EP1440603A2

EP1440603A2 - Bass frequency amplifying apparatus

Info

Publication number: EP1440603A2
Application number: EP02765287A
Authority: EP
Inventors: Loic B. Tanghe
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2001-10-17
Filing date: 2002-10-14
Publication date: 2004-07-28
Also published as: WO2003034781A2; AU2002329002A1; US20040258246A1; CN1582602A; KR20040052236A; WO2003034781A3; JP2005506781A

Abstract

The invention concerns an apparatus for amplifying frequencies of bass sounds of an audio signal. The bass components are sent to an adjustable amplifier. The comparator circuit compares the amplifier input and output signals. When the feed forward signal level is higher than the feedback signal level, the bass performance is reduced, otherwise it is increased. The oscillation peak values are reduced in time and frequency for flat loudspeakers.

Description

Device for amplifying bass frequencies

The invention relates to a device for the amplification of bass frequencies of an audio signal, which has first means for splitting the audio signal into a first signal part and into a second signal part, an amplifier which has a peak value compressor with a controllable nerp gain circuit and a control circuit for compressing the second signal part has and with second means for of the first signal part and the compressed second signal part.

Such a device is known from US Pat. No. 6,359,655. This device has an amplifier that can be regulated via feedback, the amplification of which varies depending on the bass components. When the level is low, the amplification of the bass signals is higher, and at high levels it is lower. This type of bass amplification is sufficient to achieve good acoustics via electrodynamic loudspeakers with a conical loudspeaker membrane.

This type of bass enhancement is not suitable for sound reproduction via flat speakers. Flat loudspeakers have a radiation surface lying in one plane, hereinafter also referred to as a panel, and are also referred to as loudspeakers operating in distributed mode (distributed mode speakers). Flat speakers are being used more and more for aesthetic reasons. With the flat speakers the bass reproduction can be satisfied less and less. According to the mechanical construction, vibration of a flat speaker is limited compared to an electrodynamic speaker with a conical membrane. Amplification of the bass frequencies by electrical means can quickly lead to distortion, in particular mechanical clipping and acoustic interference, often enough before electrical clipping by the amplifier.

The invention is therefore based on the object of specifying an amplifier for flat loudspeakers.

This object is achieved in accordance with the features of claim 1. According to the invention, the amplifier has a comparison circuit with a feed forward loop part to an input and with a feedback loop part to an output of the peak compressor. The feedforward loop part is part of a feedforward control that prevents a lethal peak signal from being fed to the amplifier and then passed on to the speaker. This specifies a forward-regulated, bass-compressing audio system and method that increases the bass performance and at the same time prevents perceptible disturbances such as mechanical cutting of the loudspeaker. The average value of the bass signal is maximized in both time and frequency. The idea is to add bass signals to the surface speakers that maximize the perceptible sound while reducing vibration peaks in time and frequency for surface speakers.

The comparison circuit advantageously applies input values from the feed-forward loop part with a factor of 0.3-0.7, advantageously between 0.4-0.6 and in particular with a factor of 0.5. If a peak value detected in the forward control is amplified by a factor of 0.5, which corresponds to a compression rate of 50%, the volume control can be switched more than 3 dB higher than when the compression peak value is switched off. The regulation of the volume is also dependent on the decay time of the gain control in the peak value compression device, the decay time is also referred to below as the decay time.

The first means for splitting the audio signal advantageously have a bandpass. With band-limited signals, the compressor peak value can work properly. In addition, there is no need to inflate frequencies, especially frequencies that are too low, that the loudspeaker cannot reproduce. Advantageously, the first means for splitting the

Audio signal an addition circuit. If frequencies are amplified which are below the resonance frequency of the loudspeaker, i.e. frequencies whose source a listener cannot locate, bass frequencies of two or more channels can be combined in one or more addition circuits, processed by a single compressor and reproduced via one or more loudspeakers ,

While the bass signals are being processed, the signals on the direct path can be filtered by a freely selectable high-pass filter above the resonance frequency of the loudspeaker. That is why only the individually processed, minimized vibration peak values of the bass signal are fed to the loudspeaker. The amplifier advantageously has an automatic gain control. The gain control checks the output levels of the audio arrangement and reduces the power in the bass path if the checked levels are above a defined target level. This prevents mechanical clipping at very high output levels. The output levels are the levels that result after the addition of the first and the second compressed signal part and after the manually adjustable volume control.

