Harmonic and Individual Lines and Noise

{{Unreferenced, date=December 2017 Harmonic and Individual Lines and Noise (HILN) is a parametric

codec A codec is a device or computer program that encodes or decodes a data stream or signal. ''Codec'' is a portmanteau of coder/decoder. In electronic communications, an endec is a device that acts as both an encoder and a decoder on a signal or da ...

for

audio Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to: Sound *Audio signal, an electrical representation of sound *Audio frequency, a frequency in the audio spectrum *Digital audio, representation of sound ...

. The basic premise of the encoder is that most audio, and particularly speech, can be synthesized from only

sinusoids A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...

and noise. The encoder describes individual sinusoids with amplitude and frequency, harmonic tones by fundamental frequency, amplitude and the spectral envelope of the partials, and the noise by amplitude and spectral envelope. This type of encoder is capable of encoding audio to between 6 and 16 kilobits per second for a typical audio bandwidth of 8 kHz. The framelength of this encoder is 32 ms. A typical codec extracts sinusoid information from the samples by applying a

short-time Fourier transform The short-time Fourier transform (STFT), is a Fourier-related transform used to determine the sinusoidal frequency and phase content of local sections of a signal as it changes over time. In practice, the procedure for computing STFTs is to divid ...

to the samples and using that to find the important harmonic content of a single frame. By matching sinusoids across frames, the encoder is capable of grouping them into harmonic lines and individual sinusoids. The matching can take amplitude, frequency and phase into account when trying to match sinusoids across frames. Differences between amplitude and frequency within a track can be coded with fewer bits than each individual single sinusoid would require, thus the longer a track the encoder can find, the better it will be able to reduce the final

bitrate In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...

. The decoder uses an add-and-overlap strategy: each frame in the bitstream contains parameters for 32 ms, however, the next frame starts halfway through the current frame. By filtering the synthesized segments with a

Hanning filter The Hann function is named after the Austrian meteorologist Julius von Hann. It is a window function used to perform Hann smoothing. The function, with length L and amplitude 1/L, is given by: : w_0(x) \triangleq \left\. For digital si ...

, adding two overlapping frames together will produce a smooth transition between the two. This also applies to the encoder because the short Fourier transform gives better results when the data is pre-processed with a Hanning filter. Synthesizing only the sinusoids sounds artificial and metallic. To mask this, the encoder subtracts the synthesized sinusoids from the original audio signal. The residual is then matched to a linear

filter Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component tha ...

that is excited with

white noise In signal processing, white noise is a random signal having equal intensity at different frequencies, giving it a constant power spectral density. The term is used, with this or similar meanings, in many scientific and technical disciplines, ...

. The extracted parameters can then be quantized, coded and multiplexed into a

bitstream A bitstream (or bit stream), also known as binary sequence, is a sequence of bits. A bytestream is a sequence of bytes. Typically, each byte is an 8-bit quantity, and so the term octet stream is sometimes used interchangeably. An octet may ...

. Audio codecs