8.1.1 SMPTE Requirements and Specifications
SMPTE specifies several key requirements for the Serial Digital Interface to transport digital video over coaxial cables. Such requirements include return loss, AC coupling, and data rate dependency with rise and fall times.
- Return Loss: This specification details how closely the port resembles 75-Ω impedance across a specified frequency band. The LMH1208 features a built-in 75-Ω return-loss network on SDI_OUT1 and SDI_OUT2 to minimize parasitics and improve overall signal integrity.
- AC Coupling: AC-coupling capacitors are required for transporting uncompressed serial data streams with heavy low-frequency content. The use of 4.7-μF, AC-coupling capacitors is recommended to avoid low-frequency DC wander.
- Rise/Fall Time: Output 75-Ω signals are required to meet certain rise and fall timing depending on the data rate. This improves the eye opening observed for the receiving device. The LMH1208 SDI_OUT1 and SDI_OUT2 cable drivers feature programmable edge rate adjustment to meet SMPTE rise and fall time requirements.
TI recommends placing the LMH1208 as close as possible to the 75-Ω BNC ports to meet SMPTE specifications.