Signal processing device and method
View Patent ↗A bitstream converter for converting a 1-bit pulse density modulated (PDM) bitstream signal into an analog audio signal, the bitstream converter comprising: a processor configured to process the 1-bit PDM bitstream signal using a return to zero clock having a frequency higher than a sampling frequency of the 1-bit PDM bitstream signal to output a corresponding 1-bit return to zero signal, wherein the processor is configured to process the 1-bit PDM signal to ensure a portion of each bit of the 1-bit PDM bitstream signal is zero for a duration which is based on the frequency of the return to zero clock; and signal processing means configured to extract the analog audio signal from the 1-bit return to zero signal by filtering the 1-bit return to zero signal.
1. A bitstream converter for converting a 1-bit pulse density modulated (PDM) bitstream signal into an analog audio signal, the bitstream converter comprising:
a processor configured to process the 1-bit PDM bitstream signal using a return to zero clock having a frequency higher than a sampling frequency of the 1-bit PDM bitstream signal to output a corresponding 1-bit return to zero signal, wherein the processor is configured to process the 1-bit PDM signal to ensure a portion of each bit of the 1-bit PDM bitstream signal is zero for a duration which is based on the frequency of the return to zero clock; and
signal processing means configured to extract the analog audio signal from the 1-bit return to zero signal by filtering the 1-bit return to zero signal,
wherein the return to zero clock frequency is at least 4 times the sampling frequency of the 1-bit PDM bitstream signal.
2. A bitstream converter according to claim 1 , wherein the signal processing means includes one or more filters to filter the 1-bit return to zero signal.
3. A bitstream converter according to claim 1 , wherein the signal processing means includes one or more low-pass filters to filter the 1-bit return to zero signal.
4. A bitstream converter according to claim 1 , wherein the signal processing means is configured to extract the analog audio signal from the 1-bit return to zero signal by demodulating the 1-bit return to zero signal.
5. A bitstream converter according to claim 1 , wherein: the signal processing means includes a low-pass finite impulse response filter configured to filter noise resulting from the processing of the 1-bit PDM bitstream signal to output the corresponding 1-bit return to zero signal.
6. A bitstream converter according to claim 5 , wherein the low pass finite impulse response filter is configured to attenuate frequencies above 1 MHz by at least 30 dB.
7. A bitstream converter according to claim 5 , wherein the low-pass finite impulse response filter is a moving average finite impulse response filter.
8. A bitstream converter according to claim 7 , wherein the moving average finite impulse response filter has an order of at least 2.
9. A bitstream converter according to claim 1 , wherein the 1-bit return to zero signal is a 1-bit bipolar return to zero signal.
10. A bitstream converter according to claim 9 , wherein the processor outputs the 1-bit bipolar return to zero signal as a differential signal.
11. A bitstream converter according to claim 10 , wherein the processor outputs a 1-bit unipolar return to zero signal and a complimentary 1-bit unipolar return to zero signal to differentially output the 1-bit bipolar return to zero signal.
12. A bitstream converter according to claim 1 , further comprising an oversampling stage, the oversampling stage configurable to oversample an input bitstream signal to provide the 1-bit PDM bitstream signal.
13. A bitstream converter according to claim 12 , wherein the input bitstream signal is an input multi-bit bitstream signal, the oversampling stage configured to generate the 1-bit PDM bitstream signal from the input multi-bit bitstream signal.
14. A bitstream converter according to claim 12 , wherein the oversampling stage oversamples a sampling frequency of the input bitstream signal by a factor of at least 32.
15. A bitstream converter according to claim 1 , wherein the processor is a digital signal processor.
16. A digital-to-analog converter including the bitstream converter according to claim 1 .
17. A method of bitstream conversion for converting a 1-bit pulse density modulated (PDM) bitstream signal into an analog audio signal, the method of bitstream conversion comprising:
processing the 1-bit PDM bitstream signal using a return to zero clock having a frequency at least 4 times higher than a sampling frequency of the 1-bit PDM bitstream signal, to output a corresponding 1-bit return to zero signal, by processing the 1-bit PDM signal to ensure a portion of each bit of the 1-bit PDM signal is zero for a duration which is based on the frequency of the return to zero clock; and
extracting the analog audio signal from the 1-bit return to zero signal by signal processing means by filtering the 1-bit return to zero signal.
18. A method of bitstream conversion according to claim 17 , wherein filtering the 1-bit return to zero signal includes filtering the 1-bit return to zero signal with a finite impulse response filter to remove noise produced by processing the 1-bit PDM bitstream signal to provide the corresponding 1-bit return to zero signal.
19. A method of bitstream conversion according to claim 17 including oversampling an input bitstream signal to provide the 1-bit PDM bitstream signal.
20. An audio playback device comprising a bitstream converter according to claim 1 .