In this paper we develop a framework for collecting the ENF (electric network frequency) variations in a database, which can be used in digital media authentication and time stamping. The proposed framework allows the recording of the ENF variations and of the hum signal. The discontinuities introduced in the hum signal by the probe used for data acquisition are studied and methods for minimizing these discontinuities are proposed. In the end a method for time stamping audio recordings is used, based on the created ENF database.