GB2310577A

GB2310577A - Automatic switching amplifier

Info

Publication number: GB2310577A
Application number: GB9603958A
Authority: GB
Inventors: Darby Hadley
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1996-02-24
Filing date: 1996-02-24
Publication date: 1997-08-27
Also published as: GB9603958D0

Abstract

An audio amplifier for a vehicle entertainment system is described having, two amplification stages 20, 22 for driving loudspeakers LSF, LSR connected to the output terminals, a control circuit 24 for automatically determining when a loudspeaker is connected to an output terminal, and switching means SW2, SW3 responsive to the output signal of the control circuit 24 and serving to connect each amplification stage to an associated output terminal when the presence of front and rear loudspeakers is detected and to connect the two amplification stages to only one of the output terminals when it is sensed that no loudspeaker is connected to the other output terminal. In a modification (figure 3 not shown) a further amplifier has an input switched to ground when both loudspeakers are present and to one of the audio input channels when either of the loudspeakers is detected to be absent.

Description

Automatic Switching Amplifier Field of the invention The present invention relates to an audio amplifier for use in a motor vehicle entertainment system.

Background of the invention Vehicle manufacturers often produce several variants of the same vehicle model that have different levels of equipment.

In so far as concerns the in-vehicle entertainment system, vehicle models with higher specifications often have front and rear loudspeakers while less expensive models have only two front loudspeakers. It is not however cost effective in mass production to provide different wiring harnesses and different head units (the part of the radio that is fitted in the vehicle dashboard) for the different variants and it is common to use not only the same wiring harness throughout a vehicle range but also the same head unit.

It is known for a head unit to have a switch to allow the person installing the head unit to select whether the unit is to be connected to two or four loudspeakers. The internal configuration of the audio amplifier section of the head unit is modified by the switch so that if only two front loudspeakers are connected then they receive more power than would be the case if four loudspeakers are connected. There is however a risk in this case of a vehicle being delivered with only the front loudspeakers working if the head unit is incorrectly configured during installation notwithstanding the fact that the vehicle has been fitted with four loudspeakers and all four loudspeakers have been correctly wired to the head unit.

ObJect of the invention The present invention therefore seeks to provide an audio amplifier that avoids the problem of incorrect configuration of the a head unit with a switchable audio amplifier during installation.

Summary of the invention According to the present invention, there is provided an audio amplifier for a vehicle entertainment system having output terminals for front and rear loudspeakers, two amplification stages for driving loudspeakers connected to the output terminals, a control circuit for automatically determining when a loudspeaker is connected to an output terminal, and switching means responsive to the output signal of the control circuit and operative to connect one amplification stage to each respective output terminal when the presence of front and rear loudspeakers is detected and to connect the two amplification stages to amplify the audio signal applied to only one of the output terminals when it is sensed that no loudspeaker is connected to the other output terminal.

The switchable amplifier of the invention therefore avoids the problem of errors in installation by automatically detecting the loudspeaker configuration and switching between driving four loudspeakers with lower power and two loudspeakers with higher power in dependence upon which loudspeakers are sensed to be present.

In the preferred embodiment of the invention, the control circuit is operative to determine the presence or absence of loudspeakers at the output terminals only during a short period following the initial switching on of the amplifier.

If only front loudspeakers are sensed1 it is desirable for the control circuit to disable the FADE function of the amplifier that allows the audio level difference between the front and rear loudspeakers to be varied.

In certain circuit configurations, the presence of a loudspeaker can be detected simply by measuring the d.c.

voltage at the loudspeaker terminal. To simplify installation and the external wiring, it is preferred to use a common amplifier to act as an a.c. ground for the two amplification stages. In this case, it is possible to determine if a loudspeaker is connected during the short period immediately after the amplifier is first switched on, by any suitable alternative technique.

The manner in which the presence of a loudspeaker is detected is not at the heart of the present invention and need not therefore be described in greater detail. The invention resides in the fact that once it has been determined whether one or two loudspeakers are connected to the amplifier, the amplifier automatically configures itself either to divide the power between the loudspeakers if they are both present or to apply all the available power to the single loudspeaker if only one is present.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a first amplifier of the invention connected to two external loudspeakers, Figure 2 shows the amplifier of Figure 1 when connected to only one external loudspeaker, and Figure 3 shows an alternative embodiment of an audio amplifier in accordance with the invention.

Detailed description of the preferred embodiments A complete head-unit to which the invention may be applied is described in US Patent 5,042,070. This patent describes how the absence of loudspeakers can be sensed and used to suppress the FADE function (control of the audio level difference between front and rear loudspeakers) of the micro-processor automatically to avoid confusion to the operator when there is only one loudspeaker present.

The audio amplifier of Figures 1 and 2 may be used in place of the output stage shown in the head unit of the latter patent for one of the two stereo channels. Input signal are received at the input terminals IN1 and IN2 from the fader network that allows the relative amplitude of the signals applied to the front and rear loudspeakers to be varied by the operator. The input signals are applied directly to two amplifiers 20 and 22.

