Multi-channel audio signal limiter with shared clip detection

Details Multi-channel audio signal limiter with shared clip detection Multi-channel audio signal limiter with shared clip detection

2002-10-24

2004-07-06

Mei, Xu (Department: 2644)

Electrical audio signal processing systems and devices

Including amplitude or volume control

With amplitude compression/expansion

C333S014000, C381S107000

Reexamination Certificate

active

06760452

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable.
BACKGROUND OF THE INVENTION
The present invention relates in general to audio systems which prevent distortion from amplifier clipping by employing dynamic gain limiting, and, more specifically, to a multi-channel audio system wherein separate channels are individually gain limited in response to a shared clip detection signal from a power amplifier.
Typical audio reproduction systems include a variable-gain amplification stage followed by a fixed-gain power amplifier which feeds an output transducer such as a speaker. A volume or gain command (e.g., a control voltage) provided to the variable gain stage (i.e., pre-amplifier) controls the output volume heard by a listener. In one type of known audio system, a digital signal processor (DSP) is used to process the audio signals, including the variable-gain pre-amplification. The digitally processed audio signals are converted to analog signals and applied to a multi-channel analog power amplifier for driving the speakers.
An important objective in designing an audio system is to provide minimum distortion in signal reproduction. However, there is always some distortion, especially at high sound levels. As the magnitude of the signal provided from the variable gain amplifier stage to the power amplifier increases above a certain level, the power amplifier becomes overdriven. This situation occurs when the input signal to the power amplifier multiplied by the fixed gain of the power amplifier approaches the supply voltage level provided to the power amplifier. As a result, the power amplifier becomes saturated and signal peaks of the audio signal are distorted by clipping.
The problem of power amplifier clipping is aggravated in automotive audio systems. Less voltage headroom (i.e., safety margin) is available to the power amplifier since the automobile is limited to a 12-volt electrical supply. Although a DC/DC converter can be used to obtain a higher DC voltage, such converters are relatively expensive. Also, bass boost is needed in the automotive environment to overcome low frequency road and engine noise, making clipping more likely in the bass range of the audio signal.
It is known to employ voltage limiting or compression to the input of a power amplifier to limit the occurrence of clipping (but some amount of clipping up to the limit is desirable; otherwise it may seem that the audio system does not play loud enough). In prior art voltage limiters, the pre-amplifier gain is reduced when the power amplifier exhibits a specific percentage of total harmonic distortion (THD) as measured by the proportion of time over many audio cycles that the amplifier is overdriven. A distortion detector in the power amplifier generates a clip detect signal that is fed back to the pre-amplifier to control the gain reduction. When the clip detect signal is asserted or active, the system enters an attack mode wherein the gain is progressively reduced (from the user-controlled volume setting) at a relatively fast rate for as long as the active clip detect signal is present (but usually only up to a maximum gain cut). When the clip detect signal is no longer asserted (i.e., inactive), the system enters a decay mode wherein the gain is progressively increased at a relatively slower rate until the user-controlled volume is restored (unless the clip detect signal again becomes active).
A typical power amplifier contains multiple audio channels, such as stereo left and right. In automotive audio systems, four channel power amplifiers are common for separately driving left and right speakers in both the front and rear of a vehicle. A fifth channel may be added for a bass channel or a separate power amplifier may also be used.
In the typical power amplifier configuration, distortion above the THD threshold is detected in each channel but only one clip detect signal is generated by OR'ing together the distortion detection of all the channels. This is done in order to reduce the number of output pins of the integrated circuit on which the typical amplifier is constructed and because audio system designers typically choose to treat all channels equally when applying gain cuts. Thus, in response to the clip detect signal, pre-amplifier gain is reduced in all the channels even though not all channels may have exceeded the distortion threshold. For example, the fader controls in an automotive audio system may be set to provide greater amplification to the rear speakers than to the front. When excessive distortion commences in the rear power amplifier channels, both the front and rear channels gains are cut. In an audio system where all channels are playing the same audio source material for listening from all areas of a vehicle, then a consistent gain cut to all channels is desirable.
Automotive audio systems have recently been introduced having dedicated “rear seat entertainment” features wherein listeners in the front of the vehicle may select different audio source material at a different volume setting than listeners in the back of the vehicle (referred to as “dual play”). It is desirable to continue to use the same quad channel power amplifier IC in a dual play audio system, but the practice of prior art systems to limit all channels equally and simultaneously causes undesirable consequences for the audio channels that have not exceeded the distortion threshold. Providing for separate clip detect signals for each channel, however, undesirably increases the output pins of the power amplifier raising the cost of the audio system.
Eight channel audio systems are also popular wherein two quad amplifiers are used to drive a variety of tweeters, mid-range speakers, and woofers. The actual configuration of speakers may differ between different vehicle models (e.g., different numbers of tweeters), although it is desirable to use the same (unmodified) power amplifier IC's in the amplifier module. Because it may be desired to gain limit different types of speakers differently (e.g., limiting the high frequency content of the tweeters differently than the low frequency content of the woofers), the two available clip detect signals have so far not been able achieve optimal gain limiting.
SUMMARY OF THE INVENTION
The present invention has the advantage of limiting individual audio channels having a shared clip detect signal from a multi-channel power amplifier such that only channels likely to be exceeding the distortion threshold are gain limited. In addition, any arbitrary channels may be grouped together for common gain limiting. The invention monitors the power level of each audio channel, compares the power level to a power threshold that indicates whether a high power condition with the potential to cause excess distortion exists or not, and makes a decision whether to activate each channel's gain limiter based on whether the corresponding clip detect signal is active and the high power condition exists simultaneously for that channel.
In one aspect of the invention, an audio system has at least first and second audio channels. First and second gain multipliers receive first and second pre-limiter audio signals and generate first and second post-limiter audio signals in response to first and second scalar gain commands, respectively. A multi-channel power amplifier is responsive to the first and second post-limiter audio signals for generating first and second amplified audio signals. The multi-channel power amplifier includes a distortion detector generating a clip detect signal which is active if either the first or second amplified audio signal exhibits clipping distortion above a distortion threshold. First and second gain command blocks are coupled to the first and second gain multipliers, respectively, for generating the first and second scalar gain commands in response to respective reference values and respective first and second limit command signals. The first and second gain command blocks each enter an attack mode

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