Method and apparatus for improving the effective signal-to-noise ratio of analog-to-digital conversion for multi-band digital signal processing apparatus - Patents.com

Abstract

A method for improving the effective signal-to-noise ratio (SNR) of an analog-to-digital converter (ADC) for active loudspeakers uses the two available channels of a stereo ADC to process the low- and high-frequency components of the audio signal separately. Because the power spectral density of music approximates a pink-noise spectrum, the high-frequency components of the signal have peak levels low enough to avoid exceeding the maximum ADC input level. The audio signal is analog high-pass filtered, and the resulting high-frequency signal components are sent directly to the first ADC channel without attenuation. The remaining low-frequency components are attenuated and sent to the second ADC channel. The digital signal is processed, converted back to analog, amplified, and reproduced by the loudspeaker driver. Because low-frequency noise and distortion are less audible than high-frequency noise, improving the SNR in the high frequencies provides a significant practical improvement in audio fidelity.

Description

[Technical field]

[0001] The present invention relates generally to analog-to-digital signal conversion apparatus and methods, and more particularly to methods for improving the effective signal-to-noise ratio of analog-to-digital conversion for active loudspeakers and other multi-band digital signal processing devices by using two channels of a stereo analog-to-digital conversion device to separately process low and high frequency components of an analog input signal, and active loudspeaker apparatus employing those methods. [Background technology]

[0002] An active loudspeaker is a loudspeaker that combines one or more amplifiers in one unit so that a separate audio amplifier is not required for line-level audio input signals. Typically, active loudspeakers include crossover filters ("crossovers") to separate the audio signal into two or more frequency bands (e.g., high and low frequency components in a two-way active speaker, or high, mid, and low frequency components in a three-way active speaker) to be reproduced by separate speaker drivers (e.g., a tweeter for the high frequency components and a woofer for the low frequency components). Crossovers can be applied to the audio signal after amplification ("passive crossovers") or before amplification ("active crossovers"). Active speakers that use passive crossovers only require one amplifier, while active speakers that use active crossovers require separate amplifiers for each frequency band (i.e., two amplifiers for a two-way active speaker).

[0003] Crossovers can be implemented with either analog or digital circuits. High-end active loudspeakers typically implement active crossovers using digital signal processor ("DSP") circuitry. The use of a DSP allows processing of analog audio input signals without the loss, signal degradation, and additional component cost introduced by analog signal processing circuits. By performing the crossover function digitally, greater selectivity and sharper frequency cutoffs can be achieved compared to analog filters. DSPs also allow additional functionality to be easily and inexpensively added to active loudspeakers without the need for additional circuitry, including audio effects such as equalization, dynamic range compression, delay, reverb, modulation, or the mixing and playback of multiple simultaneous signal inputs.

[0004] Figure 1 shows a schematic diagram of an example of a two-way active loudspeaker 101 that uses a DSP to implement a crossover. A conventional active speaker such as that shown in Figure 1 is typically implemented as a chain of electronic components including an analog audio input stage 102, an analog attenuation stage 103, a stereo analog-to-digital converter (ADC) 104, a DSP 105, a stereo digital-to-analog converter (DAC) 106, an analog signal booster stage 107, one or more power amplifiers 108, and one or more loudspeaker drivers 109. Two or more of these components may be combined within a single integrated circuit chip (e.g., the ADC and DAC may be included with additional DSP circuitry within a single chip).

[0005] The maximum peak signal levels of line level inputs and outputs in professional audio equipment are typically on the order of 12 volts or more, while the maximum peak input and output line levels of most commonly available audio ADC and DAC devices are in the 2 volt range, limited by the 5 volt or 3.3 volt standard supply voltages typically used in digital circuits. Thus, an analog attenuation stage 103 is required to accommodate the limited peak-to-peak input range of the ADC 104 when connected to a professional audio equipment source via the analog audio input 102. The amount of attenuation required is typically 15 dB (i.e., a gain of -15 dB). Similarly, an analog signal booster stage 107 amplifies the output of the DAC 106 by approximately +15 dB to match the 12 volt professional audio equipment signal level required by the loudspeaker power amplifier 108.