The head unit has two pairs of output terminals for front and rear loudspeakers LSF, LSR and two further output terminals that are connected to a.c. ground through RC circuits R1, C1 and R2, C2, respectively. The outputs of the two amplifiers 20, 22 are directly connected to two of the output terminals that are intended to be connected directly to the loudspeakers LSF and LSR. The output terminal intended to be connected to the return line from the loudspeaker LSR is permanently connected to the a.c. ground constituted by the capacitor C2. The output terminal intended to be connected to the return line from the loudspeaker LSF is connected by way of a switch SW3 to the a.c. ground constituted by the capacitor C1 and by way of a switch SW2 to the output of the amplifier 22.The amplifier further includes a control circuit 24, which may be a microprocessor, that controls the switches SW2 and SW3 in dependence upon the presence or absence of the rear loudspeaker LSR.

The control circuit 24 in Figures 1 and 2 can determine if the loudspeaker LSR is present by monitoring the d.c.

voltage at one of the output terminals. If the loudspeaker LSR is found to be present, then the switches SW2 and SW3 are placed in their positions shown in Figure 1. In this case, the two amplification stages are completely separate with the amplifier 20 amplifying the signal applied to the front loudspeaker LSF and the amplifier 22 amplifying the signal applied to the rear loudspeaker LSR. During this time, the fade network remains operational and the split between front and rear loudspeakers can be varied by the operator. The return lines from the loudspeakers LSF and LSR are in this case each connected to a respective one of the two RC circuits acting to provide an a.c. ground.

If the loudspeaker LSR is found to be absent, then the switches SW2 and SW3 are placed in their positions shown in Figure 2. In this case, the two signals from the fader network are separately amplified by the amplifiers 20 and 22 and applied in opposite phase to the single loudspeaker LSF thereby doubling the voltage drop across the loudspeaker LSF and increasing the output power from this loudspeaker. To avoid confusion to the operator by having the head unit provide a fader control that does not affect the output sound, the control circuit 24 can at the same time suppress the fade function in the manner described in US Patent 5,042,070.

The circuit of Figures 1 and 2 thus automatically detects the presence of the rear loudspeaker LSR and reconfigures appropriately each time that the amplifier is switched on to make the best use of the speakers present.

Figure 3 shows a preferred embodiment of the invention that allows either of the two loudspeakers to be omitted and that has only a single output terminal for each loudspeaker and a common ground terminal for both loudspeakers.

The input terminals IN1 and IN2 are again directly connected to the inputs of two amplifiers 10 and 12. The two amplifiers 10 and 12 are each permanently connected to a respective one of the two loudspeakers LSF and LSR, the return lines from the loudspeakers being connected to the output of a common amplifier 14. The common amplifier 14 can receive its input signal from one of three sources depending upon the position of a switch SW6 controlled by the control circuit 16 in dependence upon which of the loudspeakers LSF and LSR is sensed to be present.

When both speakers are present, then the input of the amplifier 14 is connected to ground and the amplifier acts only to provide an a.c. ground having the ability to sink the current outputs of the amplifiers 10 and 12.

If the loudspeaker LSR is absent then the switch SW6 is set to position 1 and if loudspeaker LSF is absent then it is set to position 3. When the switch SW6 is in one of these positions, then the circuit is the same as that described in Figure 2 in which the single loudspeaker present receives at its two terminals signals in phase opposition one being received directly from one of the amplifiers 10 and 12 and the other being received from the common amplifier 14 so that the power of the signal applied to the single loudspeaker present is automatically increased.

The control circuit 16 in this case can again disable the fade function to avoid the confusion that could result from having a control that makes no perceptible difference to the sound generated.

The sensing of the absence of one of the loudspeakers LSF and LSR is carried out by the control circuit each time that the amplifier is switched on so that if a speaker is added or removed at any time, then no action is required to reconfigure the head unit. The sensing can be carried in any known manner and it need only take place when the amplifier is first switched on so as not to interfere with the normal operation of the amplifier.

Claims

1. An audio amplifier for a vehicle entertainment system having output terminals for front and rear loudspeakers, two amplification stages for driving loudspeakers connected to the output terminals, a control circuit for automatically determining when a loudspeaker is connected to an output terminal, and switching means responsive to the output signal of the control circuit and operative to connect one amplification stage to each respective output terminal when the presence of front and rear loudspeakers is detected and to connect the two amplification stages to amplify the audio signal applied to only one of the output terminals when it is sensed that no loudspeaker is connected to the other output terminal.

2. An audio amplifier as claimed in claim 1, wherein the control circuit is operative to determine the presence or absence of loudspeakers at the output terminals only during a short period following the initial switching on of the amplifier.

3. An audio amplifier as claimed in claim 1 or 2, wherein the control circuit is further operative to disable a FADE function of the vehicle entertainment system in the absence of one of the loudspeakers.

4. An audio amplifier as claimed in any preceding claim, wherein the presence of a loudspeaker is detected by measuring the d.c. voltage at the loudspeaker terminal.

5. An audio amplifier as claimed in any of claims 1 to 3, further comprising a current amplifier serving as a common a.c. ground for the two amplification stages when front and rear loudspeakers are present and serving when only one loudspeaker is present to apply to the return line of the loudspeaker that is present a signal in phase opposition to the audio signal from the associated amplification stage.

6. An audio amplifier constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in Figures 1 and 2 or Figure 3 of the accompanying drawings.