Any information system must ensure the confidentiality of its sensitive data during its operation. Among the vulnerabilities, TEMPEST attacks appear to be an important threat as this sensitive data can be emitted through unwanted electromagnetic waves generated by hardware operation and can therefore be intercepted by any distant radio frequency receiver. The menace is worst when sensitive data is mixed with an internal communication device as the leak is hidden within a legitimate communication channel and difficult to find. It can happen for instance when an audio processing unit like a microphone or amplifier is close to a transmission security (TRANSEC) transmitter, leading to the emanation of the audio signal through the TRANSEC which follows a frequency hopping scheme. This paper focuses on detection of the used channel of a frequency hopping system, when the system bandwidth is very large and the number of hops per second is very high (TRANSEC case). By obtaining this channel information, the audio stream can be recovered, leading to a potential high security break.