MFCCs were the standard feature for automatic speech recognition systems using HMM classifiers. MFCCs work by framing an audio signal, calculating the power spectrum of each frame, applying a Mel filterbank to group frequencies, taking the logarithm of the filterbank energies, and computing the DCT to decorrelate the features. The Mel scale relates perceived pitch to actual frequency in a way that matches human hearing. MFCCs were effective for GMM-HMM systems and helped speech recognition performance by representing audio signals in a way aligned with human perception.