Correct Answer - Option 1 : Theoretical limit on compressibility of particular signals
Transform coding, in conjunction with psycho-acoustic modeling, is a common technique used to compress digital audio. But the use of such a technique can lead to a coding artifact known as a "pre-echo".
- Pre-echoes typically appear in transient situations, situations where silence is broken by a sharp attack. For example, firecrackers, chimes, and castanets all produce sharp attacks that can generate pre-echoes.
- This is a measure of perceptually relevant information contained in any audio record. Expressed in bits per sample, PE represents a theoretical limit on the compressibility of a particular signal.
- Premasking has been exploited in conjunction with adaptive block-size transform coding to compensate for pre-echo distortions.
The PE estimation process is accomplished as follows:
- The signal is first windowed and transformed to the frequency domain.
- A masking threshold is then obtained using perceptual rules.
- Finally, a determination is made on the number of bits required to quantize the spectrum without injecting perceptible noise.
- The PE measurements are obtained by constructing a PE histogram over many frames and then choosing a worst-case value as the actual measurement.
