Sound reproduction system

Abstract: The invention relates to a sound reproduction system comprising a source of a sound signal in electrical form, a first filter group (13) for dividing the signal derived from the sound signal source into several sound signals occurring in different frequency bands and an adjustable gain amplifier (2) for each frequency band for amplifying the sound signal occurring in said frequency band. To take into account the noise existing in the environment of the sound reproduction system in the adjustment of the system, the system further comprises means (3) for generating an electrical signal proportional to the background noise existing in the environment and means (CS1...CSN) for adjusting the gain of the amplifiers in response to said electrical signal proportional to the background noise.

Description: The present invention relates to a sound reproduction system comprising a source of a sound signal in electrical form and an adjustable gain amplifier for amplifying said sound signal. A system of this type is e. g. described in EP 0027519 as an on-vehicle automatic loudness control apparatus. This system contains a microphone producing an output signal representative for the combined wanted signal level from the loudspeaker and the unwanted noise from motor, tires, wind etc. present in the passenger space of a car. The system further contains a summing circuit adding the modified (squared) signals from the audio input and the microphone, one of which with reverse polarity. It is the object of the described system to provide automatic volume control depending on the level of disturbing noise. The system described in the above mentioned EP 0027519 works independent from the frequency or frequency range of wanted audio signal or unwanted noise. Another similar system is described in US 5 077 799. This system contains a microphone producing a signal representative for only the ambient noise level. The microphone output signal is used to generate a control voltage for a variable resistance device. The control voltage circuit has different time constant elements for controlling volume up or down. Also this circuit does not work depending on the frequency or frequency range of wanted input signal or ambient noise signal. A further similar system is described in DE 4021969 which can be explained as a refinement of the solution as described in EP 0027519, before summing microphone and audio signal are filtered and 2 different signal paths are provided, one for the (normal) wanted audio signal and a parallel one for an effect sound derived from the wanted audio signal. Also this system works independent from input audio frequency or frequency range and ambient noise frequency or range. The present invention makes use of the insight, that it is not necessary to increase the audio level over the whole audio frequency range when the unwanted noise signal level increases at a certain frequency or frequency range if the wanted audio signal at this moment contains a signal in the same frequency range strong enough to mask this unwanted noise. The object of the present invention is to provide a sound reproduction system capable of taking into account changes in the background noise and divide the wanted audio signal frequency range into several bands each reacting independently from each other on the noise detected in the listener space. Conventionally, such sound reproduction systems comprise a source of a sound signal in electrical form, a first filter group for dividing the signal derived from the sound signal source into several sound signals occurring in different frequency bands and an adjustable gain amplifier for each frequency band for amplifying the sound signal occurring in said frequency band. To achieve background noise correction in accordance with the invention, such a sound reproduction system further comprises at least one microphone for generating an electrical signal proportional to the total sound existing in the environment of the sound reproduction system, another filter group for dividing the total sound signal obtained from the microphone into several band signals occurring in different frequency bands corresponding to the frequency bands of the first filter group, means for each frequency band for determining the band sound level signals corresponding to the band signals of the total sound signal which exist in said bands and means for each frequency band for subtracting the electrical signal representing the sound level generated by the amplifier of each frequency band from the band sound level signal occurring in the respective frequency band for generating signals proportional to the level of background noise existing in the different frequency bands, and means for each frequency band for adjusting its amplifier gain in response to a signal proportional to the level of background noise existing in said frequency band. In the following, the sound reproduction system of the invention will be described in closer detail with reference to the accompanying drawing in which Figure 1 illustrates a sound reproduction system of the prior art, Figure 2 shows an exemplary circuit for a sound reproduction system of the