Recordings produced in accordance with RIAA standards are characterised by distortion whereby high frequencies are enhanced and base frequencies are suppressed.

Manufacturers of reproducing equipment have made attempts to correct this distortion in recordings, but have met the problem that correction of the bass region can lead to reproduction of turntable rumble while finding that very substantial suppression of the treble region is necessary in order to eliminate hiss.

The main efforts of manufacturers of reproducing equipment have therefore been directed towards improvements in the mechanical properties of the equipment.

The present invention therefore, in one aspect, aims to provide a noise reduction circuit which can, amongst other possible applications, be used in sound reproduction equipment substantially to correct the distortion present in RIAA recordings and at the same time substantially suppress noise due to mechanical deficiencies in the reproducing equipment. The invention can also be applied, for example, to correction of distortion in tape and CD recordings, especially when an analogous circuit is used to produce the recording.

Thus, in another aspect, the present invention relates to record and playback circuitry, for example applicable to tape recorders, but also finding application in various fields where it is required to clean up a signal which is being recorded on a recording medium and played back, being a signal emanating from a musical instrument, for example, or even in unrelated fields when a signal is produced in vibration or temperature measurement.

Recordings on to tape typically introduce noise into the wanted signal and give rise to amplitude fall off at base and treble frequencies. Playback from tape can both emphasise this noise and undesirably emphasise treble frequencies.

Analogously, loudspeaker units in general use, regardless of cost, have characteristics such that high (treble) and low (bass) frequencies are lost to a greater or lesser extent.

It is another aim of the present invention to provide a pre-amplifier circuit which can compensate for the deficiencies of such loudspeaker units.

According to the present invention, in its most general aspect, there is provided a pre-amplifier incorporating a noise reduction circuit, comprising an operational amplifier in combination with capacitative/resistive tuned circuits to adjust the gain of predetermined ranges of frequencies emanating from a source to compensate for deficiencies of the source and/or downstream equipment so that the source sound is modified to result in improved eventual sound production within a room, with reduced sound boom, rejection of unwanted sounds and enhanced clarity of wanted sound.

The component values of the capacitative/resistive tuned circuits are chosen accordingly.

Thus, in contrast to conventional pre-amplifier circuitry, which aims to produce a flat or even response over the range of frequencies detectable by the human ear, the pre-amplifier of the present invention produces a non-flat response, to compensate for distortion in RIAA recordings and distortion in tape or other recordings generally, and/or to compensate for the deficiencies of loudspeaker units.

According to one more specific aspect of the present invention, therefore, there is provided a noise reduction circuit comprising an operational amplifier receiving at one input a signal received from a sound signal generating unit and receiving at its second input a feedback signal via a filter network which includes a non- linear broad band high frequency tuned circuit and a non-linear broad band low frequency tuned circuit respectively operative above a selected frequency near the top of the wanted audio frequency range and below a selected frequency near the bottom of the wanted audio frequency range, wherein the non-linear characteristics of the tuned circuits are such as to increase the amplitudes of wanted noise signals in relation to the amplitudes of resonant frequency signals, with increasing effect the further beyond the selected frequencies.

In relation to sound reproducing equipment, the noise reduction circuit is preferably located before the final pre-arnplifier stage. The broad band tuned circuits may cut in at about 4000 Hz and 200 Hz respectively, becoming increasingly effective upwards and downwards, respectively, of these selected frequencies. Record hiss, which is a resonant (constant) frequency signal usually of the order of 10,000 Hz or above, results in a cancellation signal at the second input of the operational amplifier, as does turntable rumble, typically a resonant frequency signal of the order of 50 Hz. Non-resonant signals, i. e. wanted noise signals, are not affected by this characteristic of the circuit. However, the high frequency tuned circuit does also have the increasing effect, from 4000 Hz upwards, of increasingly cutting the amplitude of high frequency wanted noise signals, while the low frequency tuned circuit increasingly has the increasing effect, from 200 Hz downwards, of amplifying low frequency wanted noise signals. It can therefore be arranged that the distortion in RIAA recordings is compensated for, to a chosen extent dependent on the selection of component values.

