Motorola developed Digital Coded Squelch (their Digital Private Line or DPL) in band subaudible selective squelch system. The idea was to allow more users share a radio channel and reduce co-channel interference from distant users.
DPL used a 132 baud low speed data signal that was added to the voice frequency, and when receiving stations found a match in the code they were receiving and their own code, their loudspeaker opened. For the most part radios using Digital or Tone coded squelch were the same except the digital signal needed much better low frequency response to handle the components of the digital squelch wave form. Much like DSTAR, radio servicing stayed the same. Frequency generation and off frequency operation remained the same until synthesized radios came of being. Rf and IF amps were the same given the operating bandwidths involved. The only big difference was testing the squelch system. Much applies to a DSTAR radio. You can still send an FM audio signal into the receiver and recover the audio at the discriminator. You can still look at the SINAD up to the discriminator. You can key the transmitter, measure power, and look at the modulation waveforms and transmitter frequency deviation. And, sadly to say, if there is a problem in the radio, you will have to send it to a depot. Digital or analog, because much of the density of the circuitry and miniaturization of components don't allow for field repairs. I am sure there are some with microscopes and hot air work stations, but most hams will box and ship, as do most commercial users. Regardless of digital or analog. 73 de steve nu5d -- "It is a poverty to decide that a child must die so that you may live as you wish." Mother Teresa
