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I was chatting to my boss today and he said:

My first job was electrical working in a factory for a modem company. The last model modem we made was the 1200 baud. After that they needed programmable logic and went to VLSI chips. We didn't have the scale for that and so we had to close down.

My question is: What triggered the need for VLSI chips in modems post 1200 baud?

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    \$\begingroup\$ They got more complicated? \$\endgroup\$
    – Andrew
    Feb 6, 2017 at 9:57
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    \$\begingroup\$ Exceeding 1 bit per baud required advanced techniques like adaptive line equalisation, which required DSP. I saw a paper from Queens University of Belfast, implementing a 9600 Baud modem on a Motorola DSP56000. \$\endgroup\$
    – user16324
    Feb 6, 2017 at 10:08
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    \$\begingroup\$ Thanks @BrianDrummond - could you expand that into an answer? \$\endgroup\$
    – hawkeye
    Feb 6, 2017 at 10:42
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    \$\begingroup\$ One thing that really made my life difficult with a 600 bps fsk modern was the need then (40 years ago) for a 10 pole receive filter and a 6 pole transmit filter. Getting to 1200 on voice grade lines required two bits per baud and that, added to the rest, was very close to enough to break the camel's figurative back. \$\endgroup\$
    – jonk
    Feb 6, 2017 at 12:50
  • \$\begingroup\$ Cost is not insignificant. If your volumes are in the millions, ASICs are cheaper than discreets if your product can live a few years with no or minimal changes. \$\endgroup\$
    – winny
    Feb 6, 2017 at 18:47

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The higher data rates require the modem to understand, to emit test patterns and characterize, the phone line. That requires, essentially, a computer, a DAC, a ADC. The complete system will digitize, perform Fourier Transform, and examine frequency bin Magnitude&Phase. The modem became an embedded system.

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