| 摘要 |
<p>A conversion system using a lookup table of adjustement values for converting non-linear, logarithmic pcm codes between attenuated and unattenuated formats. A computer accesses attenuated pcm codes from a compression device, such as that found in the transmit portion of a digital speakerphone, and retrieves an adjustment value from the lookup table using the measured pcm code and a gain code. The gain code corresponds to a gain setting asserted by the computer to a gain device in the transmit portion of the speakerphone. The lookup table is preferably implemented in hardware, such as a ROM, RAM, PLA or similar type device, or is implemented in software or firmware executed by the computer. The retrieved adjustement value is added to the attenuated pcm code to determine the corresponding unattenuated pcm code. The present invention is also applicable for the reverse process for calculating attenuated pcm codes from the corresponding unattenuated pcm codes, where a separate lookup table is used for the reverse process. The reverse process is used, for example, to derive the attenuated pcm code equivalents at the output of a receive gain device in a speakerphone by measuring the unattenuated pcm code received from a digital subscriber loop. The calculated values for the transmit and receive cases are used in equations for purposes of acoustic calibration, switching functions, echo cancellation and the like. The lookup tables are substantially reduced for lower performance applications to determine rough estimates for the calculations.</p> |
