Data processor for recording and reading out check data and true data in a cassette tape

A data recorder for recording data in the form of audio or digital signals into a cassette tape comprises a first circuit for recording check signals and a second circuit for recording true signals. The first circuit allows the check signals to be recorded in a first section of the cassette tape to check the amplitude of a sound volume in which the check signals are sounded. The second circuit enables the true signals to be recorded in a second section of the cassette tape under the condition that the amplitude of the sound volume of the check signals is adequately adjusted. A data reader for reading out the thus recorded data includes a first detection circuit for determining whether the read-out signals have a sufficient level for reading out purposes, an announcing device for announcing the need to adjust a sound volume, and a read-out circuit for reading out true data from the cassette tape.