Decoder for a recorder-decoder system

An improved decoder for a recorder-decoder system having a recorder which records an audio input and records a data signal that includes a count of zero crossings of the recorded audio occurring during sequential increments of time with a threshold information bit provided for each count indicating whether a threshold level in the recorder was satisfied by the recorded audio at least once during a count, the decoder for the system providing for playback of the recorded audio and recorded data signal with a count of zero crossings of the recorded audio made during sequential increments of time with the data signal count for an increment of time when the threshold level was indicated as being met being compared with the count obtained by the decoder for the same increment of the recording to provide a measure of whether the recorder audio for such increment was tampered subsequent to its being recorded, the improvement including:

• a threshold circuit included in the decoder which is operable during playback of the recorded audio and recorded data signal for detecting whether the recorded audio exceeds the threshold level of said threshold circuit while a zero crossing count of the recorded audio is being made, said threshold level being higher than the threshold level of the recorder whereby an additional check for tampering of the recorded audio is provided in that tampering of the recorded audio is considered to have occurred if the threshold information bit for a count of zero crossings in the recorded data indicates the threshold level was not met and the threshold level indication provided by the threshold circuit of the decoder during the playback of the recorded audio of the corresponding increment of the recorded audio signal indicates the threshold level was met.