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 for producing a digital audio signal from a signal representing an audio capture, the encoder having:
a downsampler receiving the signal representing the audio capture at a first sample rate and downsampling the signal to provide the digital audio signal at a transmission sample rate which is half the first sample rate;
an encapsulation filter providing attenuation of at least 45 dB at frequencies within 10 kHz of a first Nyquist frequency corresponding to the first sample rate; and
a flattening filter having an amplitude response that is symmetrical about the transmission Nyquist frequency and that provides amplitude boost to compensate at least 50% of an amplitude droop of the encapsulation filter at frequencies between 80% and 100% of the transmission Nyquist frequency.
2. An encoder according to claim 1 , wherein a combined response of the encapsulation filter and the flattening filter is flat over the frequency range 0-20 kHz.
3. An encoder according to claim 1 , wherein a z-transform of the flattening filter contains only even powers of z.
4. A system for conveying the sound of an audio capture, the system comprising:
an encoder, the encoder producing a digital audio signal from a signal representing the audio capture, the encoder having a downsampler receiving the signal representing the audio capture at a first sample rate and downsampling the signal to provide the digital audio signal at a transmission sample rate which is half the first sample rate, the encoder having an encapsulation filter providing attenuation of at least 45 dB at frequencies within 10 kHz of a first Nyquist frequency corresponding to the first sample rate and a flattening filter having an amplitude response that is symmetrical about the transmission Nyquist frequency and that provides amplitude boost to compensate at least 50% of an amplitude droop of the encapsulation filter at frequencies between 80% and 100% of the transmission Nyquist frequency; and
a decoder, the decoder receiving the digital audio signal and producing a reconstructed signal.
5. A system according to claim 4 , wherein a combined response of the encapsulation filter and the flattening filter is flat over the frequency range 0-20 kHz.
6. A system according to claim 4 , wherein a z-transform of the flattening filter contains only even powers of z.
7. A system according to claim 4 , wherein the flattening filter is an Infinite Impulse Response (IIR) filter having a pole in its z-transform, and wherein the decoder comprises a filter having a zero in its z-transform whose z-plane position coincides with that of the pole, the effect of which is thereby cancelled in the reconstructed signal.
8. A system according to claim 4 , wherein an impulse response of the encoder and decoder in combination has 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.