An improved catheter is provided that is particularly useful for simultaneously mapping electrical activity at multiple locations within the heart. In one implementation, inpainting is the vision-based technique selected to augment a conventional signal-processing-based technique. The holder insert assembly includes a flat disc portion, a plurality of end cap members fixedly secured to the disc portion, and a plurality of hollow tubular members having a first end and a second end. Comparison is made between the chord presented to the user and the detected chord to make a determination as to whether at least the root note of the detected chord matches the root note of the presented chord. In still another embodiment, the electrode has a porous carbon layer, wherein the layer is formed of a plurality of particles bound together.