EC Explained: Echo cancellation is one of the key enabling technologies for Voice over Packet (VoP).

ECHO

The vast majority of the public telephone system local loop wiring is done using two-wire connections whereby the same pair of wires carries voice signals in both directions. In the telephone company's central office or in an office PBX, a two-to-four wire conversion is done using a hybrid circuit. Hybrid circuits do not perform perfect impedance matches. The imperfection results in echo.

The echo canceller's goal is to detect and remove echo as quickly and effectively, thereby minimizing any loss in voice quality due to the echo. The echo canceller must perform this function under all conditions including double-talk (when both parties are speaking at the same time) and in the presence of background noise. Furthermore, the echo canceller must not cause detriment to signaling tones (DTMF, etc.) or fax and modem transmissions.

Adaptive Digital's G.168 software provides: an algorithm with less than 50ms convergence, large double talk range, advanced comfort noise, and works with 128ms tail-length without performance penalty.

ECHO TYPES

Hybrid - The most prevalent cause of echo in PSTN is impedance1 mismatches within the hybrid where 4-wire phone circuits are converted to 2-wire circuits. This electrically generated echo occurs when the incoming energy from the far end speaker is reflected back toward the speaker as a slightly altered and delayed replication due to impedance mismatch in the hybrid (Figure 1). The presence of echo occurs whenever the replicated signal delay exceeds 10 msecs, and becomes apparent to the speaker as reflected voice when the delay exceeds as little as 16 msec. It is manifested to the far end as an altered replica of the speaker original.
Active hybrid circuits provide some echo reduction, but not enough when the end-to-end circuit delay is even moderate. The signal being reflected back is measured as ERL (Echo Return Loss), the higher the ERL, the lower the reflected signal back to the speaker.

Figure 1. Hybrid

Active hybrid circuits provide some echo reduction, but not enough when the end-to-end circuit delay is even moderate. The signal being reflected back is measured as ERL (Echo Return Loss), the higher the ERL, the lower the reflected signal back to the speaker.1. Impedance: the apparent opposition in an electrical circuit to the flow of an alternating current that is analogous to the actual electrical resistance to a direct current and that is the ratio of effective electromotive force to the effective current- Acoustic Echo

Another type of echo, acoustic echo, is the undesired voiceband energy transfer from the speaker to microphone in a hands free telephone set or speakerphone.

Delay - Delay in telephony context is the time during which voice signals travel across the network. End-to-end delay is the sum of delays required for a voice signal generated by the speaker's mouth to cross the different network devices and network links in order to reach the listener's ear. Roundtrip delay (Fig 2) is the sum of end-to-end delay in both directions as it is reflected back to the speaker's ear. The two directions can take separate paths, one through a satellite and one via land-lines, for example.


Figure 2. Roundtrip delay

• Contributing factors -The PSTN telephony network "cloud" itself is a source of delay, as are many points along the voice network: examples:
  
  Compression delay:

1. G.723.1: 37 msec

2. G.729: 15 msec

3. G.728: 2.5 msec

4. Inter-process hand-offs: approx. 10 msec at each end

5. Transmission lines: typically 1 msec per 100 miles of cable

6. Satellite links: 250 to 270 msec, multiple hops can yield even longer delays

7. Serial delay2: from 0.5 msecs for a 128K packet on a 2Mbps line, to 128.6 msecs

8. For a 1024K packet on a 64Kbps line

9. VoIP gateway node: 50 to 100 msec

10. Decompression delay: typically 10 msecs or less

A point to remember is that any end-to-end phone connection can pass through many of the above components several times along the way. Delays introduced in voice traffic path are most pronounced in long distance and wireless telephone calls.2. Serial Delay: the amount of time taken to put a packet on a transmission line. This is dependent on packet size and line speed.

