than "Port Activity" detection. Fewer than 0.5% of onsets/hour (2.5 per hour) for modest noise condition (-45 dBm) and fewer than 2% (10 per hour) for high noise condition (-35 dBm) have perceptible clipping. 3. False detection performance (i.e., detecting noise as "speaker active") exceeds those of "port activity" due to tSUSTAIN criteria. No more than 1% (36 seconds per hour) of noise segments is misclassified as "sustained" for modest noise conditions, and no more than 2% (72 seconds per hour) of "silence" is detected as port active for noisy conditions. "Port active" and "Speaker active" detection in the presence of echo: 1. Less than 1% of residual echo is detected as "port active" - (i.e. 36 sec. per hour) during normal operation of canceller. 2. Less than 0.1 % (i.e. 3.6 sec per hour) of residual echo is detected as "speaker active" or "port sustained" during normal operation of canceller. 3. Clipping of input speech in the presence of echo is no higher than clipping in the presence of modest to high level of noise. Software License D2 Technologies, Inc. and Maintenance Agreement August 4, 1997 CONFIDENTIAL 22 A.3 DTMF Detection A.3.1 Functional requirements: Table A-1 specifies the nominal frequencies for the DTMF digits that must be detected. -------------------------------------------------------------- Nominal High Group Frequencies (Hz) 1209 1336 1477 1633 -------------------------------------------------------------- Nominal 697 1 2 3 A Low Group 770 4 5 6 B Frequencies 852 7 8 9 C (Hz) 941 * 0 # D -------------------------------------------------------------- Table A-1: Nominal DTMF Frequencies 1. Detect the presence of all 16 DTMF digits that are produced by different phones on the market under a broad range of network conditions. 2. DTMF digit information is provided as soon as the minimum duration is met. This information is called leading edge detection. This allows the