According to another aspect of the invention there is provided record and playback circuitry which comprises a record circuit comprising an operational amplifier receiving a source signal at one input and having its second input firstly connected in a capacitative/resistive feedback circuit for boosting base frequencies and secondly connected to a capacitative/resistive filter network for filtering noise and producing treble boost, and a playback circuit comprising an operational amplifier receiving a source signal at one input, which is connected to a capacitative/resistive filter network for cutting treble frequencies and firstly connected in a resistive/capacitative feedback circuit for boosting base frequencies and secondly connected to a capacitative/resistive filter network for filtering out residual noise.

In the case of a tape recorder, the source signal for the playback circuit is the tape output signal.

The general aim of the invention, in this case, is to provide, from the playback circuit, a signal linearised with respect to the original source signal applied, for example, to the tape, for supply to a power amplifier, and the component values used in the filter networks are selected acordingly.

According to still another aspect of the invention, there is provided a pre- amplifier circuit comprising an operational amplifier which at its first input receives audio signals to be amplified encompassing the range 20 Hz to 20 kHz, and which has connected to its second input wide band, capacitative/resistive, frequency dependent control circuits respectively operative from an intermediate point in the said range upwards with increasing effect and from an intermediate point in the said range downwards with increasing effect, whereby increasingly to amplify audio signals towards the upper end and the lower end of the range in order to compensate for loudspeaker unit characteristics.

The invention can also be applied to correct errors in tape or CD recordings on playback, especially if the recording is made using an analogous circuit. It can also be applied to correct distortions arising in microphones and musical instruments.

Preferably, there is also connected, in parallel with each of the frequency response control circuits, a potentiometer enabling adjustment of the characteristic of the corresponding feedback circuit inversely to match, at least approximately, the characteristics of an individual loudspeaker unit.

The lower frequencies control circuit preferably comprises a feedback circuit, while the upper frequencies control circuit preferably comprises a coupling to the negative (zero) side of the power supply.

The invention is further described with reference to the accompanying drawings, in which:- Figures 1 to 3 are circuit diagrams of three differing forms of pre-amplifier providing noise reduction; and Figure 4 comprises graphs showing the amplification characteristics of the circuit of Figure 3.

The pre-amplifier shown in Figure 1 is an RIAA compensation and auto-mix circuit.

The input 10 to the RIAA compensation circuitry is typically from a reord deck and the output 12 from the auto-mix circuitry typically leads to a pre-amplifier circuit. The circuit shown is for a stereo system, and is therefore identically duplicated on the left and right hand sides of the diagram, so that only one side need be described. The present invention is especially concerned with the RIAA compensation circuit, say that shown in the bottom right hand corner of the circuit diagram, and this will now be described.

The compensation circuit comprises an operational amplifier 14, one input of which receives a signal from the input 10 via a 4.7 uf electrolytic capacitor 16.

The input level is centralised at the mid-point of the power supply by a biassing circuit comprising 220 K resistor 18,200 K resistor 20 and 82 K resistor 22.

The second input of the operational amplifier 14, which is tied to the zero potential line of the power supply through a 1 K resistor 24 and a 10 uf electrolytic capacitor 26 for removing any ripple on the power supply, receives a feedback signal from the output of the operational amplifier via a filter network.

This filter network comprises two tuned circuits, one for high frequencies (treble) and one for base frequencies (bass). The high frequency tuned circuit comprises a 330 K resistor 28 and a 10 nf capacitor 30 and the low frequency tuned circuit comprises a 27 K resistor 32 and a 2 nf capacitor 34.

