EndOfPOTS.jpgLast week Spectrum Magazine interviewed Daniel Beringer about an initiative to end traditional landline telephones by June 2018.  To those of us for whom the public switched telephone network (PSTN), which often goes by the nickname plain old telephone service (POTS), was once the primary means of communication, this feels like a radical change. 


One advantage to an all-VoIP system is audio quality could be improved.  A POTS line runs at a constant data rate of 8k samples/sec * 8 bits/sample.  POTS encoding schemes, mu-law in the US and A-law in Europe, have smaller increments at lower signal levels to reduce quantization noise.  This is an improvement over a linear ADC, but it's nothing compared to modern audio compression.  The audio must be band-limited to prevent aliasing.  A POTS line only passes 300-3300Hz. 


The Audible Spectrum and Human Speech

Human hearing covers 20Hz to 20,000Hz, although the high end degrades with age.  Hearing sensitivity peaks at just over 3kHz, which is the primary frequency in a baby’s cry. 


The fundamental frequency produced by someone’s vocal cords determines the pitch the voice.  Voice pitch is typically 50Hz to 250Hz for men and 120Hz to 500Hz for women.  Note that a typical man’s voice fundamental falls outside the range of a POTS connection.  The human ear detects only harmonics of a male voice and perceives hearing the low pitch that is actually not present. 


The human vocal system produces speech phonemes by varying the vocal tract to create resonances that change the higher frequency components.  The spectral peaks in the 250 to 3500Hz range determine vowel sounds.  These peaks  fall mostly under 3300Hz, so we are able to distinguish them on the telephone. 


Spectral components above 3300Hz provide a richness to voice that isn’t conveyed over a POTS line. 


The English sounds /s/ and /f/ are similar to Gaussian noise, with a constant amplitude in the frequency domain.  These sound are hard to distinguish on a band-limited connection.


All of this means a POTS line is good enough, but more bandwidth and lower noise would improve intelligibility.


Why are we still using POTS?

I asked Daniel Beringer why we’re still using POTS.  He explained that even calls that originate from a VoIP line must be switched through a phone switching circuit.  Governments control the switching between phone companies, and no one can agree on an upgrade path. 


With modern compression, you could transmit “high-definition telephone”, with a spectrum from 70Hz to 7000Hz, using 13.5kbps of data.  Lack of bandwidth is not a reason for staying with POTS. 


Roadblocks to Upgrading

Phone companies could install equipment that allows legacy phones to be used with an all VoIP network.  It seems like the transition will cause some minimal frustration for legacy users in urban areas.  I asked Mr. Beringer about rural users.  He says there are gaps and rural areas, but they will be filled in by 2018.  My understanding is rural users will have an IP connection in their house, not an analog connection to a phone station with VoIP.  This seems hard to believe, but maybe rural data networks are being built faster than I realize. 


In internet searches for roadblocks, I found quite a few conspiracy theories.  One example is that government conspirators want to shut down the internet and they want to prevent BBS use too.  Presumably the POTS, whose switching connections are still controlled by governments, is beyond their ability to control without a sneaky VoIP initiative.  I only mention this because it shows how ingrained the POTS is in our minds.  On some emotional level I can understand the reaction of “this must be a conspiracy.”