G.168 ECHO CANCELLER - LINE, NETWORK, & PACKET ECHO CANCELLATION

Adaptive Digital's high performance G.168 compliant echo canceller: Toll-quality echo cancellation provided for configurable tail lengths of 16ms (128 taps), 32ms (256 taps), 64ms (512 taps), and 128ms (1024 taps) on a per channel basis. G.168 Plus™Packet handles round trip delays up to 512 msecs.

Adaptive Digital's G.168 Echo Canceller is certified as
   "Toll-quality" by AT&T Labs
      Line, Network, Acoustic, & Packet



Adaptive Digital’s G.168 echo cancellation algorithm comes in three variants, each of which is designed to suit a specific type of application. The three variants are Line echo cancellation (EC), Network EC, and Packet EC.

Line Echo Canceller
The Line EC (LEC) is designed to operate inside a device that has a local hybrid circuit. The hybrid circuit is the circuit that converts between a4-wire and a 2-wire interface. Hybrid circuits include:SLIC (Subscriber Line Interface Circuit)FXO (Foreign Exchange – Office)FXS (Foreign Exchange – Subscriber)DAA (Data Access Arrangment)Examples of equipment that need a line echo canceller include:
  • •   ATA
  • •   VoIP Gateway with FXO and/or FXS interfaces

The significance of the local hybrid is that there is little delay between the hybrid, the source of the echo, and the echo canceller.

Network Echo Canceller
The Network Echo Canceller (NEC) is designed to operate in a piece of network equipment that may not have an internal echo source. The echo source may be elsewhere in the network. Examples of equipment that may need an echo canceller include:
  • •   VoIP Gateway with digital trunks
  • •   T1/E1 echo canceller

Since the echo source is elsewhere in the network, the network echo canceller must be able to deal with longer echo delays than the line echo canceller. It must also be able to handle signaling tones that are found in the telephone network. It may also need to cancel reflections caused by more than one hybrid circuit in the network. Finally, the network echo must be able to deal with echo-free circuits or circuits in which the echo is already cancelled at the source.

Packet Echo Canceller
The conventional wisdom tells us that an echo canceller should be as “close to” the echo source (hybrid) as possible. For a variety of reasons, this makes the echo canceller perform better. But the introduction of the Voice –Over-IP network, the conventional wisdom has been turned on its side. The added delay incurred by the packet network can cause echo that is not otherwise perceptible to become a problem. This may include not only echo due to hybrid circuits but also some residual acoustic echo from handsets or hands-free devices. As a result, there could be equipment in the field that did not require echo cancellation before VoIP entered the picture. Since it is not feasible to modify all such equipment, it is sometimes necessary for an echo canceller in a packet network to cancel echo whose source is on the opposite end of the packet network. A packet network echo canceller must be able to handle excessively long echo delays. It must also be able to operate well whether there is echo or not. In fact, it is far more common that a connection be echo free because the echo is cancelled at the source. Furthermore, a packet echo canceller must be able to deal with the presence of speech compression algorithms within the VoIP network. Examples of equipment that may need a packet echo canceller include:
  • •   VoIP Gateway
  • •   VoIP Conference Server

AVAILABILITY

ADT G.168 Echo Canceller is available on the TMS320™ DSP Family

C54x™DSP Generation: Line and Network EC

C55x™DSP Generation: Line, Network and Packet EC

C64x™DSP Generation: Line, Network and Packet EC

C64x+™DSP Generation: Line, Network and Packet EC

C62x™DSP – Short Tail (LEC) Only

   C67x™DSP, C67x+™DSP, C647x™DSP – Please inquire

   PC/Windows – Please inquire

 Linux/x86

ARM7, ARM9

DATA SHEET .pdf

ECHO CANCELLER FEATURE COMPARISONS

Adaptive Digital’s echo cancellation software is offered in three variants, which are compared by feature set in the following table.