The treble frequencies tuned circuit is adapted to become operative above about 4000 Hz and the bass frequencies tuned circuit is adapted to become operative below about 200 Hz. Between these selected frequencies, which comprise the major part of the audio frequencies range, the filter network has no effect because no compensation is required. It is only at the top and bottom of the audio frequencies range that correction is required for distortion present in the RIAA recording being played by the record deck providing the input signal.

This distortion is compensated for at the high frequency end by the tuned circuit 28,30, which, increasingly at frequencies from 4000 Hz upwards, acts to clip the amplitude of the wanted sound signals, and at the low frequencies end by the tuned circuit 32,34, which, increasingly at frequencies from 200 Hz downwards, acts to amplify the wanted sound signals.

The tuned circuits also have another most important and related effect. This is that resonant (constant) frequency signals above 4000 Hz and below 200 Hz produce feedback signals at the second input of the operational amplifier substantially equal and opposite to the resonant frequency signals as received at the input, so that these resonant frequency signals are substantially cancelled or removed. Thus, such resonant frequency signals as those due to record hiss and turntable rumble are substantially eliminated even though, by the inherent nature of the capacitors incorporated in the tuned circuits, all wanted sound signals of non-resonant nature, i. e. signals with sharp edges as distinct from signals of sine wave form, are transmitted.

The output of the compensation circuit is fed to the auto-mix circuit, which is unique in that it provides for auto-selection, i. e. without switches, of the record deck and of auxiliary inputs 40 from a CD player, tape deck, tuner, etc., and provides the signal output 12 to a pre-amplifier circuit, at a uniform volume level regardless of the source.

The summing mixer circuit on the top right of the circuit diagram comprises an operational amplifier 44 having its first input level adjusted with respect to the power supply by 22 K resistor 46, and connected to an a. c. rejection circuit comprising 22 K resistor 48 and 4.7 uf capacitor circuit 50 for ensuring constant tone. At its second input, the operational amplifier 44 receives the output from the RIAA compensation circuit via 4.7 uf electrolytic capacitor 52 and 100 K resistor 54, and the auxiliary inputs via a circuit comprising 4.7 uf electrolytic capacitor 56 and 200 K resistor 58. Feedback to the second input via 100 K resistor 60, together with the aforesaid components 52,54,56,58, ensures the uniform level output to the pre-amplifier without the use of selection switches, which are frequently sources for introduction of unwanted noise. Generally, it may be stated that whereas the total output of the compensation circuit is passed to the operational amplifier 44, only one half of the output of the auxiliary inputs is passed through.

The pre-amplifier shown in Figure 2 comprises record and playback circuitry for a tape recorder.

The drawing shows at the top a tape recording circuit and at the bottom a tape playback circuit. In both cases the circuit shown is for a stereo system, and is therefore substantially identically duplicated on the left and right hand sides of the drawing, so that only one side need be described.

Referring to the tape recording circuit first, the source signal 110, for example from a microphone, a musical instrument or from measurement circuitry, is fed to the first input of an operational amplifier 112 via a 4.7 KF capacitor 114. The input level is centralised at the midpoint of the power supply by a 47 K/47 K resistor line 116. The second input of the operational amplifier 112 receives a feedback signal from the amplifier output via a 20 N capacitor 118 and a 20 K resistor 120. This feedback path has the effect of boosting low (bass) frequencies present in the source signal. The second input of the operational amplifier 112 is, however, also connected to a filter network leading to the negative (zero) power line; this filter network comprises 4.7 K resistor 122,3.3 K resistor 124 and 4.7 NF capacitor 126, whereby to filter out noise and boost higher (treble) frequencies.

Output signal 128 from the operational amplifier 112 is the signal recorded on the tape of the tape recorder, using electrolytic capacitor 130 for impedance matching.

The tape playback circuit at the bottom of the diagram comprises an operational amplifier 132 receiving the signal derived from the tape at its first input, via a 4.7 uF capacitor 134. This input is centralised with respect to the voltage supply by 47 K/47 K resistor line 136. The first amplifier input is also connected to the negative (zero) power line via a filter network comprising parallel connected 100 NF and 4.7 NF capacitors 138,140 and series variable resistor 142, for cutting higher (treble) frequencies, to a factory preset or optionally user-variable extent.

