A signal processing chain, such as an audio chain, produces an analog output signal from a digital input signal. The signal processing chain is operated by generating a first flag signal for the analog output signal and one or more second flag signals for the digital input signal. Each flag signal assumes a first level or a second level and is set to the first level when a signal from which the flag is generated has a value within an amplitude window. An amount the first flag signal for the analog output signal and the second flag signal for the digital input signal match each other may be calculated for issuing an alert flag which indicates an impaired operation of the signal processing chain.