Feature G.168 LEC G.168 NEC G.168+ Packet EC
Maximum Tail Length 16 msec 128 msec 512 msec
Tandem Free Operation -- X X
High Efficiency Tandem Free Operation -- -- X
Packet Loss Concealment -- -- X
PCM Slip Correction -- X X
Dynamic NLP X X X
Comfort Noise Generator X X X
Rapid Convergence X X X
Automatic Tail Search -- X X
Cancels multiple reflectors X X X
Convergence Monitor X X X
Tone Disabler X X X
SS7 Tone Detection -- X X
Double-talk Detection X X X
Stationary Signal Detector X X X
Pre-NLP Signal Available X X X
Split Pre- and Post- Processing X X X
Handles Residual Acoustic Echo X X X
eXpress DSP™ compliant X X X
Functions are “C” callable X X X
User Configurable Parameters X X X

SPECIFICATIONS

C54x

All Memory usage is given in units of 16-bit word.

Tail Length
MIPs (Peak)
Program Memory
Data Memory
Per-Channel Data Memory
8 msec
2.7
4399
413
433
16 msec
3.5
4399
477
629
32 msec
5.0
4399
605
1021
64 msec
6.0
5728
920
1620
128 msec
6.6
5728
1528
2484

NOTE: Characterized with PCM Slip Correction disabled.

C55x

All Memory usage is given in units of byte.

Tail Length
MIPs (Peak)
Program Memory
Data Memory
Per-Channel Data Memory
8 msec
3.8
9833
1406
968
16 msec
4.7
9833
1662
1376
32 msec
6.3
9833
2174
2192
64 msec
6.6
12663
3594
3266
128 msec
7.4
12663
5642
5122
*256 msec
8.2
13686
9738
8834
*384 msec
10.4
13686
13834
12546
*512 msec
12.4
13686
17930
16258

* NOTE: Tail lengths of 256, 384, & 512 msec. are only supported in the G.168Plus™ Packet EC.


C62x

All Memory usage is given in units of byte.

Tail Length
MIPs (Peak)
Program Memory
Data Memory
Per-Channel Data Memory
16 msec
3.4
17321
852
1332
32 msec
4.2
17321
1108
2114

C64x

All Memory usage is given in units of byte.

Tail Length
MIPs (Peak)
Program Memory
Data Memory
Per-Channel Data Memory
16 msec
3.5
24640
1330
1574
32 msec
4.1
24640
1736
2628
64 msec
4.5
31296
2344
3556
128 msec
4.7
31296
 3560 5412
*256 msec tail 5.1 31264
5992
9124
*384 msec tail 5.6 31264
8424
12836
*512 msec tail 6.2 31264
10856
16548

* NOTE: Tail lengths of 256, 384, & 512 msec. are only supported in the G.168Plus™ Packet EC.

C64x+

Tail Length
MIPs (Peak)
Program Memory
Data Memory
Per-Channel Data Memory
C64x+ - Tail Lengths of 256, 384, & 512 msecs are supported only in the G.168 Packet EC.
16 msec
3.4
23296
1330
1574
32 msec
4.0
23296
1736
2628
64 msec
4.4
29280
2344
3556
128 msec 4.6 29280 3560 5412
*256 msec
4.3
29280
5992
9124
*384 msec
4.7
29280
8424
12836
*512 msec
5.1
29280
10856
16548

* NOTE: Tail lengths of 256, 384, & 512 msec. are only supported in the G.168Plus™ Packet EC.

Linux x86

CPU Utilization

Varient
Platform
Average MIPS*
Peak MIPS
LEC
32 bit
--
72
LEC
64 bit
--
28
NEC
32 bit
--
124
NEC
64 bit
--
48
PEC
32 bit
19
148
PEC
64 bit
8
60

* PEC average MIPS characterized with smart packet optimization set to maximum on echo-free circuit

FUNCTION

LEC_ADT_g168Init(…) Initializes echo canceller channel

LEC_ADT_g168Cancel(…) Executes cancellation function

LEC_ADT_g168echoCancel(..) Executes pre-NLP function (split API)

LEC_ADT_g168postCancel(..) Executes NLP function(split API)

LEC_ADT_g168RuntimeConfig Changes modifiable EC parameters

LEC_ADT_g168GetConfig(…) Gets parameters and status of the EC

LEC_ADT_move( ) Inform EC that it’s data has moved. This is used for memory optimization.


