Mobile phones now transmit video signals at relatively slow bit rates. The signals transmitted are compressed using inter-frame prediction where sufficient similarity exists between successive picture frames. The receiver includes a decoder which reconstitutes the compressed signals received using signals stored on a buffer representing a previous picture frame. A mode decision circuit compares the signal at the output of the decoder with the previous signal stored and using the same decision criterion as used for the original compression generates a signal which indicates that either the signal is suitable for inter-frame prediction or not. This signal is then compared with the signal from the encoder indicating whether or not the current signal was inter-frame predicted or not. If equality is not detected, then an error may have taken place. Then, error concealment techniques are used.