flattening of the channel for one parameter does not entirely distort the channel for the other. Automatic equalization for both amplitude and delay are effective, particularly for switched data systems. Such devices are self-adaptive and require a short adaptation period after switching, on the order of <1 sec (Ref. 13). This can be carried out during synchronization. Not only is the modem clock being "averaged" for the new circuit on transmission of a synchronous idle signal, but the self-adaptive equalizers adjust for optimum equalization as well. The major drawback of adaptive equalizers is cost. Figure 10.18 shows typical envelope delay response of a voice channel along with the opposite response of a delay equalizer to flatten the envelope delay characteristics of the voice channel. Typical channel Equalized delay delay characteristic Equalizer delay characteristic Frequency (Hz) Figure 10.18 A delay (phase) equalizer tends to flatten the delay characteristic of a voice channel. (From Ref. 14. Copyright BTL. Reprinted with permission.) 10.9 DIGITAL TRANSMISSION ON AN ANALOG CHANNEL 10.9.7 Data Transmission on the Digital Network Many data users only have analog access to the digital network. In other words, their connectivity to the network is via a subscriber loop to the local serving exchange. It is commonly at this point where the analog channel enters a PCM channel bank and the signal is converted to the standard digital signal. This class of data users will utilize conventional data modems, as described in Section 10.9.5. Other data users will be within some reasonable distance (some hundreds of feet) from a digital network terminal. This may be a PABX SO equipped, providing access to a DSO or E0 line (Chapter 6), where the line rate is 64 kbps, the standard digital voice channel. In some instances in North America the digital line will only provide 56 kbps. The digital network transmission rates are incompatible with standard transmission rates in the data environment. Standard data rates are based on