9.3.9.1 Pre-Emphasis/De-Emphasis

The SNx5DP159 provides De-emphasis as a way to compensate for the ISI loss between the TMDS outputs and the receiver it is driving. There are two methods to implement this function. When in pin strapping mode the PRE_SEL pin controls this. The PRE_SEL pin provides –2-dB, or 0-dB de-emphasis, which allows output signal pre-conditioning to offset interconnect losses from the SNx5DP159 device outputs to a TMDS receiver. TI recommends setting PRE_SEL at 0 dB while connecting to a receiver through a short PCB route. When pulled to ground with a 65-kΩ resistor –2-dB can be realized, see Figure 9. When using I²C, Reg0Ch[1:0] is used to make these adjustments.

As there are times true pre-emphasis may be the best solution there are two ways to accomplish this. If pin strapping is being use the best method is to reduce the Vsadj resistor value increasing the V_OD and then pulling the PRE_SEL pin to ground using the 65-kΩ resistor, see Figure 28. If using I²C this can be accomplished using two methods. First is similar to pin strapping by adjusting the Vsadj resistor value and then implementing –2-dB de-emphasis. Second method is to set Reg0Ch[7:5] = 011 and the set Reg0Ch[1:0] = 01 which accomplishes the same pre-emphasis setting. See Figure 29.

Figure 28. Pre-Emphasis Using Pin Strapping Method

Figure 29. Pre-Emphasis Using I²C Method