Echo Cancellation in VoIP Applications. Jeffrey B. Holton. Texas Instruments June, 2002

Transcription

1 Echo Cancellation in VoIP Applications Jeffrey B. Holton Texas Instruments June, 2002

2 Abstract Voice-over-IP networks present a significant cost savings over traditional PSTN solutions. However, the technologies involved also introduce new, or at least more complex, challenges. Increased delays inherent to IP networks translate into greater needs placed on echo cancellers. TI s Voice software portfolio incorporates a robust echo canceller that exceeds G.168 standards. This echo canceller will accurately detect up to three instances of signal reflection simultaneously. It will analyze a moving stream of up to 128ms in length. The reflected portion is then perceptively eliminated from the waveform under analysis. What causes echo echo echo? There are two categories of echo. The first type is known as acoustic echo. This is normally associated with handsfree speakerphones. It occurs when sound from the speaker is reflected by a source and reintroduced to the voice path. This source can be the walls of the room in which the speakerphone is placed. It can also be a consequence of a poorly-designed and constructed enclosure. This occurs when there is no adequate shielding and distance between the microphone and speaker. Acoustic echo is not the focus of the echo canceller under discussion in this paper. More relevant to this discussion is what is known as line echo. Line echo is an electrical phenomenon inherent in analog telecommunications. Analog telephones with RJ-11 connections are known as two-wire phones. However, the interface to the central office is four-wire (with separate transmit and receive pairs). The connection between the two is known as a hybrid. At the point where the hybrid is placed on the far end, a reflection often occurs. This is due to an inherent impedance mismatch. The analog data bounces as it is moved from a four-wire line to a two-wire line. This results in the perception of an echo. We call this far-end echo. hybrids PSTN Figure 1--Far end echo occurs at the hybrid on the receiving side Echo can also be generated and detected on the near end. page 2

3 hybrids PSTN Figure 2--Near end echo occurs at the local hybrid The time delay caused by the echo depends on the distance between the phone and the hybrid. One millisecond of echo corresponds roughly to 60 miles. Echo is subjectively considered to be annoying after about 50ms. However, it can be perceived by a trained ear after only 8 to 10ms. Near-end echo is sometimes a negligible, trivial consideration. Far-end echo is rarely so. These depend on the position of the hybrids in relation to the terminating endpoints. Line echo is not an issue for a true IP Phone-to-IP Phone call involving only IPbased traffic. There are no hybrids in such an implementation. However, gateways allow interconnectivity between IP-based telephony and the PSTN. At the point at which the PSTN places the hybrid, echo will occur. It is normally considered the job of the far end gateway to eliminate this echo. Although the IP network has no line echo issues, network delay must also be considered. Echo is more apparent where the turnaround delay is large. Because of network hops, the delay on an IP network can be quite significant. Implementing a robust echo canceller in a VoIP application is imperative. IP Network PSTN IP phone delay gateway analog telephone Figure 3--Echo occurs in an IP application when it interconnects with the PSTN Implementation and Challenges It is necessary for a gateway or endpoint to incorporate a method for removing echo. Referring to Figure XXXX, an echo canceller must perform the following tasks: 1. Analyze the signals (R in [=R out ], S in ) on either side of the hybrid. 2. Determine the inherent signal loss through the hybrid (illustrated by curved arrow). 3. Determine the inherent delay through the network. page 3

