Telos ZephyrExpress Redundancy Reduction, Bit Reservoir Buffering, Ancillary Data, Bands

main bands into 18 more. At a 32 kHz sampling rate, the resulting bandwidth is 27.78 Hz

–allowing very accurate calculation of the masking threshold values. Sufficient frequency resolution is available to exceed the width of the ear’s critical bands (100 Hz below 500 Hz; 20% of the center frequency at higher frequencies) across the audible spectrum, resulting in better hiding of noise than would otherwise be possible.

Redundancy Reduction

Redundancy reduction is accomplished by a Huffman coding process to take advantage of the statistical properties of the simplified signal output from the psychoacoustic stage. Values that appear more frequently are coded with shorter words, whereas values that appear only rarely are coded with longer words. This results in an overall decrease in the data rate, with no degradation, since it is a lossless reduction scheme.

Notice that this redundancy reduction process is the ideal supplement to psychoacoustic masking. In general, maskers with high tonality have more redundancy but allow less masking, while noise-like signals have low redundancy and high masking effect.

32 bands 576 Bands

Polyphase MDCT

Filterbank Filters

1024 point FFT/ Perceptual Model

Huffman

Non-uniform Coding

Quantization

Distortion

Control

Side

Information

External Control

Bitstream Formattting CRC Check

Block diagram of the Layer III coding process.

Bit Reservoir Buffering

Often, there are some critical parts in a piece of music that cannot be encoded at a given data rate without audible noise. These sequences require a higher data rate to avoid artifacts. Layer 3 uses a short time “bit reservoir” buffer to address that need. Similar to a savings account, this buffer is filled in “easy times” with data bits that are not required for the actual frame. If a critical part occurs, the encoder can use the saved bits to code this part with a higher data rate.

Ancillary Data

The bit reservoir buffer offers an interesting capability: an effective solution for the inclusion of such ancillary data as text or control signaling. The data is held in a separate buffer and gated onto the output bitstream using the bits allocated for the reservoir buffer when they are not required for audio.

5-6 CODING