invention for one of the sound signal bands. Figure 3 is a block diagram of the sound reproduction system of Figure 2, and Figure 4 is a block diagram of the sound reproduction system of the invention. Figure 1 shows a conventional sound reproduction system comprising a source 1 of a sound signal in electrical form and an amplifier 2 amplifying the signal derived from said source. The gain of the amplifier 2 is adjusted by means of signal V, which is shown as applied via a multiplier 11, one of the inputs of said multiplier having the value 1. The sound is reproduced by loudspeaker 15. The sound signal source 1 may be any conventional sound source, such as a tuner, a tape recorder or a CD player. Figure 2 shows a schematic circuit diagram for one channel of a sound reproduction system of the invention. It differs from the system of Figure 1 in that in addition to the amplifying signal V, another signal is applied to the multiplier 11, said another signal correcting the gain of the amplifier 2 in such a way that the gain will be set in accordance with the noise level existing in the environment of the sound reproduction system. In Figure 2, the system effecting the correction is indicated with the reference CS. An essential part of this correction system CS is a microphone 3 which is mounted in the interior of the car and thus generates a signal proportional to the total sound existing in the listening space. In the circuit of Figure 2, this signal generated by the microphone 3 is processed in block 7, which computes the level of the sound from the microphone 3, i.e. produces the total sound level signal. To generate a signal proportional to the level of the music coming from the loudspeaker 15 of the sound reproduction system, the system comprises blocks 10, 5 and 6, the input of block 10 being connected to the output of the amplifier 2. Block 10 is a time-delay means delaying the electrical signal for a delay time corresponding to the travel time of sound from loudspeaker 15 to microphone 3. Thus the signals from the microphone and the time-delay unit 10 are simultaneous. After the unit 10, the signal is applied to a multiplier 5 wherein it is multiplied by a gain correction constant vk to achieve a suitable signal level. This gain correction vk is largely dependent on the sensitivity level of the microphone 3. From the multiplier 5, the signal is supplied to block 6 wherein a power level corresponding to the signal is calculated. The power levels obtained from blocks 6 and 7 are subtracted from one another in a summing unit 4, an electrical signal proportional to the background noise level being obtained from the output of the summing unit. The useful signal, i.e. the signal generated by the sound reproduction system, has thus already been subtracted from said electrical signal. This noise level signal is applied to block 8 wherein it is processed to eliminate too abrupt changes from the signal. By such processing, the occurrence of too abrupt changes in the gain of the amplifier 2 is prevented. From block 8, the signal proportional to the noise level is supplied to multiplier 9 wherein it is multiplied by signal EL, which is a correction intensity level signal and makes it possible to adjust the signal level so that the signal proportional to the noise level can influence the amplifier 2 in the desired ratio. From multiplier 9, the signal is applied to a summing unit 12 wherein the value 1 is added thereto. Thereafter the signal is applied to multiplier 11 wherein it is multiplied by the normal amplification signal V of the amplifier. The purpose of the summing unit 12 is to produce a situation wherein the output of the multiplier 11 comprises two factors, one of these being proportional to the gain V only, and the other being proportional to the noise level signal multiplied by the gain V and the signal EL. In this way, when the noise level is very low or close to zero a signal proportional to the gain V only is obtained for the amplifier 2. If, on the other hand, the background noise intensifies and thereby the value of the noise level signal increases, this term including the noise level signal will become increasingly dominant in the control input signal of the amplifier 2. In this way, a gain level for the amplifier 2 that is proportional to the noise existing in the environment of the sound reproduction system is obtained. Figure 3 shows a block diagram of the system of Figure 2, the system CS of Figure 2 being shown as a single box having as inputs the electrical signal derived from the sound signal source 1 and the electrical signal derived from the microphone 3. The output is constituted by the signal to the loudspeaker 15, respectively. Figure 4 shows a sound reproduction system according to the invention. In this system, one control system CS is assigned for each band of the equaliser, said control system CS thus corresponding to the control system shown in Figure 2. In the circuit of Figure 4, these control systems are indicated with reference numerals CS1,...CSn and are n in number. The system of Figure 4 further comprises a source 1 of an electrical sound signal, but now the signal is applied to a filter group 13 prior to being