[0006] Discrete package ADC devices typically have two audio channels so that one device can be used to digitize a stereo audio signal. However, individual loudspeakers typically have a mono signal input (e.g., either the left or right channel of a stereo signal) and therefore only require a single ADC channel. Assuming a second ADC channel exists in a mono active loudspeaker, it is desirable not to leave it unused (and thus "waste" it). Thus, a second channel is often used in an attempt to improve the signal-to-noise ratio (SNR) in conventional active loudspeakers. This is achieved by inverting the polarity of the attenuated input signal from attenuation stage 103 that is fed to ADC channel 111 compared to the polarity fed to ADC channel 110. The outputs of the two ADC channels are subtracted 112 from each other by program instructions executed on DSP 105. This method can theoretically reduce the uncorrelated noise at the ADC input by 3 dB. In practice, however, there is not much uncorrelated noise at the ADC input. Because both ADC channel circuits are typically laid out closely together on a single silicon chip and are therefore nearly equally affected by external noise sources, the noise reduction achieved with this method is typically much less than 3 dB.

[0007] The difference signal from the two ADC channels is fed along path 113 to a high pass filter stage 114 and along path 115 to a low pass filter stage 116 of the DSP 105. The high pass / low pass filter stages 114, 116 form a crossover filter inside the DSP 105 that separates the input signal into high and low frequency components. FIG. 1 shows a typical example of two cascaded second order filters in each of the high frequency filter stage 114 and the low frequency filter stage 116. After processing, each of the filtered high and low frequency components is converted back to a separate analog signal via the stereo DAC 106. The separated high and low frequency analog signal components are then boosted by the analog signal booster stage 107 to restore the 12 volt audio signal level and sent to the power amplifier 108 which drives the high and low frequency loudspeaker converters 109.

[0008] Typically, the limited peak-to-peak input and output signal ranges of available ADC and DAC devices present a significant drawback in terms of signal-to-noise ratio. Pre-ADC attenuation stage 103 and post-DAC gain stage 107 each introduce noise into the audio signal. This noise limits the published SNR specifications of ADC 104 and DAC 106. Furthermore, signal attenuation inherently reduces the effective audio resolution and ultimately the fidelity of the loudspeaker. For example, if ADC 104 has 24-bit resolution and attenuation stage 103 reduces the input signal level by 15 dB, the input signal resolution will be 10 times the unattenuated input signal (Log2[10 15dB / 10dB Since the resolution of the input signal to the 24-bit ADC 104 is effectively reduced by 5 bits relative to the input signal resolution (because 19-bit sigma-density [because 19-bit sigma-density] ≈ 5 bits), the input signal resolution of the 24-bit ADC 104 is effectively only 19 bits. This reduces the headroom available for signal processing in the DSP 105, which can result in an audible degradation in sound quality.

[0009] Therefore, there is a need for analog-to-digital signal conversion that does not reduce the input signal resolution, thereby improving the effective signal-to-noise ratio in active loudspeakers and other devices. Summary of the Invention [Problem to be solved by the invention]

[0010] A method has been presented for improving the effective signal-to-noise ratio of analog-to-digital and digital-to-analog conversion for active loudspeakers and other multi-band digital signal processing devices. In one or more embodiments, the method of the present invention uses two available channels of a stereo analog-to-digital conversion device to process the low and high frequency components of the signal separately.

[0011] The power spectral density of music approximates that of the pink noise (also known as 1 / f noise) spectrum, i.e., a spectrum in which the power density of a signal is inversely proportional to frequency. Thus, for pink noise as well as for typical music, the power density for signal frequencies above the middle of the audio spectrum (i.e., near and above 600 Hz) is about 15 dB lower than the power density for signal frequencies near 20 Hz. Thus, the higher frequency components of the audio signal (i.e., audio frequencies above about 600 Hz) do not need to be attenuated by 15 dB before entering the ADC stage, because they already have peak signal levels at least 15 dB lower than the peak signal level at 20 Hz. [Means for solving the problem]

[0012] In one or more embodiments, an active speaker with digital signal processing circuitry exploits this property of the 1 / f power density spectrum to improve the effective signal-to-noise ratio. In one or more embodiments, the high frequency components of the audio signal are separated from the audio signal before any attenuation or analog-to-digital conversion. The high frequency components are formed by high-pass filtering the unattenuated input signal in the analog domain with an analog high-pass filter stage. The resulting high frequency components are then sent to one channel of the ADC without attenuation, thereby increasing the effective SNR of the high frequency components by 15 dB, or 5 bits of resolution.

