GB2502136A - An output filter for a DAC in an audio playback system - Google Patents

An output filter for a DAC in an audio playback system Download PDF

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Publication number
GB2502136A
GB2502136A GB1208739.1A GB201208739A GB2502136A GB 2502136 A GB2502136 A GB 2502136A GB 201208739 A GB201208739 A GB 201208739A GB 2502136 A GB2502136 A GB 2502136A
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GB
United Kingdom
Prior art keywords
predetermined
frequency
predetermined frequency
filter circuit
filter
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
GB1208739.1A
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GB201208739D0 (en
Inventor
Colin James Wonfor
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
TELLURIUM Q Ltd
Original Assignee
TELLURIUM Q Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by TELLURIUM Q Ltd filed Critical TELLURIUM Q Ltd
Priority to GB1208739.1A priority Critical patent/GB2502136A/en
Publication of GB201208739D0 publication Critical patent/GB201208739D0/en
Publication of GB2502136A publication Critical patent/GB2502136A/en
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H03M1/06—Continuously compensating for, or preventing, undesired influence of physical parameters
    • H03M1/0617—Continuously compensating for, or preventing, undesired influence of physical parameters characterised by the use of methods or means not specific to a particular type of detrimental influence
    • H03M1/0626—Continuously compensating for, or preventing, undesired influence of physical parameters characterised by the use of methods or means not specific to a particular type of detrimental influence by filtering
    • H03M1/0631—Smoothing
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H03M1/06—Continuously compensating for, or preventing, undesired influence of physical parameters
    • H03M1/0617—Continuously compensating for, or preventing, undesired influence of physical parameters characterised by the use of methods or means not specific to a particular type of detrimental influence
    • H03M1/0626—Continuously compensating for, or preventing, undesired influence of physical parameters characterised by the use of methods or means not specific to a particular type of detrimental influence by filtering
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H03M1/06—Continuously compensating for, or preventing, undesired influence of physical parameters
    • H03M1/08—Continuously compensating for, or preventing, undesired influence of physical parameters of noise
    • H03M1/0854—Continuously compensating for, or preventing, undesired influence of physical parameters of noise of quantisation noise
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H03M1/66—Digital/analogue converters
    • H03M1/661—Improving the reconstruction of the analogue output signal beyond the resolution of the digital input signal, e.g. by interpolation, by curve-fitting, by smoothing
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H03M1/66—Digital/analogue converters

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  • Engineering & Computer Science (AREA)
  • Theoretical Computer Science (AREA)
  • Tone Control, Compression And Expansion, Limiting Amplitude (AREA)

Abstract

An electronic filter device is described which is stated to effectively remove noise caused by digital to analogue converter clocking frequencies and harmonics thereof. It is stated that an analogue audio signal with natural high-quality sound is provided.  The filter device comprises a pre-emphasis filter 2 followed by a low-pass filter 5.