applied to the amplifier, and the filter group divides it into several sound signals occurring in different frequency bands, said sound signals being respectively applied to the different control systems CS1,...CSn. Correspondingly, the signal from the microphone 3 is similarly divided to the same frequency bands by means of filter group 14. The outputs from these filters are again applied to a control system CS1,...CSn representing the respective frequency band. Hereby it is possible to combine the sound signal existing in each frequency band and the noise occurring in said frequency band so as to enable adjustment of the gain of said frequency band in such a way that the noise occurring in said frequency band can be compensated. The outputs of the control systems CS1,... CSn are combined in a summing unit 16, supplying the loudspeaker 15. As stated previously, the control systems CS1,...CSn comprise exactly the same units as the control system CS illustrated in Figure 2. Thus these control systems comprise means for determining a band sound level signal corresponding to the total sound signal occurring in said frequency band (which is obtained from the filter group 14). They further comprise means for subtracting an electrical signal representing the sound level generated by the amplifier of the pertinent frequency band from the band sound level signal occurring in said frequency band, to provide an electrical signal proportional to the level of background noise occurring in said frequency band. The control systems further comprise means for adjusting the gain of the amplifier of said frequency band, so that the background noise occurring in said frequency band is taken into account in the adjustment of the amplifier. In the foregoing, the sound reproduction system of the invention has been described only by means of one exemplary circuit diagram, and it is to be understood that particularly the circuit shown in Figure 2 may be modified in a variety of ways without departing from the scope defined by the appended claims.

Claims: A sound reproduction system, comprising: a source (1) of a sound signal in electrical form; a first filter group (13) for dividing the signal derived from the sound signal source (1) into several electrical sound signals, each electrical sound signal occurring in different frequency bands; an adjustable gain amplifier for each frequency band for amplifying the electrical sound signal occuring in said frequency band and for providing an electrical sound level signal representing a sound level for each frequency band; first power level calculation means for each frequency band responsive to said electrical sound level signal for providing a first power level signal indicative of the power level corresponding to the electrical sound level signal for each frequency band; at least one microphone (3) for generating an electrical total sound signal proportional to the total sound existing in the environment of the sound reproduction system; another filter group (14) for dividing said electrical total sound signal into several band signals occurring in frequency bands corresponding to the frequency bands of the first filter group (13); second power level calculation means for each frequency band responsive to said electrical total sound signal for determining a second power level signal indicative of the power level corresponding to the band signal for each frequency band; subtraction means for each frequency band for subtracting the first power level signal from the second power level signal occurring in the respective frequency band for generating error signals proportional to the level of background noise existing in the different frequency bands; and adjustment means for each frequency band for adjusting its amplifier gain in response to a signal proportional to a desired gain value (V) and in response to a signal proportional to the product of said desired gain value and a scaled error multiplication signal, said adjustment means for each frequency band including first multiplication means for providing said scaled error signal as the product of said error signal for each frequency band and a gain adjustment signal (EL), the magnitude of said gain adjustment signal (EL) being selected to modify the magnitude of said error signal such that said scaled error signal for each frequency band adjusts the amplifier gain for each frequency band. A sound reproduction system according to claim 1, further comprising time-delay means for each frequency band for delaying each of the electrical sound level signals for each frequency band prior to being provided to said first power level calculation means for a delay time corresponding to the travel time of sound from said loudspeaker to said microphone. A sound reproduction system according to claims 1 or 2 further comprising means for correcting the scaled error signal according to the masking abilities of the human auditory system whereby amplifier gain adjustments are not made when enough masking audio signal level is found in that band.

Inventors: KUUSAMA JUHA, KUUSAMA, JUHA
Applicants: NOKIA TECHNOLOGY GMBH
Application year: 1993
Main classification: H03G3
IPC codes: H03G3, B60R11