The second input of the operational amplifier 132 receives a feedback signal from the amplifier output via a filter network comprising 20 NF capacitor 144 and 20 K resistor 146, again for boosting lower (bass) frequencies, as in the record circuit. The second amplifier input is further connected by 10 K resistor 148 and 2.2 uF electrolytic capacitor 150 to the negative (zero) side of the power supply, whereby to filter out residual noise. Moreover, the second input of the amplifier 132 is further optionally connected to a 100 K variable resistor 152 enabling factory preset or user-variable adjustment of the amount of base boost. Variable resistor 52, when provided, serves for both sides of the stereo circuit.

Also serving both sides of the stereo circuit is a volume control sub-circuit 154.

The output signal 156 from the playback circuit is intended to be fed to a power amplifier circuit via a 4.7 uF electrolytic capacitor 158 providing impedance matching.

The above-described circuitry is able to record onto and playback from tape an output signal which is linearised with respect to the original input signal, notwithstanding the inherent non-linear characteristics of tape recordings. By some variation of component values, the circuitry can be used for recording and playback from other recording media. Typically the input signal will be a music signal, but the invention also finds applicability to the recording and playback of other signals, such as temperature or vibration measurement signals.

Figure 3 shows a stereo pre-amplifier circuit.

At the top of Figure 3, the basic pre-amplifier circuit in accordance with the invention is duplicated for left-hand and right-hand loudspeaker units.

Adjustable components are shown at the bottom of Figure 3 and these are the same for both basic pre-amplifier circuits.

That part of the pre-amplifier circuit shown at the top right-hand side of Figure 3 comprises an operational amplifier 210 receiving at its first input, held at the centre level of the power supply by 47 K resistors 212 and 214, input signals in the audio range (encompassing 20 Hz to 20 kHz) via a 10 uf electrolytic capacitor 216.

The second input of the operational amplifier 210 receives feedback signals from the output of the amplifier via a lower frequencies control circuit comprising a 20 K resistor 218 and a 20 nf capacitor 220 for gain limitation. The second input is also tied to the negative (zero) side of the power supply through a 10 K resistor 222 and a 2.2 uf electrolytic capacitor 224, thus providing higher frequencies control.

The lower frequencies control circuit is operative, by selection of the component values, with increasing effect from 1 kHz upwards and the higher frequencies control circuit is operative, by selection of the component values, with increasing effect from 1 kHz downwards. However, there can be a frequency bend, at the centre of the audio frequencies range, over which neither frequency control circuit is operative.

Connected in parallel with each of the frequency dependent control circuits is a dual sided potentiometer enabling the frequency response of the operational amplifier to be matched to a particular loudspeaker system. Referring to the lower part of the circuit diagram, the bass frequencies potentiometer (100 K LIN) is referenced 226 and the treble frequencies potentiometer (100 K LIN) is referenced 228. Dual sided potentiometers are employed to serve both the operational amplifier 210 and the operational amplifier 210A required in a stereo system. Operational amplifier 210A (top left of the circuit diagram) is controlled identically to operational amplifier 210. The potentiometers 226,228 are respectively connected in series with 4.7 nf capacitors 230,232.

Additionally, a dual volume control 234 is incorporated, from which an output is taken to a power amplifier. Volume control 234 (47 K LOG) is connected in series with 10 uf electrolytic capacitor 236 between the output side of each operational amplifier 210,210A and the negative (zero) side of the power supply.

Figure 4 shows typical response curves for the stereo pre-amplifier circuit. It can be seen that the gain gradually increases on both sides of the 1 kHz intermediate value to compensate for the fall offs in the response of loudspeaker units at higher (treble) and lower (bass) frequencies.