Digital encapsulation of audio signals
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.
1. 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,
wherein the encoder is adapted to transmit information identifying the selected downsampling filter to a decoder as metadata; 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 of the transmission sample rate.
2. An encoder according to claim 1 , wherein the selected downsampling filter has a steeper attenuation response at the transmission Nyquist frequency if the analysed spectrum is rising rapidly at the transmission Nyquist frequency.
3. An encoder according to claim 1 , comprising a flattening filter having a symmetrical response about the transmission Nyquist frequency.
4. An encoder according to claim 3 , wherein the flattening filter has a pole.
5. An encoder according to claim 1 , wherein the downsampling filter 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.
6. An encoder according to claim 5 , wherein there exists a comparison filter having the same alias rejection as the decimation filter, and an impulse 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.
7. An encoder according to claim 1 , wherein the encoder comprises an Infinite Impulse Response (IIR) filter having a pole, wherein the encoder is adapted to transmit information to a decoder, wherein 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.
8. An encoder according to claim 1 , 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.
9. An encoder according to claim 2 , wherein the transmission Nyquist frequency is 44.1 kHz and the encoder's frequency response droop does not exceed 1 dB at 20 kHz.
10. 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,
wherein the encoder comprises an Infinite Impulse Response (IIR) filter having a pole,
wherein the encoder is adapted to transmit information to a decoder, and
wherein 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.
11. 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,
wherein the downsampling filter 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; and,
wherein there exists a comparison filter having the same alias rejection as the decimation filter, and an impulse 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.