 (or any symmetrical passband, for that matter), delay increases toward band edge and is minimum about band center (around 1800 Hz). As the bit rate is increased on the channel, there is more and more opportunity for the delayed energy from bit 1 to spill into the principal energy component of bit 2. When it does, we call it ISI (intersymbol interference). 8 Angular frequency is 2 X frequency in hertz. It is sometimes called the radian frequency. 10.9 DIGITAL TRANSMISSION ON AN ANALOG CHANNEL Figure 10.10 Typical differential delay across a voice channel, with FDM equipment back-to-back. "Back-to-back" means that the transmit and receive portions of the equipment are placed on a test bench and appropriately interconnected. Phase or delay distortion is the major limitation of modulation rate on the voice channel. The shorter the pulse width (the width or period of 1 bit in binary systems), the more critical will be the EDD parameters. As we discuss in Section 10.9.5, it is desirable to keep delay distortion in the band of interest below the period of 1 baud. 10.9.3.2 Attenuation Distortion (Frequency Response). Another parameter that seriously affects the transmission of data and can place definite limits on the modulation rate is attenuation distortion. Ideally, all frequencies across the passband of a channel of interest should undergo the same loss or attenuation. For example, let a - 10-dBm signal enter a channel at any given frequency between 300 and 3400 Hz. If the channel has 13 dB of flat attenuation, we would expect an output at the distant end of -23 dBm at any and all frequencies in the band. This type of channel is ideal but unrealistic in a real working system. In Rec. G.132 (Ref. 9), the CCITT recommends no more than 9 dB of attenuation distortion relative to 800 Hz between 400 Hz and 3000 Hz. This figure, 9 dB, describes the maximum variation that may be expected from the reference level at 800 Hz. This variation of amplitude response is often called attenuation distortion. A conditioned chan- nel, 