[0013] In one or more embodiments, the original audio signal (including both high and low frequency components) is attenuated by 15 dB in an analog attenuation stage to generate an attenuated audio signal that is then sent to the other channel of the ADC. The attenuated audio signal is processed and digitally low pass filtered in a DSP to generate the low frequency component of the audio signal. The high frequency component is processed separately and filtered in the DSP. Both the high frequency and low frequency components are then converted back to analog signals by a DAC. The low frequency component is boosted back to its pre-attenuation level in an analog signal booster stage. Both signal components are then amplified in separate power amplifiers and audibly reproduced through separate loudspeaker drivers.

[0014] Because human hearing is less sensitive to noise and distortion in the low frequencies than in the mid and high frequencies, improving the SNR and effective bit depth in the mid and high frequencies provides a significant practical improvement in overall loudspeaker performance and audio fidelity.

[0015] The present invention may be better understood, and its features made apparent to those skilled in the art, by reference to the accompanying drawings, in which: [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram of a conventional two-way active loudspeaker with digital signal processing circuitry. [Diagram 2] 4 is a graph showing the spectral power density versus frequency of pink noise, as well as the attenuation versus frequency of a high pass filter used in an embodiment of the present invention. [Diagram 3] FIG. 1 is a schematic diagram of a two-way active loudspeaker with digital signal processing circuitry having an improved effective signal-to-noise ratio in one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The use of the same reference numbers in different drawings indicates similar or identical items.

[0018] A method has been presented for improving the effective signal-to-noise ratio of analog-to-digital and digital-to-analog conversion for active loudspeakers and other multiband digital signal processing devices. In one or more embodiments, the method of the present invention uses two available channels of a stereo analog-to-digital conversion device to process low and high frequency components of the signal separately.

[0019] The power spectral density of music approximates that of pink noise (also known as 1 / f noise) spectrum, i.e., a spectrum in which the power density of a signal is inversely proportional to frequency. Thus, for pink noise as well as for typical music, the peak signal power (and therefore the peak signal level) at a particular frequency falls by 3 dB for every doubling of frequency, which corresponds to a 30 dB difference between peak signal levels across the range of the audible spectrum from 20 Hz to 20 kHz.

[0020] 2 is a graph 201 showing the power density versus frequency of the spectrum of pink noise, as well as the attenuation versus frequency of a high pass filter used in an embodiment of the invention. The left vertical scale 202, right vertical scale 203, and horizontal scale 204 of graph 201 are all logarithmic. The left vertical scale 202 represents the power density in dB at a particular frequency, with the zero dB level normalized to the maximum power level of the entire signal. The right vertical scale 203 represents the signal attenuation in dB at a particular frequency for a high pass filter with a cutoff frequency of approximately 600 Hz as used in an embodiment of the invention.

[0021] Figure 2 shows that the power density for signal frequencies above the middle of the audio spectrum (i.e., near and above 600 Hz) is about 15 dB lower than the power density for signal frequencies near 20 Hz. This is because the power density at a particular frequency, denoted Pf, is proportional to 1 / f, so the difference in power density between 600 Hz and 20 Hz is:

number

[0022] Figure 3 is a schematic diagram of a stereo active speaker with digital signal processing circuitry that exploits this property of the 1 / f power density spectrum to improve the effective signal-to-noise ratio. In the embodiment of Figure 3, an active loudspeaker 301 includes an analog audio input stage 302, an analog high-pass filter stage 303, an analog attenuation stage 304, a stereo analog-to-digital converter (ADC) 305, a DSP 306, a stereo digital-to-analog converter (DAC) 307, an analog unity gain stage 308, an analog signal booster stage 309, a power amplifier 310, and a loudspeaker driver 311.

