In a forensic authenticity examination of an audio recording, being able to prove a recording is an original or some form of copy is of paramount importance. This article reports on a method to detect if a digital recording has been copied using an analogue interface between recording systems. Making a number of initial assumptions about the recorded signals and recording systems, it will be shown that in its simplest form, recorded digital audio data may be considered to be an acoustic signal added to white noise and convolved with the low-pass response of the recording system. This allows an estimate of the magnitude squared transfer function of the low-pass response of the recording system to be made purely from the recording under analysis. The application of a novel transform to the estimate leads to a single parameter descriptor, and when the make and type of recorder are known, this parameter value distinguishes between an original recording and a copy recording.