Video and signal conversion, part 1

Diversity

There are so many options for converter features, performance and packaging that it is impossible to describe all the functions and details in one article. A quick market survey would indicate that users prefer modular, or glue, products. While they can require time to reconfigure cables and firmware settings, they are easier to upgrade than rack-based products and usually cost less as well. As requirements change for video production and processing, the features for conversion can change as well. Upgrading to better features and functions is a never-ending process; nothing is ever perfect.

Optical converter

The optical converter is usually targeted towards video signals running at 270Mb/s, 1.5Gb/s and 3Gb/s, and soon 6Gb/s and 11.88Gb/s (10G-SDI), but they can support data rates from 15Mb/s to 11.88Gb/s depending on configuration of the internal signal processing blocks. (See Figure 1.)

This is the simplest optical-to-electrical and electrical-to-optical converter. The basic blocks for the electrical-to-optical converter are:

• Laser driver. Electrical digital signals feed the laser driver generating output currents that bias the laser for the correct average power output and modulate the laser to generate the digital 1s and 0s. The video pathological signal has a large DC component, so it is crucial to have a laser driver that supports this unbalanced power pattern. (See Figure 2.)
• Laser or LED. This device converts electrical currents into photons, or light. Optical output power is in direct proportion to the current level. An article covering the detailed physics of this conversion will be published later in Broadcast Engineering.

For the optical-to-electrical converter, the blocks are:

• Photodiode. It receives the photons and converts them to an electrical current output.
• Trans-impedance amplifier (TIA). It converts the photodiode current to a voltage output, which feeds a limiting amplifier. The photodiode current is small; it might only be in the pico-amp range. The TIA converts this current to a voltage and amplifies the signal, but the gain is designed so TIA output is distortion-free. The high-gain limiting amplifier converts the low-level TIA output into digital 1s and 0s.

This processing is necessary to ensure distortion-free operation even with the video pathological signal. To reduce jitter, advanced optical converters integrate reclockers just before the laser driver or right after the limiting amplifier. This reduces jitter in order to achieve the best signal quality and the lowest possible bit error rate.

Diagnostic functions such as jitter measurement, or eye diagram analysis, can be included along with image and signal processing capability. These advanced features are available as options in some SFPs, providing engineers nonintrusive, real-time signal performance monitoring and status at the edge of their signal distribution network. Engineers can view all the signals in the studio and know exactly the point where any failure might occur.