Description

ELECTRONIC FILTER DEVICES
The present invention relates to electronic filter devices.
BACKGROUND OF THE INVENTION
Playback of digitally stored audio recordings, particularly music recordings, can suffer from harmonic distortion caused by high frequency limited sampling resolution inherent in all digital to analogue conversion techniques.
Digital systems typically operate at clocking frequencies of 44.1 kHz, 48 kHz, or 192 kHz.
These clocking frequencies, and harmonics of these frequencies, can produce unwanted noise in the analogue playback signal. Current solutions that are intended to prevent or reduce this noise make use of a low pass filter which passes signals having frequencies in the range 20 Hz to 20 kHz, and reduces gain at 6dB per octave thereafter. As such, the gain at 40 kHz is only -3dB (i.e. attenuation of 3dB). Such techniques do not effectively limit the amount of noise due to the clocking frequency of the digital circuit in the output analogue signal. In addition, existing solutions cause a change in phase of the analogue output signal across the frequency range. Such changes in phase angle cause audio imaging errors (i.e. placement in the perceived sound stage of instruments and/or performers appears to move from its original location). These drawbacks can lead to lower quality output, and to audio fatigue when listening for extended periods.
It is, therefore, desirable to provide a filter device that can overcome the drawbacks of the previously considered solutions, and that can provide an audio output signal of desirable quality and natural sound.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided an electronic filter device comprising a first filter circuit for receiving an output from a digital to analogue converter as an input signal, and operable to generate an intermediate signal from a received input signal, a second filter circuit for receiving an intermediate signal from the first filter circuit, and operable to generate an output signal from an intermediate signal received from the first filter circuit, wherein the first filter circuit is a pre-emphasis filter having a first frequency response, and is operable to increase amplitudes of signals above a first threshold frequency, which first threshold frequency is below a digital clock frequency of such a digital to analogue converter, and wherein the second filter circuit is a low pass filter having a second frequency response, and is operable to amplify signals below a second threshold frequency, which second threshold frequency is below a digital clock frequency of such a digital to analogue converter, and is operable to attenuate signals above the second threshold frequency.
In one example, the second frequency response is substantially the inverse of the first frequency response.
In one example, the first filter circuit is configured to have substantially zero gain at a first predetermined frequency, a predetermined attenuation at a second predetermined frequency lower than the first predetermined frequency, and a predetermined gain at a third predetermined frequency higher than the first predetermined frequency.
In one example, the second filter circuit is configured to have substantially zero gain at a fourth predetermined frequency, a predetermined gain at a fifth predetermined frequency lower than the fourth predetermined frequency, and a predetermined attenuation at a sixth predetermined frequency higher than the fourth predetermined frequency.
In one example, the fourth predetermined frequency is substantially equal to the first predetermined frequency, and wherein the fifth predetermined frequency is substantially equal to the second predetermined frequency, and wherein the sixth predetermined frequency is substantially equal to the third predetermined frequency.
In one example, the predetermined gain of the first filter circuit is substantially equal to the predetermined gain of the second filter circuit, and wherein the predetermined attenuation of the first filter circuit is substantially equal to the predetermined attenuation of the second filter circuit.
In one example, the first filter circuit is configured to have substantially zero gain at a first predetermined frequency, a predetermined attenuation at a second predetermined frequency lower than the first predetermined frequency, and a predetermined gain at a third predetermined frequency higher than the first predetermined frequency, and wherein the second filter circuit is configured to have substantially zero gain at the first predetermined frequency, a predetermined gain at the second predetermined frequency, and a predetermined attenuation at the third predetermined frequency.
