The telephone system was completely analog at its beginning in the nineteenth century, of course. Over the past 30 years or so, it has gradually been converted to digital technology and the process is not yet complete.
The digital techniques were originally designed to work in conjunction with the existing analog system. This, and the fact that many of the standards for digital technology are quite old, should help the reader understand some of the rather peculiar ways things are done in digital telephony.
We can summarize the results here as they apply to the North American system. The numbers vary slightly in some other countries but the principles are the same.
The analog voice signal is low-pass filtered at about 3.4 kHz and then digitized, using 8-bit samples at a sampling rate of 8 kHz. The signal is compressed, either before or after digitization, to improve its signal-to-noise ratio.
The bit rate for one voice signal is then ƒb(voice) = 8 bits/sample × 8000 samples/second = 64 kb/s. The sample rate is determined by the maximum frequency to be transmitted, which was chosen for compatibility with existing analog FDM transmission (which uses SSBSC AM with a bandwidth of 4 kHz per channel, including guardbands between channels).
An upper frequency limit of about 3.4 kHz has long been considered adequate for voice transmission. A much lower bit rate could be used for telephone-quality voice using data compression and vocoders.
These techniques are not employed in the ordinary telephone system, though data compression is used in special situations where bandwidth is limited and expensive, as in intercontinental undersea cables.
The connection of wireless systems to the PSTN requires conversion of standards in both directions. In general, wireless systems use ordinary telephone quality as a guide, though many of these systems fall short of what has been considered toll quality, that is, good enough to charge long-distance rates for.
Until now, users have been so delighted to have portable telephones that they have been willing to put up with lower quality. This situation is changing quickly now that wireless phones are everywhere.
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