Amplifiers – With semiconductor amplifying device – Including gain control means
Reexamination Certificate
2001-04-16
2004-02-17
Shingleton, Michael B (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including gain control means
C330S086000, C330S254000, C330S284000, C341S154000
Reexamination Certificate
active
06693491
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to techniques for controlling the level of a signal. More specifically, the present invention provides methods and apparatus for controlling the level of a signal of amplifiers.
2. Description of the Prior Art
Virtually all audio amplification systems require a means of controlling the overall gain of the signal path. Such systems are described in described in U.S. Pat. No. 5,777,512 for METHOD AND APPARATUS FOR OVERSAMPLED, NOISE-SHAPING, MIXED-SIGNAL PROCESSING issued on Jul. 7, 1998, the entire disclosure of which is incorporated herein by reference for all purposes as well as in U.S. patent application Ser. No. 09/156,262 for METHOD AND APPARATUS FOR CONTROLLING AN AUDIO SIGNAL INPUT LEVEL filed on Sep. 18, 1998, the entire disclosure of which is incorporated herein by reference for all purposes.
Such gain control enables the system engineer to optimize signal levels to fit the dynamic range of the system and allows end-users to adjust the loudness or volume of the amplified sound to suit comfort levels or taste. Volume control can be achieved by means including variable resistive elements, e.g., potentiometers, in the analog audio path, variable gain amplifiers (VGAs) in the analog audio path, and multiplication of digitized audio by a digital volume control word.
In multi-channel systems such as 2-channel conventional stereo or 4-6 channel surround audio systems, potentiometers (“pots”) are often ganged on a common rotational shaft such that all channels receive roughly the same degree of gain control. Alternatively, multiple VGAs can be used for a plurality of channels, each receiving the same gain control signal as all others, in order to achieve a uniform overall gain setting. In the digital domain, each audio channel receives the appropriate digital gain parameter value.
In the case where individual channel gains need to have a relative offset but still track together globally, such as in left/right balance (pan) or interchannel trimming, additional potentiometers can be added to the pot-based system in series with the master volume control or a slip clutch mechanism can be used to allow individual pot adjustability in the ganged pot configuration. Also, per-channel offsets can be added to the global gain control signal in VGA-based systems, and numerical offsets can be similarly implemented between the channel gain parameters in the digital system.
Audio volume control circuits must typically satisfy a broad range of requirements. For example, such circuits should have a logarithmic transfer function to match the nature of human loudness perception. A logarithmic transfer function is achieved in pot-based systems by the use of “audio taper” pots which have a logarithmic variation in their resistance as a function of shaft rotation. This works well in most applications, with the disadvantage that it can be difficult to inexpensively manufacture pots which can precisely match one another for ganged use. As a result, in modestly priced audio systems differences between channel volumes at low volume settings are often easily perceived. In VGA-based systems, the logarithmic volume variation is implemented by mapping of the control signal from linear input (e.g., a voltage from a potentiometer or DAC) into the appropriate logarithmic form. Alternatively, the control voltage can be derived from an audio taper pot. In digital volume control systems, the logarithmic volume steps can be achieved with a mapping function, e.g., from a simple look-up table.
Audio volume control circuits must also exhibit low noise. Potentiometers, since they are passive devices, contribute no active noise to the signal path, but can degrade audio signals with resistor thermal noise, and discontinuity noise from, for example, a dirty wiper contact. In some cases pots also allow electromagnetic interference to enter the audio path due to inadequate shielding. VGAs are active devices akin to operational amplifiers and therefore inherently contribute some degree of noise. This noise can be minimized with adequate design techniques, but this comes at additional cost due to larger signal handling transistors or increased bias current in gain stages. In digital volume control systems, if implemented exclusively in the digital domain, the system noise is governed by the bit resolution of the system following the volume control block. For example, if one has a 16-bit digital audio system with digital-only volume control, this implies that full output loudness correlates to activity in all 16 bits at the DAC that drives the power amplifier. If one then sets the volume to be ¼ of the maximum available dynamic range, i.e., a volume reduction of 2 bits, one is left only using 14 bits at the DAC—a distinct cut in resolution. Also, noise and distortion products contributed by the DAC and any subsequent EQ are not attenuated when volume is reduced. The noise floor can thus become apparent at even normal volume settings. It is for this reason that all-digital volume controls are usually not used, and instead a hybrid of pre-DAC digital control and post-DAC analog control is used if the system is to include any volume control in the digital domain.
Audio volume control circuits must also exhibit low distortion. Passive, potentiometer-based volume control systems are essentially distortionless. The exception to this might be very slight voltage-based resistor value dependencies. VGAs are active amplification devices and are therefore subject to the usual set of non-ideal characteristics inherent in any active gain block that can contribute to overall distortion. All-digital volume controls could potentially suffer from distortion due to truncation or rounding errors in the multiplication process.
Audio volume control circuits must also exhibit transition smoothness. Changes from one gain setting to another should be done on a gradual scale to prevent the introduction of audible artifacts into the audio. After all, gain control is actually the multiplication of the hi fidelity audio signal(s) with a quasi-static control signal, and any aberrant behavior in the control signal will produce modulation products in the audio signal. The “trick” is to keep all variations in the gain control signal sufficiently gradual, e.g., with frequency components below 10 Hz, and/or small in amplitude, such that the modulation products remain unnoticeable or unobtrusive. Hand-operated potentiometers inherently provide a relatively slow and smooth transition from one gain setting to the next by virtue of the limited rate at which the human operator can turn the control knob. If, however, the potentiometer(s) is (are) operated by a stepper motor (as in the case of remote control) there is more risk that the individual stair steps of volume change will be noticeable, depending on the servo-stepper design. In VGA systems, the gain control signal must be made to ramp smoothly between gain settings even if the command to change is a step function. This is easily achieved if the signal is derived from a continuous source such as a pot, but requires more care if the control signal is derived from a more coarse source such as a DAC. In digital volume control systems, smaller, intermediate gain steps can be added between allowed volume setting increments, permitting the control algorithm to more closely approximate a smooth ramp during volume changes. For example, if the volume control is only allowed to sit on integer dB positions such as 0 dB, −1 dB, −2 dB, and so forth, it could be made to micro-step between these steps in ¼ dB increments to reduce the audibility of the 1 dB steps.
Audio volume control circuits must also enable precise tracking between channels in multi-channel systems. That is, where two or more channels are involved, it is usually desirable that the volume control function for each channel closely match that of the others throughout its entire gain control range. Offsets can be intentionally introduced between ch
Beyer Weaver & Thomas LLP.
Shingleton Michael B
Tripath Technology Inc.
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