Digital encapsulation of audio signals
AssigneeMQA Limited
InventorsPeter Graham CRAVEN, John Robert STUART
Life of this patent
Estimating the term…
Abstract
Encoding and decoding systems are described for the provision of high quality digital representations of audio signals with particular attention to the correct perceptual rendering of fast transients at modest sample rates. This is achieved by optimising downsampling and upsampling filters to minimise the length of the impulse response while adequately attenuating alias products that have been found perceptually harmful.
Claims39
A system comprising an encoder and a decoder for conveying the sound of an audio capture, wherein the encoder is adapted to furnish a digital audio signal at a transmission sample rate from a signal representing the audio capture, and the decoder is adapted to receive the digital audio signal and furnish a reconstructed signal,
- wherein the encoder comprises a downsampler adapted to receive the signal representing the audio capture at a first sample rate which is a multiple of the transmission sample rate and to downsample the signal to furnish the digital audio signal;
- and,
- wherein an impulse response of the encoder and decoder in combination is characterised by a duration for its cumulative absolute response to rise from 1% to 95% of its final value not exceeding five sample periods at the transmission sample rate.
A system comprising an encoder and a decoder for conveying the sound of an audio capture, wherein the encoder is adapted to furnish a digital audio signal at a transmission sample rate from a signal representing the audio capture, and the decoder is adapted to receive the digital audio signal and furnish a reconstructed signal,
- wherein the encoder comprises a downsampler adapted to receive the signal representing the audio capture at a first sample rate which is a multiple of the transmission sample rate and to downsample the signal to furnish the digital audio signal;
- and,
- wherein an impulse response of the encoder and decoder in combination is characterised by a duration for its cumulative absolute response to rise from 1% to 50% of its final value not exceeding two sample periods at the transmission sample rate.
A system according to any preceding claim, wherein the downsampler comprises a decimation filter specified at the first sample rate, wherein the alias rejection of the decimation filter is at least 32 dB at frequencies that would alias to the range 0-7 kHz on decimation.
A system according to any preceding claim, wherein the decoder comprises a filter having a response which rises in a region surrounding the Nyquist frequency corresponding to the transmission sample rate and the encoder comprises a filter having a response that falls in said region, thereby reducing downward aliasing in the encoder of frequencies above the Nyquist frequency to frequencies below the Nyquist frequency.
A system according to any preceding claim, wherein the transmission sample rate is selected from one of 88.2 kHz and 96 kHz and the first sample rate is selected from one of 176.4 kHz, 192 kHz, 352.8 kHz and 384 kHz.
A method of furnishing a digital audio signal for transmission at a transmission sample rate by reducing the sample rate required to convey the sound of captured audio, the method comprising the steps of:
- filtering a representation of the captured audio having a first sample rate that is a multiple of the transmission sample rate using a decimation filter specified at the first sample rate;
- and,
- decimating the filtered representation to furnish the digital audio signal, wherein an impulse response of the decimation filter has an alias rejection of at least 32 dB at frequencies that would alias to the range 0-7 kHz on decimation,
- where there exists a second filter having the same alias rejection as the decimation filter, and a response having a duration for its cumulative absolute response to rise from 1% to 95% of its final value not exceeding five sample periods at the transmission sample rate.
A system for conveying the sound of an audio capture, the system comprising:
- an encoder adapted to receive a signal representing the audio capture and to furnish a digital audio signal at a transmission sample rate, said encoder characterised by an impulse response having a duration for its cumulative absolute response to rise from 1% to 95% of its final value;
- and,
- a decoder adapted to receive the digital audio signal and furnish a reconstructed signal, said decoder characterised by an impulse response having a duration for its cumulative absolute response to rise from 1% to 95% of its final value,
- wherein the combined response of the encoder and decoder produce a total system impulse response having a duration for its cumulative absolute response to rise from 1% to 95% that is less than the characterising duration of the impulse response of the encoder alone and the characterising duration of the impulse response of the decoder alone.
An encoder adapted to furnish a digital audio signal at a transmission sample rate from a signal representing an audio capture, the encoder comprising a downsampling filter characterised by a frequency response having a double zero at each frequency that will alias to zero frequency and having a slope at the transmission Nyquist frequency more positive than minus thirteen decibels per octave.
A system comprising an encoder and a decoder for conveying the sound of an audio capture, wherein the encoder is adapted to furnish a digital audio signal at a transmission sample rate from a signal representing the audio capture, and the decoder is adapted to receive the digital audio signal and furnish a reconstructed signal,
- wherein the encoder comprises a downsampler adapted to receive the signal representing the audio capture at a first sample rate which a multiple of the transmission sample rate and to downsample the signal to furnish the digital audio signal;
- and,
- wherein the encoder comprises an Infinite Impulse Response (IIR) filter having a pole, and the decoder comprises a filter having a zero whose z-plane position coincides with that of the pole, the effect of which is thereby cancelled in the reconstructed signal.
An encoder adapted to furnish a digital audio signal at a transmission sample rate from a signal representing an audio capture, the encoder comprising a downsampling filter adapted to receive the signal representing the audio capture at a first sample rate which a multiple of the transmission sample rate and to downsample the signal to furnish the digital audio signal, wherein the encoder is adapted to analyse a spectrum of the captured audio and select the downsampling filter responsively to the analysed spectrum.
A decoder for receiving a digital audio signal at a transmission sample rate and furnishing an output audio signal, wherein the decoder comprises a filter having an amplitude response which increases with frequency in a frequency region surrounding the Nyquist frequency corresponding to the transmission sample rate
Dependencies are inferred from the claim text.