For a better understanding of the invention, an embodiment is explained below with reference to the drawing. Show it

Fig. 1 is a block diagram of a device for amplifying bass frequencies with a peak compressor and

Fig. 2 is a circuit diagram of the peak compressor

FIG. 1 shows a device 1 for amplifying bass frequencies with two inputs 2 and 3, an adder 4, a high pass 5, a band pass 6, a second high pass 7, an amplifier 8, two further adders 9 and 10, two outputs 11 and 12 and two further inputs 13 and 14. The amplifier 8 has a peak value compressor 15 and an automatic gain control 16. The peak value compressor 15 has a controllable amplifier circuit 17, a control circuit 18 and a comparator 19. A feedforward loop part 20 connects an input 21 of the controllable amplifier 17 to the comparator 19 and a feedback loop part 22 connects an output 23 of the controllable amplifier 17 to the comparator 19. The automatic gain control 16 has a further controllable one

Amplifier circuit 24, a further control circuit 25 and a second comparator

26 on. At outputs 11 and 12 there are two manually operated volume controls

27 and 28 connected, which emit signals to (not shown) speakers. The outputs of the volume controls 27 and 28 are connected to the inputs 13 and 14, which control the second comparator 26 by means of electrically conductive connections.

The function of the device is as follows: Two digital audio signals AL and AR of a stereo system with a left and a right channel are applied to inputs 2 and 3 of device 1. Both audio signals AL and AR are fed to the adder 4, summed up and a summed signal is in the bandpass 6 in the band limited. With the bandpass 6, the forward-regulated peak value compressor 15, which compresses the peak values of the amplitude of the bass signal, and the automatic gain control 16, which reduces the values when the output levels are too high, the device 1 has a singularly executed bass path 29 , The bandpass 6 is a DR filter with an irifinite impulse response (ILR = infinite impulse response). The peak value compressor 15 calms the peak values of the signal, while at the same time protecting or maintaining the tail of the pulse. At the input of the bass path 29 is the adder 4, which adds the signals of the left and right channels. The output signal of the bass path 29 is then added directly to the high-pass filtered signals of a left and right path 30 and 31.

FIG. 2 shows the peak value of the compressor 15 with the amplifier circuit 17, the control circuit 18 and the comparator 19. The comparator 19 has a level germination circuit 33 at its input 32, which is located towards the feedforward loop part 20. Circuit 33 detects the largest signal peak by forward control with a very short response time of 0.1 ms and a very long decay time of 3 seconds. The level detection circuit 33, also called filter below, is supplied with a digital input signal x of an n value and the detection circuit 33 outputs a digital signal y of the n value, which can be calculated as follows with constants Ta and Tr:

y (n) = | x (n) | + Kp * (y (n-l) - | x (n) |) + Km * | (y (n-l) - | x (n) |) | With

Kp = (Kr + Ka) / 2

Km = (Kr-Ka) / 2 Kr = exp (-l / (Tr * pattern frequency))

Ka = exp (-l / (Ta * pattern frequency))

The absolute value of the input signal is thus supplied to a first-order attack and release filter 33. With a short settling time Ta and a longer settling time Tr, the peak value of the input signals x (n) is present at the output of the filter 33 with the signal y (n). The pattern frequency is defined by the time interval between the occurrence of two successive patterns.

This is followed by an amplifier 34. The amplifier 34 amplifies the input signal y (n) with a factor of 0.3-0.7, advantageously between 0.4-0.6 and in particular with a factor of 0.5. On a second, the feedback loop Another level detection circuit 36 is located at the input 35 facing the loop part 22. The function of this level detection circuit 36 works according to the same function as explained for the level detection circuit 33. The detection circuit 33 sends signals via the amplifier 34 and the detection circuit 36 sends signals directly to a comparison circuit 37. From the comparison circuit 37 an output signal reaches the control circuit 18 with a gain reduction circuit 38 and a gain increase circuit 39. The circuits 33-39 operate as follows:

In the comparison circuit 37, the filter response of the filter 33 reduced by 50% is compared with the filter response of the filter 36. If the value changed by 50% is greater than the filter response from the feedback loop of the filter 36, the gain of the amplifier is reduced in the reducing circuit 38 with the constant DEC_GALN. If the value is larger, the amplification of the amplifier 17 in the amplification boost circuit 39 is increased with the constant INC_GAIN. Gain factor limits are 1.0 and 0.01.

