A Time-Varying Transmitter Filter for a High-Speed Modem

This paper contains an analysis of the stability criterion for a simple signal generator for a proposed high-speed voiceband modem. This modem is proposed to be synchronized to a PCM network, and is capable of learning the quantization levels of the A/D converter at the network channel bank, thereby delivering data transparently to the network, as 8-bit words at 125 micron-sec interval. Consequently, data transmission at much higher rates than heretofore may be possible. One important problem for this modem is having to transmit at the network rate of 8 kHz, which is more than twice the bandwidth of the transmission medium due to the presence of the channel bank filters bandlimited to about 250 Hz-3500 Hz, and is against the Nyquist condition. This paper analyzes a solution to this problem which is accomplished by using time-varying filters and transmitting data at an average rate as specified by the Nyquist condition.