[0023] 3, the audio input signal is provided along path 312 to high pass filter stage 303 and separately provided along path 313 to analog attenuation stage 304 before any attenuation or analog to digital conversion. The input signal provided along path 312 to high pass filter stage 303 is high pass filtered in the analog domain by analog high pass filter stage 303 and then sent to a first ADC channel 314 of ADC 305 to generate an unattenuated digital high frequency component of the signal.

[0024] In the embodiment of FIG. 3, the analog high-pass filter stage 303 is an active second-order high-pass filter with unity gain including an operational ramp and a resistor-capacitor circuit, with electronic component values ​​selected to have a cutoff frequency of approximately 600 Hz. In one or more alternative embodiments, the analog high-pass filter stage 303 can have a gain greater than or less than unity. For example, the analog high-pass filter stage 303 can attenuate the signal by a small amount, but much less than the 15 dB of attenuation applied by the analog attenuation stage 304. Alternatively, in one or more embodiments, the analog high-pass filter stage 303 can be a passive high-pass filter or any other type of audio frequency filter. In one or more embodiments, the high-pass cutoff frequency required to avoid exceeding the input signal level limits of the ADC 305 without signal attenuation is typically a value between 400 and 800 Hz, but the analog high-pass filter stage 303 can have a higher or lower cutoff frequency required to avoid exceeding the input limits of the ADC 305.

[0025] In the embodiment of Figure 3, analog high-pass filter stage 303 performs a function similar to one of the second-order high-pass filters 114 shown in Figure 1. As such, only one digital second-order high-pass filter 315 is included in DSP 306, and unity filter stage 316 is used in place of one of the second-order high-pass filters 114 shown in Figure 1. In one or more alternative embodiments, unity filter stage 316 may be omitted, or additional or alternative first, second, or higher order high-pass filter stages may be included in either or both of analog high-pass filter stage 303 or DSP 306 as necessary to achieve the desired crossover filtering function.

[0026] 3, the digital high frequency components output from the high pass filter 315 are then converted back to analog high frequency signal components in the first channel of the DAC 307. The analog high frequency components then pass through an analog unity gain stage 308, are amplified by a first power amplifier 310, and are audibly reproduced by a first loudspeaker driver 311 (i.e., a tweeter). In one or more alternative embodiments, the analog unity gain stage 308 may be omitted, such that the output of the first channel of the DAC 307 is sent directly to the power amplifier 310.

[0027] In one or more embodiments, the lack of attenuation of the high frequency audio signal components allows the DSP 306 to process the high frequency components with a higher effective bit resolution. Furthermore, the high frequency signal components do not need to be boosted after the DAC 307, thereby avoiding the introduction of additional noise and distortion to the high frequency components of the audio signal. In the embodiment of FIG. 3, the effective SNR of the high frequency components is increased by 15 dB, or 5 bits of resolution.

[0028] As shown in Figure 2, the low frequency components of the audio signal must be attenuated so that the larger peak signal amplitudes do not exceed the input range of the ADC 305. In the embodiment of Figure 3, the audio input signal that does not pass through the analog high pass filter 303 is attenuated by 15 dB in the analog attenuation stage 304 and then sent to the second ADC channel 317 of the ADC 305. In the embodiment of Figure 3, the attenuated audio signal then passes through a digital second order low pass filter 318 in the DSP 306 to generate the digital low frequency components. In one or more embodiments, additional or alternative first, second or higher order low pass filter stages may be included in the DSP 306 as needed to achieve the desired crossover filter function.

[0029] In the embodiment of FIG. 3, the digital signal output from low pass filter 318 Low FrequencyThe components are then converted back to analog low-frequency signal components in the second channel of the DAC 307. The analog low-frequency signal components are then boosted by the analog signal booster stage 309, amplified by the second power amplifier 310, and audibly reproduced by the second loudspeaker driver 311 (i.e., woofer). In one or more embodiments, the analog attenuation stage 304 and the analog signal booster stage 309 introduce some additional noise and distortion to the low-frequency components of the audio signal, but the noise and distortion is less audible than for high-frequency audio content because human hearing is less sensitive to noise and distortion at low frequencies than at mid- and high frequencies. Thus, in the embodiment of FIG. 3, the improved SNR and effective bit depth in the mid- and high-ranges provide a significant practical improvement in overall loudspeaker performance and audio fidelity.