In one example, the first predetermined frequency is 1 kHz, the second predetermined frequency is 20 Hz, and the third predetermined frequency is 20 kHz, and wherein the predetermined attenuation is 20 dB, and the predetermined gain is 20 dB.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure us a block diagram illustrating an electronic filter circuit embodying one aspect of the present invention; Figure 2 illustrates a frequency response of a first filter circuit of the device of Figure 1; Figure 3 illustrates a frequency response of a second filter circuit of the device of Figure 1; Figure 4 is a block diagram of a first filter circuit for use in the filter device of Figure 1; and Figure 5 is a block diagram of a second filter circuit for use in the filter device of Figure 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 1 illustrates a filter device 1 embodying one aspect of the present invention. The device 1 includes a first filter circuit 2 having an input 3 for receiving an input signal to be filtered. The input signal is an analogue signal generated as an output of an digital to analogue converter. The first filter circuit 2 operates to filter the input signal received through the input 3, to produce an intermediate signal. The intermediate signal is supplied from an output 4 of the first filter circuit ito a second filter circuit 5.
The second filter circuit 5 comprises a differential amplifier 6, having first and second differential inputs 7 and 8, and an output 9. The output 9 serves as the output of the second filter circuit Sand of the filter device 1 as a whole. A feedback network 10 is connected to receive a signal supplied on the output 9, and operates to filter that signal to provide a feedback signal ii to the differential amplifier 6.
The second filter circuit 5 is, therefore, an active filter circuit, due to the presence of the differential amplifier 6. As is well known, the differential amplifier 6 operates to amplify the difference between the respective inputs received at the first and second differential inputs 7 and 8, to produce an output signal for supply from the output 9.
The first filter circuit 2 may be provided by a passive filter circuit, or by an active filter circuit.
Both first and second filter circuits are analogue filter circuits.
Figure 2 illustrates the frequency response of the first filter circuit 2 of Figure 1. The frequency response is shown as gain (y-axis) plotted against frequency (x-axis). The first filter circuit 2 has a gain of 0dB at a first predetermined frequency 10, as indicated by the crossing of the x-axis by the frequency response at frequency 10 in Figure 2.
At a second predetermined frequency 12, below the first predetermined frequency 10, the first filter circuit 2 exhibits attenuation of the input signal. That is, the first filter circuit 2 has a predetermined reduced gain 13 at the second predetermined frequency 12, as illustrated in Figure 2.
At a third predetermined frequency 14, above the first predetermined frequency 10, the first filter circuit 2 operates as a pre-emphasis filter that increases the amplitude of the input signal. The first filter circuit 2 has a predetermined increased gain 15 at the third predetermined frequency 14, as illustrated in Figure 2.
This combination of frequency characteristics described above lead to the first filter circuit 2 being described as an "S-curve" filter, with a centre atthefirst predetermined frequency, predetermined attenuation at the second predetermined frequency! and predetermined gain at the third predetermined frequency.
Figure 3 illustrates the frequency response of the second filter circuit 5 of Figure 1. The frequency response is shown as gain (y-axis) plotted against frequency (x-axis). The second filter circuit 5 has a gain of 0dB at a fourth predetermined frequency 16, as indicated by the crossing of the x-axis by the frequency response at frequency 16 in Figure 3.
At a fifth predetermined frequency 18, below the fourth predetermined frequency 16, the second filter circuit 5 exhibits gain of the intermediate signal supplied as an input to the second filter circuit 5 from the first filter circuit 2. The second filter circuit 5 has a predetermined gain 19 at the fifth predetermined frequency 18, as illustrated in Figure 3.
At a sixth predetermined frequency 20, above the fourth predetermined frequency 16, the second filter circuit S exhibits attenuation of the intermediate signal. The second filter circuit has a predetermined reduced gain 21 at the sixth predetermined frequency 20, as illustrated in Figure 3.
This combination of frequency characteristics described above lead to the second filter circuitS also being described as an "S-curve" filter, with a centre at the fourth predetermined frequency, predetermined gain at the fifth predetermined frequency, and predetermined attenuation at the sixth predetermined frequency.
