Stimmen - 3, Durchschnittliche Bewertung: 4.3 ( )

Hersteller: Polycom Dateigröße: 75.37 kb Dateiname: 241757ef-9bc1-9d44-c170-4e78ba0802ee.pdf Unterrichtssprache:

• AEC for medium to large rooms - these products may contain microphone inputs and record inputs / outputs in addition to the standard audio inputs and outputs required for AEC operation. • AEC for videoconferencing - these products may contain multiple inputs and outputs, or incorporate “phone add” modules to permit the addition of a 2-wire conference (telephone call into the videoconference). Step 2: Eliminate the products that don’t meet G.167 or the tail length needs of the application. It is relatively easy to determine how well an AEC cancels echoes. Most AEC products are based on the same algorithm: the adaptive LMS digital filter. This is a very well-defined algorithm that has been used for years. Since this process is well established, it is fairly easy to determine whether a manufacturer has done an adequate job of implementing it. The performance of the AEC can basically be judged by two criteria. • First, the product must be compatible with the ITU G.167 recommendation for AEC. • Second, the AEC must have an adequate tail length for the environment it is to be used in. Although these criteria are necessary, they are not sufficient to determine whether an AEC is good enough. There will most likely be several AEC solutions that meet these specifications. These are the specifications that can be compared on paper. What remains are the characteristics that can only be evaluated by comparative listening, and will make the most difference in how an AEC sounds. G.167 Compliance or Compatibility The ITU G.167 Recommendation for Acoustic Echo Controllers gives criteria for a number of performance characteristics typically listed on manufacturers’ data sheets. These include such specifications as initial convergence time (or rate of convergence), amount of cancellation, and bandwidth. G.167 compliance is a good indication that the LMS algorithm (the actual echo canceling filter) has been implemented reasonably well. It also means that the manufacturer has subjected the product to a series of standard tests, and that the specifications are most likely based on valid experimental data. This makes the selection process easier, because it sums up many different characteristics. Products can be eliminated easily based on G.167 compliance, rather than by evaluating each performance characteristic individually. When an echo canceller is G.167 compliant, the following specifications commonly found on data sheets have met the requirements of the standard in the room in which the echo canceller was tested: • Bandwidth • Weighted Terminal Coupling Loss (or total cancellation) • Initial Convergence Time (or convergence rate) • Recovery Time After Echo Path Variation Since most of the specifications found on data sheets are covered by G.167, it is not important to consider each of these specifications in detail. The manufacturer’s equipment should have already been verified to meet the requirements of the standard. If the product exceeds any of the requirements, this may improve the audio quality to some degree. This improvement, however, will not be as significant as the effects of the tail length and state machine. Therefore, all G.167 compliant devices should be considered equally until the other factors have been evaluated. Room Acoustics Because of differences in room acoustics, a device may be G.167 compliant in one room and not another! G.167 testing is performed in real rooms. If the product meets the requirements in these rooms, it is compliant. A device that is G.167 compliant in one room, however, might not be compliant in another. This is because the acoustics of all rooms are different. This flexibility allows manufacturers to test their products in the types of rooms they were designed for and claim compliance. However, this also means that the customer has the responsibility of determining whether the AEC will operate in his or her particular environment. An AEC solution that was designed to operate in an office may not work properly in a conference room. If an echo canceller were compliant in one room and not another, it would most likely be due to a tail length that was too short for the second room. Since tail length is not specified by G.167, it must be evaluated separately. Tail Length The tail length of an AEC is the length of time over which it can cancel echoes. The tail length of the echo canceller should meet the requirements of the room it is to be operated in. This is directly related to the reverberation time of the room. As the room reverberation time increases, a longer tail length will be needed in that room. If the reverberation time is much longer than the tail length, a significant amount of the echo will remain audible. However, excess tail length will not improve or degrade the performance of the canceller. Determine the minimum tail length requirements for an application based on the typical acoustics of rooms the product will be used in. Any products that do not meet or exceed tha...

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