[0030] In one or more embodiments, the audio signal may be split into three or more components. For example, in a three-way loudspeaker, the audio signal is split into low, mid and high frequency components. In one or more embodiments, the low frequency components are attenuated, digitized, digitally low pass filtered, converted back to analog, amplified and sent to a woofer speaker driver as described above, but with a lowered low pass filter cutoff, e.g., 300 Hz. Similarly, the high frequency components are analog high pass filtered, digitized, digitally high pass filtered, converted back to analog, amplified and sent to a tweeter speaker driver as described above, but with an elevated high pass filter cutoff, e.g., 2000 Hz.

[0031] Since mid-frequency components require less attenuation than low-frequency components, the mid-frequency components may be analog bandpass filtered and attenuated less than the low-frequency components before entering the ADC. For example, in one or more embodiments, an analog mid-band bandpass filter may have a lower cutoff of 300 Hz and an upper cutoff of 2000 Hz, with only 3 dB of mid-frequency attenuation. This is because there is a difference in power density between 300 Hz and 20 Hz:

number

[0032] In one or more embodiments, the audio signal on path 313 may be analog low-pass filtered before attenuation to remove the energy content of the high frequency components of the signal, thereby slightly reducing the attenuation required in analog attenuation stage 304 and the subsequent boost in analog signal booster stage 309. Although the high frequency components of the signal add only a small amount of additional energy to the signal, filtering them out can save approximately 2 dB of headroom. This can reduce the required attenuation and increase the low frequency resolution by 0.66 bits. In embodiments requiring the highest audio fidelity, this improvement may be worth the additional cost and complexity of an additional analog low-pass filter.

[0033] In the embodiment of FIG. 3, analog audio input stage 302, analog high-pass filter stage 303, analog attenuation stage 304, and ADC 305 are shown with balanced signal inputs and outputs, which are commonly used in line-level signal inputs and outputs of professional audio equipment to reduce the effects of external electromagnetic noise on the audio signal. In one or more alternative embodiments, analog audio input stage 302, analog high-pass filter stage 303, analog attenuation stage 304, and / or ADC 305 may instead use unbalanced signal inputs and / or outputs (i.e., single-ended signal wires and grounds as commonly used in consumer-grade audio equipment) for all or part of the pre-DSP signal path. Similarly, in the embodiment of FIG. 3, gain stages 308 and 309 and power amplifier 310 are shown with unbalanced signal inputs and outputs, but may instead use balanced signal inputs and / or outputs for all or part of the post-DSP signal path in one or more alternative embodiments.

[0034] Thus, a method is described for improving the effective signal-to-noise ratio of analog-to-digital and digital-to-analog conversion for active loudspeakers and other multiband digital signal processing devices by using two available channels of a stereo analog-to-digital conversion device to process low and high frequency components of the signal separately. Although the present invention has been described with respect to a particular embodiment, it will be apparent to one skilled in the art that the inventive features of the present invention are applicable to other embodiments, all of which are intended to be within the scope of the present invention. For example, the cutoff frequencies of the low pass, band pass, and / or high pass filters may be adjusted to suit the frequency response range of each speaker driver. Similarly, the amount of attenuation before the ADC and / or the amount of boost after the DAC may be adjusted according to the maximum peak-to-peak input signal level and the maximum allowable signal level of the ADC. Furthermore, the method may be used to improve the effective signal-to-noise ratio in any application using multiband digital signal processing, such as audio compressors, audio effects processors, audio and / or video recording devices, sound reinforcement or public address systems, or speech recognition, among others.

