Method of anti-spoofing

A method of training an encoder as part of an autoencoder to identify the authenticity of sub-periods of an audio or video signal, the method comprising:

• providing a set of training signals, each training signal having a plurality of sub-periods associated with a respective plurality of labels of whether the sub-period is real or fake, and
• for each training signal, performing the steps of:
• applying a first encoder to the training signal, the first encoder configured to output an n-dimensional array of continuous values parameterizing the authenticity of each sub-period;
• quantising the output of the first encoder onto vectors in a first set of codebook vectors stored in a first codebook, the first codebook representing the authenticity of the first set of codebook vectors;
• using an auxiliary classifier to output an indication of the authenticity of at least one sub-period based on the codebook vectors;
• applying a second encoder to the training signal, the second encoder configured to output an m-dimensional array of continuous values parameterizing the content of each sub-period;
• quantising the output of the second encoder onto vectors in a second set of codebook vectors stored in a second codebook, the second codebook representing the content of the second set of codebook vectors;
• applying at least one decoder to the quantised vectors from the first encoder and the second encoder to generate a reproduction of the training signal;
• and
• adjusting at least one of the encoders, the decoder, the codebooks and the auxiliary classifier using the reproduction of the training signal, the original training signal, the plurality of labels, and the output indication.