Echo Cancellation -In order to combat the echo phenomenon, an echo canceller is employed. Today's echo cancellers use sophisticated algorithms running on high speed Digital Signal Processors (DSPs) to combat the echo. By sampling the voice signal, the echo canceller can create a model of the echo path which is in turn used to estimate the echo. This estimation is then subtracted from the incoming voice signal allowing normal speech to pass through. This is a very simplistic view, as echo cancellers need to have the capability to sample multiple echoes occurring at different times and at different levels.

Figure 3. Telephone Circuit



Telephone Circuit - Phone A's transmission passes through Hybrid A, through Echo Canceller A, through the Telephone Network, through Echo Canceller B, through Hybrid B to telephone B. A similar path is established in between Phone B and Phone A.When Phone A's transmission reaches Hybrid B, part of the signal is reflected by the hybrid back towards Echo Canceller B and therefore back to Phone A. If echo cancellation is not performed (and the network delay is moderate), the speaker at Phone A will perceive his echo.It is the responsibility of Echo Canceller B to cancel the echo that is induced by Hybrid B. Likewise, it is the responsibility of Echo Canceller A to cancel the echo that is induced by Hybrid A.The terminology Near End and Far End are usually used when referring to an echo canceller. For example, the Far End signal enters echo canceller A and passes through unchanged and is sent out to the hybrid. The hybrid, which is at the Near End with respect to echo canceller A, reflects a portion of the far end signal back towards the echo canceller. The Near End signal received by echo canceller A therefore consists of the sum of Phone A's transmit signal and the echo of the far end induced by Hybrid A.

Echo Tail - A hybrid circuit does not create a brick-wall echo. A brick wall echo refers to one where the response of a far end impulse would be an echoed impulse. Since the hybrid is a circuit, the impulse response of the echo path is of a diffuse nature. The impulse response of the hybrid circuit is referred to as the echo tail. The duration of the echo tail is referred as the tail length. An echo canceller must cancel the entire tail.
To make matters more interesting, it is possible that multiple echo sources can be present within the tail circuit. This situation is referred to as one with multiple echo tails. A good echo canceller will cancel echo due to all the echo sources in the network.

Adaptive FilteringAs each hybrid circuit is slightly different, each echo tail is different as well. Many factors determine the echo path. It is even possible for an echo tail to change while a circuit is active. This could happen when a second telephone extension is taken off-hook in parallel with the first one.Due to these variations in echo tails, it is necessary for an echo canceller to adapt to the tail continuously. Adaptive Filtering is employed within echo cancellers to this end. The adaptive filters should converge quickly, but not so quickly that they might diverge under some conditions.
This is especially important when a circuit is first established. The amount of time it takes the echo canceller to adapt to an echo path is referred to as the "convergence time".

Line Echo Canceller
- Short Tail (Fig. 4)Figure 4. Line Echo Canceller

Network Echo Canceller- Long Tail (Fig. 5)Figure 5. Network Echo Canceller

 

Acoustic -

Acoustic Echo is caused by direct and indirect feedback from speaker to microphone.

 

Support and Quality Assurance

Adaptive Digital supports customers from evaluation through integration, deployment, and throughout the product lifecycle. It is this close relationship with customers in the past that has enabled us to make our canceller robust under a wide variety of real-world conditions. Many echo canceller vendors buy and re-license the underlying technology, and are therefore far less adept at supporting the canceller beyond simple software integration. Our canceller technology has been developed exclusively with in-house expertise and is supported by the same engineers who have worked to develop it over the years. We can therefore provide support at the system level as well as the software level. Adaptive Digital fielded its first echo canceller in 1995. Over the years, we have continually made improvements to both voice quality and efficiency. After having deployed the canceller for so many years and having seen the types of problems that arise under real-world conditions, we have a process in place for analyzing field problems and resolving them whether they are due to algorithm configuration parameters or due to external factors. As a result of this experience, we have incorporated the appropriate user controllable parameters, and we assist customers in adjusting these parameters based upon feedback from the lab or from the field.

• For more information on the Adaptive Digital Echo Canceller algorithm, G.168, one of our products or custom solutions, contact us at 610-825-0182