For The Love of Digital Forensics: Where Would You Find 1111 1111 1111 and “ID3"?
Well, the answer is, in an MP3 file. With audio, we normally sample at twice the highest frequency of the signal, and so if we have hi-fi quality audio we normally define a 22kHz bandwidth (as this is the range your ear can hear). Then our sampling rate becomes around 44.1kHz, which gives us one sample every 22 microSeconds. Each sample has 16 bits and two channels, and so the rate we require for raw high-quality audio is 1.4 Mbps. This would be a high requirement for many systems and is generally wasteful. So, we compress this down by performing a DCT (Discrete Cosine Transform).
Psycho-acoustic model
MPEG and Dolby AC-3 use the psycho-acoustic model to reduce the data rate, which exploits the characteristics of the human ear. This is similar to the method used in MPEG video compression which uses the fact that the human eye has a lack of sensitivity to the higher-frequency video components (that is, sharp changes of colour or contrast). The psycho-acoustic model allows certain frequency components to be reduced in size without affecting the perceived audio quality as heard by the listener.
A well-known effect is the masking effect. This is where noise is only heard by a person when there are no other sounds to mask it. A typical example is…