About Adaptive Digital's G.168 Echo Cancellation Algorithm

The Adaptive Digital Technologies patented, industry standard, carrier class G.168 echo canceller is available on many DSP and general purpose processors. Adaptive Digital’s echo canceller qualified as toll-quality at AT&T’s Voice Quality Assessment Labs in Middleton, NJ. AT&T’s Voice Quality Assessment Lab evaluated Adaptive Digital’s echo canceller using its stringent series of performance tests including AT&T’s Mean Opinion Score (MOS) subjective tests as well as the standardized set of G.168 objective tests. The subjective and objective performance of Adaptive Digital’s echo canceller surpassed even the performance of AT&T’s benchmark lab echo cancellers.

The importance of subjective testing should not be overlooked. Although the ITU’s G.168 recommendation is well thought out, this type of objective test is no guarantee that an echo canceller will sound good to a human listener. By doing both subjective (MOS) and objective (G.168) testing, Adaptive Digital ensures the optimum voice quality, which is, after all, what it’s all about!

Another important reason for solid subjective performance is that G.168 is not a bit-exact specification. When an algorithm is specified in a bit-exact way, every compliant implementation will sound identical. This is the case for most of the standardized vocoders. In contrast, G.168 specifies test conditions, excitations, and required minimum output performance. As a result, the voice quality achieved by different echo cancellers varies greatly. Beyond all the lab testing that can and should be done, there is no substitute for lessons learned through years of real-world deployment.

The overall importance of echo cancellation should also not be overlooked. From the users perspective, echo is arguably the worst type of impairment that can be encountered during a telephone conversation. Typically, if a circuit has echo, the two parties agree to hang up and dial again because it is so difficult to speak in the presence of one’s own echo.

The moral of the story is – use a top-notch echo canceller. A reputation for good voice quality is difficult to achieve. A reputation for bad voice quality is easy to achieve and difficult to overcome.

1. Overview

• 1.1 Echo Tail

• 1.2 Bulk Delay

• 1.3 Echo Cancellation

• 1.4 Tandem Free Operation

2. Additional Requirements

3. Features

4. Availability

5. Specifications

ECHO CANCELLATION OVERVIEW

Echo in the telephone network is caused by hybrid circuits that convert between two-wire analog interfaces and four-wire analog interfaces, as seen in figure 1

At the four-wire side of the hybrid, one pair of wires carries voice signals toward the hybrid (sometime called the receive path), and a second pair of wires carries voice signals away from the hybrid (sometimes called the send path). On the two-wire side of the hybrid, a single pair of wires carries voice signals in both directions. The echo comes about because hybrid circuits are not perfectly matched. As a result, some of the four-wire receive signal is leaked back into the four-wire send signal.

We will refer to the person speaking at the four-wire side as the far end speaker, and the person speaking at the two-wire side as the near end speaker.

ECHO TAIL

If we characterize a hybrid in terms of its impulse response, we see that the impulse response tends to be non-zero for a few milliseconds, but can be as long as 16 milliseconds. The impulse response is often referred to as the echo tail, and the duration of the echo tail is often referred to as the tail length. The tail length of the echo tail shown in figure 2 is 8 millseconds.

There are situations where the echo canceller is not located at the same location as the hybrid. It may be separated by one or more T-1 links or other types of links that cause the tail length appear to be even longer from the point of view of the echo canceller. Furthermore, there may be more than one hybrid in the path – resulting in an overall impulse response that is the concatenation of multiple impulse responses. In this case, each tail is sometimes referred to as a reflector, and the situation where there are multiple hybrids in a circuit is referred to as one where there are multiple reflectors.

 

BULK DELAY
There rare situations where an echo canceller may be on the opposite side of a VoIP or satellite link from the hybrid. In this case, the there is considerable delay between the echo canceller and the hybrid in both directions. In this case, the echo tail appears to begin with a segment of zeros followed by the hybrid impulse response as shown in figure 2. The duration of the segment of zeros is referred to as the bulk delay.

There are two ways to handle bulk delay. One is to place an artificial delay into the far end input to the echo canceller to effectively remove the bulk delay from the point of view of the echo canceller. This technique falls short for two reasons. First of all, it requires a-priori knowledge about the amount of bulk delay that will be encountered. Second, it does not allow for the situation where there may be a local reflector and a remote reflector. Because of both of these reasons, it may be preferred to use a second technique in which the entire possible delay window is analyzed, and any reflectors within that window are cancelled.

 

ECHO CANCELLATION

The primary job of an echo canceller is to remove the echo of the receive path that has bled through the hybrid into the send path. This is done by modeling the echo path with an adaptive filter, using that adaptive filter to predict the echo, and subtracting the predicted echo from the send signal. This is shown in figure 3.

