Second harmonic distortion (HD2): Ratio of second-order harmonic to the input signal (carrier). Often measured as dBc.
"I just wanted to let you know that I was very satisfied with the CD you made for me from a really old album my husband had. It is very clear and without background noise - he almost misses all that hissing. He was so excited to have it and couldn't believe I had found a place to transfer the music by looking on the internet. Thanks again,"