The frequency response characteristics of the first and second filter circuits 2 and 5 (of Figure 1) are complementary to one another -the second filter circuit 5 operates to amplify those signals that the first filter circuit has attenuated, and to attenuate those signals that have been emphasized. The first filter circuit 2 emphasizes the digital clocking signals, and harmonics of those signals, in order to reduce harmonic distortion in the signal.
The second filter circuit S then attenuates significantly the digital clock frequencies and harmonics thereof, whilst amplifying the frequencies in the audio range.
In a particularly advantageous example of a filter device embodying the present invention, the frequency response characteristics of the first and second filter circuits 2 and 5 are substantially the inverse of one another. In such a case, the centre (or zero gain) frequencies are substantially equal -the first and fourth predetermined frequencies 10 and 16 are substantially equal to one another. In addition, the second and fifth predetermined frequencies 12 and 18 are substantially equal to one another, and the third and sixth predetermined frequencies 14 and 20 are substantially equal to one another. Furthermore the respective aftenuations 13 and 2lat the second and sixth predetermined frequencies 12 and 18 are substantially equal to one another, as are the respective gains 15 and 19 at the third and fifth predetermined frequencies 14 and 20.
In one particular example, the centre frequencies (the first and fourth predetermined frequencies 10 and 16) are set at or about 1 kHz. The second and fifth predetermined frequencies 12 and 18 are set at or about 20 Hz and the third and sixth predetermined frequencies 14 and 20 are set at or about 20 kHz. The respective attenuations at the second and sixth predetermined frequencies 12 and 20 are both set at or about -20dB, and the respective gains at the third and fifth predetermined frequencies 14 and 18 are set at or about +20dB. This combination of frequency choice and gain set points provides an overall band pass characteristic with a pass band from around 20 Hz to around 20 kHz (+1-0.1 dB) with frequencies higher than 20 kHz effectively being removed. This naturally includes the clocking frequencies and harmonics of the digital circuit.
In this way, the filter device embodying the present invention is operable to remove or attenuate harmonic distortion caused by high frequency noise resulting from sampling resolution of digital to analogue conversion. This result is achieved by effectively increasing the harmonic distortion in the incoming signal using the first filter stage. The device is then able to produce an audio output that is closer to the sound of an audio track recorded on a vinyl disk. Such a sound is pleasing since the method of recording onto a vinyl disk is analogue, and so the resulting reproduced sound is often considered to have more natural quality than converted digital signals. The device of the present invention enables the desirable sound to be created from a digital audio source. Applications for the device include any device that makes use of digital audio storage, such as mobile devices, CD and DVD players, digital television, portable computing devices, and digital music streaming devices and systems.
The device can be produced as a discrete circuit, on one or more integrated circuits, or can be included in another circuit such as an amplifier or digital to analogue converter.
One possible example circuit diagram for the first filter circuit 2 is illustrated in Figure 4. The example shown in Figure 4 includes first and second resistor-capacitor (RC) networks SC1 and RC2. The first and second SC networks SC1 and 5C2 are connected in parallel between the input 3 and the output 4 of the first filter circuit 2. A resistor 22 connects the output 4 to ground. The first SC network RC1 comprises a first capacitor Cl connected in series with a first resistor 51, and the second SC network 5C2 comprises a second capacitor C2 connected in parallel with a second resistor R2.
Figure 5 illustrates one possible circuit suitable for the second filter circuit 5, which comprises the differential amplifier 6. The amplifier 6 receives the intermediate signal 4 at the first differential input 7, and supplies an output signal 9. The feedback network 10 includes third and fourth SC networks RC3 and RC4, and receives the output signal 9 as an input, and provides an output signal to the second differential input 8 of the amplifier 6. The third and fourth SC networks 5C3 and RC4 are connected in parallel with one another. The third SC network 5C3 comprises a third capacitor C3 connected in series with a third resistor S3. The fourth SC network RC4 comprises a fourth capacitor C4 connected in parallel with a fourth resistor 54. A resistor 24 connects the second differential input 8 with ground.

