).

Chapter 4 SOURCE OF DELAY : Transmission link (electromagnetic wave propagation)delay: Transmission line delay : This is caused by the finite propagation velocity( speed ) of electromagnetic waves which, depending on the dielectric constant of the transmission medium, is between 0.66 to 1.0 times speed of light (300,000 km/sec), the latter happens in the free space vacuum. The transmission link delay is about 0.8–0.6 per 100 miles for copper coaxial cable or fiber optics and 0.54 msec per 100 miles (speed of light) on air for wireless.

Chapter 4 Satellite link delay: The propagation time of radio signals at velocity of light in free space to a geostationary satellite at a distance of about 35,786 km is about 120 ms which would then need to be relayed down to the destination.

Chapter 4 SPEECH CODING/DECODING DELAY: Speech coder delay: 1–40 msec, includes signal segmentation delay and signal processing time for speech compression algorithm. Coder delay may include an additional look-ahead delay (typically 5 ms) where the signal in the next speech block is also used in the compression of the current block. However, this look-ahead delay is only a constant 5 ms addition to the overall delay. Speech decoder delay : typically less than 10 msec.

Chapter 4 NETWORK PROCESSING DELAY Packetisation delay : is the duration of packet, i.e. the time taken to fill a packet payload with encoded/compressed speech. This delay is a function of the sample block size required by the voice coder and the number of blocks placed in a single frame. For example with a block size of 10 ms, three blocks may be used in a frame. Voice network developers aim for a packetisation delay of no more than 30 ms.

Chapter 4 Transmission (serialization or offloading) delay: note this is not the propagation time over the communication link, this is the time it takes to clock on all the bits in a packet onto the network interface i.e. to place them on transmission line. Processing delay: In packet switching networks, processing delay is the time taken for the routers/gateways to process the packet header for sending it to the correct destination and to check (and correct) for bit errors in the packet that may have occurred during transmission. Processing delays in high-speed routers are small; typically on the order of microseconds or less.

Chapter 4 Queuing delay : Is the sum of the delays encountered by packets in transmission from source to destination. Queuing delay depends on bandwidth and demand and increases with congestion at times high network demand. Voice over IP gateway node delay : 50–100 msec.

Chapter 4 DE-JITTER DELAY : This may be accomplished with a de-jitter buffer at the receiving-end router/gateway. The de-jitter buffer transforms the variable delay of buffer into a fixed delay. It holds the first sample received for a period of time before it plays it out. This holding period is known as the initial play out delay

Chapter 4 Acoustic echo delay : Acoustic echo delay can be longer that network echo delay. The duration of acoustic echo depends on the dimensions of the room and the number of reflections off the walls that the echo goes through. For example, sound travels at a speed of 340 meters/sec at a room temperature of 25◦ C.

Chapter 4 Hence the time taken for sound to travel one meter will be about 2.94 msec. A distance of 10 meters from speaker to microphone will take about 29.4 msec and to this must be added the delay for coding and transmission through the communication network as described above.