 Playback without noise reduction produces a noticeably brighter sound, however.The calibration of the recording and playback circuitry is therefore critical for faithful reproduction of the original program content, and this is easily offset by poor quality tape, dirty recording/playback heads, or using inappropriate bias levels/frequency for the tape formulation, as well as tape speed, when recording or duplicating.This can manifest itself as muffled-sounding playback, or "breathing" of the noise level as the signal varies.