Which of the following is not useful in digital EEG recordings?

Biocalibration is a process used primarily during analog EEG recordings. It involves calibrating the equipment to ensure accurate measurements of brain activity. In digital EEG recording, biocalibration is often less applicable because digital systems use different methods to ensure signal quality and accuracy.

The other choices you mentioned play significant roles in digital EEG systems. For instance, the system reference helps in subtracting common noise signals from the data, ensuring that the recordings reflect the brain's activity more accurately. The balance of electrode impedances is crucial in both digital and analog systems to minimize noise and distortion in the recordings. Filters are used to remove unwanted frequencies, which is also essential in improving the clarity and usability of EEG data.

Thus, while biocalibration has its importance in specific contexts, it is not as relevant for digital EEG recordings, making it the correct choice in this scenario.