But, as we will explain, there is far more to an echo canceller than meets the eye for a number of reasons.

 

The ideal situation for an echo canceller to model the echo tail is when the far end speaker is speaking and the near end speaker is silent. This condition is referred to as single-talk. The reason this is ideal is that the receive signal is used as a reference signal for the echo canceller for comparison with the send signal. If both the far end speaker and the near end speaker are speaking at the same time (a condition known as double-talk condition), the near end speech will be added to the echo thereby making it more difficult for the echo canceller to compare it to the reference. In fact, a double-talk condition can cause an echo canceller’s adaptive filter to diverge. In order to prevent this from happening, echo cancellers employ double-talk detectors. When double-talk is detected, the echo canceller temporarily stops adapting it’s filter to prevent divergence.

Echo cancellers use adaptive filters that do not perfectly model the echo tail. As a result, the echo cancellers have to perform post-processing (Nonlinear Processing or NLP) to remove the residual echo caused by the imperfections. Similarly, the echo path may have some nonlinearities that the adaptive filter cannot perfectly match. The NLP is intended to take care of this also.

The NLP has to be designed carefully in order to minimize unwanted effects. The nonlinear processor is free to suppress residual echo at will in a single-talk situation. But if it is too aggressive during a double-talk situation it may suppress the near end speech – an undesirable effect.

Another more subtle effect of an NLP occurs when there is background noise at the near end. This background noise will be heard by the far end speaker when the NLP is not engaged, but the noise will be suppressed when the NLP is engaged. The appearance and disappearance of the background noise can be annoying. In order to take care of this, an echo canceller will often replace the residual signal with comfort noise while the NLP is engaged rather than blindly suppressing the signal. When an echo canceller has this feature, it is referred to as a comfort noise generator (CNG).

An overly aggressive NLP may also cause DTMF digits to be partially clipped (in duration). The possible consequence is that a DTMF detector may not detect a tone that should have otherwise been detected. In situations where an application needs an aggressive NLP, this problem can be circumvented by outputting both the pre-NLP and post-NLP signals. The DTMF detector can be fed the pre-NLP signal.

Beyond nonlinearities in the echo path, there can be other impairments that an echo canceller may need to deal with.

In the echo path such as PCM slips, packet loss (in a packet network), and residual acoustic echo from a hands-free phone. Ideally, an echo canceller should do its best to mitigate the effects of these impairments as well.

TANDEM FREE OPERATION

Yet another scenario that an echo should deal with is the one where there is another echo canceller in a circuit that is closer to the hybrid than ours. In this case, the other echo canceller should remove the echo leaving our echo canceller the appearance that there is no hybrid in the circuit. Under these circumstances, an echo canceller could actually create echo rather than leave the echo-free signal alone. Being able to handle this situation properly is referred to as tandem free operation. A similar situation is one in which an echo canceller is placed on a circuit that does not have a hybrid.

 

ADDITIONAL ECHO CANCELLER REQUIREMENT

The telephone system carries more than just voice signals. It can carry fax and modem signals, signaling tones (such as DTMF tones and inter-office signaling tones). Passing these tones properly imposes additional constraints on an echo canceller. For example, an echo canceller must detect the presence of certain modems by identifying their answer tones (as specified by ITU G.165, V.8). When these modems are present, the echo canceller must disable itself for the duration of the modem connection. Similarly, when certain interoffice signaling tones known as SS7 tones are present, the echo canceller must disable itself temporarily. This feature is known as a tone disabler.

For full data sheet download .pdf

The Asterisk® VOIP Systems - HPEC is upon Adaptive Digital's G.168 EC algorithm. By incorporating Adaptive Digital's field-proven echo cancellation technology, Digium brings better and more robust voice quality to its worldwide customer base. The High Performance Echo Cancellation (HPEC) is carrier-grade and provides host-based Toll Quality echo cancellation on a global scale.

"As Asterisk continues its rapid rate of deployment on a wide range of telephone equipment from vendors around the world, Digium is committed to delivering our customers the absolute best quality possible," said Mark Spencer, CTO of Digium. "Digium’s new high performance echo cancellation solutions once again set the industry standard for open source, carrier-class voice networks."

 

 

 

 

PRODUCTS