Claims (9)

  1. CLAIMS: 1. An electronic filter device comprising: a first filter circuit for receiving an output from a digital to analogue converter as an input signal, and operable to generate an intermediate signal from a received input signal; a second filter circuit for receiving an intermediate signal from the first filter circuit, and operable to generate an output signal from an intermediate signal received from the first filter circuit, wherein the first filter circuit is a pre-emphasis filter having a first frequency response, and is operable to increase amplitudes of signals above a first threshold frequency, which first threshold frequency is below a digital clock frequency of such a digital to analogue converter, and wherein the second filter circuit is a low pass filter having a second frequency response, and is operable to amplify signals below a second threshold frequency, which second threshold frequency is below a digital clock frequency of such a digital to analogue converter, and is operable to attenuate signals above the second threshold frequency.
  2. 2. A filter device as claimed in claim 1, wherein the second frequency response is substantially the inverse of the first frequency response.
  3. 3. A filter device as claimed in claim 1 or 2, wherein the first filter circuit is configured to have substantially zero gain at a first predetermined frequency, a predetermined attenuation at a second predetermined frequency lower than the first predetermined frequency, and a predetermined gain at a third predetermined frequency higher than the first predetermined frequency.
  4. 4. A filter device as claimed in claim 1, 2 or 3, wherein the second filter circuit is configured to have substantially zero gain at a fourth predetermined frequency, a predetermined gain at a fifth predetermined frequency lower than the fourth predetermined frequency, and a predetermined attenuation at a sixth predetermined frequency higher than the fourth predetermined frequency.
  5. 5. A filter device as claimed in claim 4, when appended to claim 3, wherein the fourth predetermined frequency is substantially equal to the first predetermined frequency, and wherein the fifth predetermined frequency is substantially equal to the second predetermined frequency, and wherein the sixth predetermined frequency is substantially equal to the third predetermined frequency.
  6. 6. A filter device as claimed in claim 5, wherein the predetermined gain of the first filter circuit is substantially equal to the predetermined gain of the second filter circuit, and wherein the predetermined attenuation of the first filter circuit is substantially equal to the predetermined attenuation of the second filter circuit.
  7. 7. A filter device as claimed in claim 1 or 2, wherein the first filter circuit is configured to have substantially zero gain at a first predetermined frequency, a predetermined attenuation at a second predetermined frequency lower than the first predetermined frequency, and a predetermined gain at a third predetermined frequency higher than the first predetermined frequency, and wherein the second filter circuit is configured to have substantially zero gain at the first predetermined frequency, a predetermined gain at the second predetermined frequency, and a predetermined attenuation at the third predetermined frequency.
  8. 8. A filter device as claimed in claim 7, wherein the first predetermined frequency is 1 kHz, the second predetermined frequency is 20 Hz, and the third predetermined frequency is 20 kHz, and wherein the predetermined attenuation is 20 dB, and the predetermined gain is 20 dB.
  9. 9. A filter device substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.
GB1208739.1A 2012-05-18 2012-05-18 An output filter for a DAC in an audio playback system Withdrawn GB2502136A (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
GB1208739.1A GB2502136A (en) 2012-05-18 2012-05-18 An output filter for a DAC in an audio playback system

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB1208739.1A GB2502136A (en) 2012-05-18 2012-05-18 An output filter for a DAC in an audio playback system

Publications (2)

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GB201208739D0 GB201208739D0 (en) 2012-07-04
GB2502136A true GB2502136A (en) 2013-11-20

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GB1208739.1A Withdrawn GB2502136A (en) 2012-05-18 2012-05-18 An output filter for a DAC in an audio playback system

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Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4755771A (en) * 1986-03-12 1988-07-05 Oki Electric Industry Co., Ltd. Equalizing circuit
US5420585A (en) * 1992-09-09 1995-05-30 Peter Qvortrup Digital to analog conversion devices
US6721427B1 (en) * 1999-06-08 2004-04-13 Zanden Audio System Co., Ltd. Analog filter for digital audio system and audio amplifier for using the same
US7038608B1 (en) * 2004-12-16 2006-05-02 Valeo Raytheon Systems, Inc. Digital to analog converter
US7390960B1 (en) * 2003-07-18 2008-06-24 Jeffrey Arnold Electronic signal processor
US8023665B2 (en) * 1998-05-04 2011-09-20 Schwartz Stephen R Microphone-tailored equalizing system

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4755771A (en) * 1986-03-12 1988-07-05 Oki Electric Industry Co., Ltd. Equalizing circuit
US5420585A (en) * 1992-09-09 1995-05-30 Peter Qvortrup Digital to analog conversion devices
US8023665B2 (en) * 1998-05-04 2011-09-20 Schwartz Stephen R Microphone-tailored equalizing system
US6721427B1 (en) * 1999-06-08 2004-04-13 Zanden Audio System Co., Ltd. Analog filter for digital audio system and audio amplifier for using the same
US7390960B1 (en) * 2003-07-18 2008-06-24 Jeffrey Arnold Electronic signal processor
US7038608B1 (en) * 2004-12-16 2006-05-02 Valeo Raytheon Systems, Inc. Digital to analog converter

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