How does the DCT (Discrete Cosine Transform) help to compress sound (or any wave-like data)? According to the DCT transform there are N input values and N output values as a result. Where is the compression achieved and how?
Think about the sequence 1,2,3,4,5,.. It will not compress using LZ (zip) at all because there is zero repetition. Now encode the sequence as differences: 1,1,1,1,1,... Zip will compress it 99% now. Every algorithm detects a certain pattern well. DCT helps to encode the data into a format that is well compressible.