4 4. Store the digital signals for the amount of time equivalent to the delay. 5. Invert the stored signals and adjust for the signal loss figure. 6. Combine the inverted, stored signal with the outgoing signal in the Subtractor. Figure 4--Arrangement of an echo canceller and a hybrid In an ideal world, the result will be a clean, echo-free signal. However, one additional piece will help. There is always some residual echo that cannot be removed. Recall that echo is caused by impedance mismatches in an inductance coil. The resistance in the coil is not a linear (nor even a regular) function. Therefore, any estimate in the signal loss will only be valid for a brief instant in time. By the time the coefficient is applied to the echo removal formula, it is in a sense no longer legitimate. Cost is realized primarily in the memory required to store the packets for analysis. The length of time to search for echo determines the number of packets to store. The MIPS overhead for processing the echo is also a factor. MIPS conservation will be dealt with in the following section. A non-linear processor (NLP) effectively eradicates this remaining echo. From a perceptual standpoint, the echo is now removed. A double-talk detector is also useful in echo cancellation. This handles cases in which the users on both ends are speaking simultaneously. Without this function, the quieter speaker will often be erroneously identified as echo. Elimination of the quieter signal is one sign of a poor echo canceller. An inferior echo canceller and a half-duplex application are directly related. Echo canceller discussions often reveal two technologies. The first category, called single-pass filters, is ideally suited for short echo tails of up to 32ms. For anything larger than 32ms, a multi-segment or full-filter implementation is necessary. <=32ms single-segment >32ms multi-segment or full-filtered In general, ideal for local calling Most-suited for long-distance communications page 4

5 The table demonstrates an inherent limitation. From a practical standpoint, implementing an echo canceller requires resources. Each echo canceller will use an amount of memory and an amount of processing cycles. A practical limit emerges at about 32ms for analyzing a single chunk of data. In order to analyze more than 32ms, multiple single-segments will be analyzed. A full-filtered echo analysis will require many single-segment streams. Memory and MIPS usage In multi-segment or full-filtered ECs, MIPS and memory consumption increases dramatically as tail length grows. In order to minimize the consumption of resources, Telogy implements a proprietary MIPS management algorithm. Certain functions are carried out only once over multiple segments of data to be analyzed. (One example would be the delay measurement. Delay should be constant throughout the conversation.) The same concept can be used to minimize memory consumption as well. For competitors who implement no memory or MIPS optimization, consumption of resources increases linearly. Our consumption runs along a curve that will always be below their line. Standards, Testing, and Comparison Testing for compliance with G.168 requires the use of CSS files. These files simulate human speech across several combinations of ethnicity, gender, and intonation. However, they do so in 300ms bursts. Compliance with G.168 therefore can indicate something other than practical usefulness. A subjective test, unfortunately, is still the most significant indicator of a great echo canceller. Echo canceller technology is very complex. Because of that complexity, every echo canceller will fail at some point where others may not. This is why it is essential that our own weaknesses not be revealed to our competition. A competitor could hypothetically construct a demonstration to take advantage of our weaknesses. Likewise, if we know our competitor s weaknesses, we can construct a similar demonstration. One key term in echo canceller comparisons is convergence. No echo canceller works immediately, since the audio data must be analyzed in order to determine the delay in the circuit. The time between connection and the elimination of echo is referred to as convergence time. Given two echo cancellers, the better is usually the one with the lower convergence time. One could construct certain real-world echo scenarios that could break a G.168- compliant EC. Telogy will still identify and eliminate the echo generated in many of these situations. TI s Telogy EC product does comply with the G.168 standard, but is shown in practical trials to be superior to it. Product Integration: Telogy meets Telinnovations Texas Instruments recently acquired the Telinnovations group from Ditech. The Telinnovations echo canceller is normally perceived in the industry as being superior to Telogy s. TI BCG intended to replace the Telogy EC with the Telinnovations product. page 5

6 However, a deeper analysis revealed that the superiority of the Telinnovations EC is conditional. Under certain circumstances, convergence time for the Telinnovations EC is greater than Telogy s. Therefore, a current project is underway to integrate the two products. The result will be a new product without the weaknesses of each of its parents. Testing will take place in Conclusion TI s existing Telogy echo canceller product eliminates echo inherent in copper-wire telephony. It is perceptively effective even at long tail lengths. Memory and MIPS management algorithms are implemented in order to outperform competitive products. Telogy s EC product is compliant with the G.168 standard. It also outperforms the standard in many tests where other ECs fail. The integration of the Telogy and Telinnovations EC products will result in an extremely robust EC product. page 6