Page 207 - Asterisk™: The Future of Telephony

Sign-bit Cord-bit Step-bit
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10 Reduced accuracy
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Amplitude 00 11 10 01 00 10 01 11 Highly accurate
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Highly accurate
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Samples
Figure 7-12. Quantized and companded at 5-bit resolution
In a digital audio system (which the modern PSTN arguably is), aliasing always occurs
if frequencies that are greater than one-half the sampling rate are presented to the an-
alog-to-digital (A/D) converter. In PSTN, that includes any audio frequencies above
4,000 Hz (half the sampling rate of 8,000 Hz). This problem is easily corrected by
‡
passing the audio through a low-pass filter before presenting it to the A/D converter. §

‡ A low-pass filter, as its name implies, allows through only frequencies that are lower than its cut-off frequency.
Other types of filters are high-pass filters (which remove low frequencies) and band-pass filters (which filter
out both high and low frequencies).

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