Gain = Gain * LNC_GALN Gain = Gain * DEC_GAIN with

INC_GAΓN = 10 exp (24 / (settling time ^ pattern frequency * 20)) DEC_GALN = 10 exp (-24 / (decay time * pattern frequency * 20))

The filter response of filter 33 reduced by 50% becomes a target level. This target level is compared to the peak detected by feedback and the control circuit 18 adjusts the gain applied to the output to set the target level with a very short decay time of 20 ms to very quickly intervene through the input signal remove. The 50 ms rise time takes longer to maintain the time frame for the entire duration of the signal intervention, regardless of which the signal is weakened. The compressor 15 calms the peak values of the signals regardless of the volume control and without adding unwanted interference. The sound is kept clean and is free of artifacts, i.e. of undesirable perceptible disturbances. Because level detection is feedforward, the peak compression depends only on the input signals AL and AR. With this device, high peak compression rates can be achieved that are less than 0.5 times the peak level detected from the feedforward control. Because the vibration of the conical loudspeaker membrane or the panel is important here, the vibration curve in relation to the frequency must be observed. This ratio is determined by speaker parameters such as the Small and Thiele parameters. From the curve and the pressure response of the speaker, the possible bass boost can be derived that maximizes the bass frequency pressure response while not increasing the maximum in the vibration curve.

With an amplification in bandpass 6, more noticeable bass frequencies can be achieved, only peaks of bass frequencies are reduced.

LIST OF REFERENCE NUMBERS

1 bass frequency amplifier 30 path

2 entrance 31 path

3 input 32 input

4 adders 33 level detection circuit

5 high pass 34 amplifiers

6 Bandpass 35 entrance

7 high pass 36 level detection circuit

8 amplifier 37 comparison circuit

9 adder 38 gain reduction circuit

10 adders 39 boosting circuit

11 exit

12 exit

13 entrance

14 entrance

15 Compressor peak value

16 Automatic gain control

17 amplifier circuit

18 control circuit

19 comparator

20 feedforward loop part

21 entrance

22 Reverse loop part

23 exit

24 amplifier circuit

25 control circuit

26 comparator

27 Volume control

28 Volume control

29 bass path

Claims

EXPECTATIONS:

1. Device (1) for amplifying bass frequencies of an audio signal (AL, AR), which has first means (4, 5, 6 and 7) for splitting the audio signal (AL, AR) into a first signal part and into a second signal part , An amplifier (8) having a peak value compressor (15) with a controllable amplification circuit (17) and a control circuit (18) for compressing the second signal part and with second means (9, 10) for bringing together the first signal part and the compressed second signal part, characterized in that the peak value compressor (15) has a comparison circuit (19) with a feedforward loop part (20) to an input (21) and with a feedback loop part (22) to an output (23) of the peak value compressor ( 15).

2. Device according to claim 1, characterized in that the comparison circuit (19) from the input (22) incoming input values with a factor of 0.3 - 0.7, advantageously from 0.4 - 0.6 and in particular with a factor of 0.5.

3. Device according to claim 1, characterized in that the first means (4, 5, 6 and 7) for splitting the audio signal (AL, AR) have a bandpass (6).

4. Apparatus according to claim 1 or 3, characterized in that the first means (4, 5, 6 and 7) for splitting the audio signal (AL, AR) have an addition circuit (4).

5. Apparatus according to claim 1 or 2, characterized in that the amplifier (8) has an automatic gain control (16).

6. Method for amplifying bass frequencies of a left and a right digital audio signal (AL, AR) of a stereo system with a left and with a right channel in the following method steps: - the audio signal (AL, AR) is bandpass limited,

the bandpass-limited audio signal is then compressed in a forward and backward-coupled peak value compressor,

- Furthermore, the audio signal (AL, AR) is high-pass filtered and

- The compressed audio signal is added to the high-pass filtered audio signal.

7. The method according to claim 6, characterized in that the left and the right audio signal are added before the bandpass limitation and the added signal is added to the respective high-pass filtered audio signal after compression.