Claims

1. 1. A method for improving an effective signal-to-noise ratio for an active loudspeaker, comprising: The method comprises: Receives analog audio signals, high pass filtering the analog audio signal to generate a first high frequency analog signal; Converting the first high frequency analog signal into a high frequency digital signal; attenuating the analog audio signal by approximately 15 dB to generate an attenuated analog signal; converting the attenuated analog signal to an attenuated digital signal; applying digital signal processing to the high frequency digital signal and the attenuated digital signal; converting the high frequency digital signal into a second high frequency analog signal; converting the attenuated digital signal into a low frequency analog signal; amplifying the second high frequency analog signal to generate an amplified high frequency analog signal; amplifying the low frequency analog signal to generate an amplified low frequency analog signal; reproducing the amplified high frequency analog signal through a first loudspeaker driver; Reproducing the amplified low frequency analog signal through a second loudspeaker driver. The method includes:

2. The method of claim 1 , wherein high-pass filtering the analog audio signal includes selecting a cutoff frequency of approximately 600 Hz.

3. The method of claim 1 , wherein high-pass filtering the analog audio signal includes selecting a cutoff frequency between 400 Hz and 800 Hz.

4. The method of claim 1 , wherein applying the digital signal processing comprises applying a digital high pass filter to the high frequency digital signal.

5. The method of claim 4 , wherein the digital high-pass filter comprises a second order high-pass filter.

6. The method of claim 1 , wherein applying the digital signal processing comprises applying a digital low pass filter to the attenuated digital signal.

7. The method of claim 6 , wherein the digital low pass filter comprises a second order low pass filter.

8. The method of claim 6 , wherein the digital low pass filter comprises a plurality of cascaded low pass filters.

9. Amplifying the low frequency analog signal includes: boosting the low frequency analog signal by approximately 15 dB to generate a boosted low frequency analog signal; Amplifying the boosted low frequency analog signal using a power amplifier. The method of claim 1 , comprising:

10. an analog audio input stage; an analog high pass filter stage including a high pass filter input stage connected to the analog audio input stage and a high pass filter output stage; an analog attenuation stage including an attenuating input stage connected to the analog audio input stage and an attenuating output stage; a stereo analog-to-digital converter (ADC) including a first ADC channel input connected to the high pass filter output stage, a second ADC channel input connected to the attenuated output stage, a first ADC channel output, and a second ADC channel output; a digital signal processor (DSP) including a first DSP channel input connected to the first ADC channel output, a second DSP channel input connected to the second ADC channel output, a first DSP channel output, and a second DSP channel output; a stereo digital-to-analog converter (DAC) including a first DAC channel input connected to the first DSP channel output, a second DAC channel input connected to the second DSP channel output, a first DAC channel output, and a second DAC channel output; a first power amplifier including a first power amplifier input connected to the first DAC channel output and a first power amplifier output; a second power amplifier including a second power amplifier input connected to the second DAC channel output and a second power amplifier output; a first loudspeaker driver connected to the first power amplifier output; a second loudspeaker driver connected to the second power amplifier output; An active loudspeaker comprising:

11. 11. The active loudspeaker of claim 10, wherein the analog high-pass filter stage further comprises a cutoff frequency of approximately 600 Hz.

12. 11. An active loudspeaker in accordance with claim 10, wherein the analog high-pass filter stage further comprises a cut-off frequency between 400 Hz and 800 Hz.

13. 11. An active loudspeaker in accordance with claim 10, wherein the analog attenuation stage provides approximately 15 dB of attenuation.

14. 11. The active loudspeaker of claim 10, wherein the DSP further comprises a digital high pass filter connected between the first DSP channel input and the first DSP channel output.

15. 15. An active loudspeaker in accordance with claim 14, wherein the digital high-pass filter comprises a second order high-pass filter.

16. 11. The active loudspeaker of claim 10, wherein the DSP further comprises a digital low pass filter connected between the second DSP channel input and the second DSP channel output.

17. 17. An active loudspeaker in accordance with claim 16, wherein the digital low pass filter comprises a second order low pass filter.

18. 17. An active loudspeaker in accordance with claim 16, wherein the digital lowpass filter comprises a plurality of cascaded lowpass filters.

19. 11. The active loudspeaker of claim 10, wherein the second power amplifier further comprises an analog signal booster stage.

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