NOTES, RESOURCES, ADDITIONAL INFORMATION | 61
Beyond Edison’s Legacy
The PSTN uses the g.711 codec. Its audio frequency response is limited to 3.4kHz. A modern
reader might ask, “Why was such a low fidelity considered to be satisfactory for digital
telephony?” Microphones, loudspeakers, and earphones have all had much better fidelity
for many decades. Indeed, when the g.711 codec was standardized in the 1960s, FM radio
was just getting going with its much superior 15kHz bandwidth. Mostly, the coding choice
resulted from the legacy of the carbon button microphones that were ubiquitous in telephones
throughout most of the early history of telephony. (Edison invented the carbon microphone
and licensed it to the Bell System, which had only an impractical liquid-based microphone in
its own portfolio.) This microphone works on the principle that acoustic pressure squeezes the
carbon granules more closely together, thus lowering electrical resistance in proportion to the
pressure. As you can imagine, it has a non-flat response curve with a 5db peak at 2kHz and
hefty roll-offs below 300Hz and above 3kHz. They were mostly abandoned for all but telephone
use in the late 1920s, but were standard in phones up to the 1980s. Old analog long-distance
lines also had a lot of high-frequency attenuation owing to capacitive effects. When microwave
radios were introduced to long-distance telephony, a decision had to be made as to what
frequency range to accommodate in their FDM (Frequency Division Multiplex) scheme. There
was, as always, a trade-off: more frequency response meant fewer channels. Since the micro-
phones weren’t producing much in the way of high frequencies, why bother carrying them
over the radio links? Thus, the radios were designed with narrow 4kHz carrier spacing. When
the first digital T-Carrier systems where invented, it must have seemed perfectly natural to stay
with the 3kHz or so audio bandwidth enshrined in the microwave link technology. A sampling
rate of 8kHz with 8-bits (compressed) depth had a nice symmetry and delivered a satisfactory
4kHz Nyquist response limit , so on with the show.
The Electret capacitor microphones ubiquitous in today’s phones have frequency response
well beyond 3kHz. IP networking combined with modern codecs allow the better fidelity to
be transported. Some mobile phone applications, such as Apple’s ‘facetime’ are beginning to
take advantage of this. The VX is ready for high-fidelity phone calls, and includes support for
g.722, a codec rapidly gaining favor with VoIP providers and users. g.722 uses about the same
amount of bandwidth as g.711, but samples audio at 16kHz - double that of g.711! It delivers
very clean, clear, natural caller audio. Future VX software releases are likely to includes support
for more high fidelity codecs.
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