Patent Application
20070067552
The present invention is related to Digital Audio Bus Switches, and to 3-bit bus switches in general.
Today's radios & AV receivers have increased requirements for supporting multiple audio sources, most of which have digital interfaces. Compared to several years ago, there are many more audio sources available today that must be supported.
Excluding SPDIF & TDM, most common digital audio buses found in consumer audio consist of a 3-wire serial interface that includes a bit clock (BCLK), serial data (SDAT), and word/channel clock (WS). IIS, Left-justified, and Right-justified are audio interface formats that fall into this category of 3-wire serial audio bus. With the increase in digital audio sources, there is a growing need to provide more switching/muxing capability to allow for multiple audio sources to be routed to a DSP or other audio processor. Most audio processors have a limited number of digital audio bus interfaces, anywhere in the 3-5 range. Adding audio bus inputs adds pins and cost to audio processors, so it is desired to minimize inputs and control bus routing external to the processor. The ability to externally switch between many different audio sources allows the audio designer flexibility in configuring his system in multiple ways using the same audio processor.
Consider Table 1 below which shows many of the digital audio sources supported on today's radios and AV receivers.
In all systems, at least one of these sources will have to be selected at any given time. Many newer systems require selecting two or more sources at a time. Examples include: multiple headphone system that allows a different audio source to be selected for each headphone; multiple audio zones in a house or automobile, where each zone can select their own audio source; and ripping audio (MP3/WMA) while listening to something else on your portable player.
Audio designers typically use one of two methods to deal with this problem today. The first method involves using multiple discrete logic muxes, such as a 74HCT153 or 74LVC257, to provide switching capability for multiple inputs. The problem with this approach is that there aren't any devices targeted at 3-wire or 3-bit buses, and so designers use other devices that aren't optimized for the application. It typically takes several devices (4 for a 4:2 mux, or 9 for a 6:3) to implement the correct functionality.
The schematic in
The second method involves using a programmable logic device, such as a CPLD or FPGA, to provide the muxing capability. While these devices offer more flexibility, they are usually expensive and can impose extra cost on the design since the devices will need to be programmed prior to assembly. FPGA's can also require additional power rails, which adds more cost to the power supply. This approach makes sense if there is a lot of extra glue logic to be reduced on the board in addition to the audio bus switching.
What is needed is a low-cost, low-power, monolithic solution to address the problem of switching 3-bit audio buses in today's audio applications. Such a device should be controllable via an I2C interface, much like the other audio IC's in the system, in order to reduce control signals, where I2C register values would control the routing of inputs to outputs. The device should have 3-bit bus interfaces on the input and output, allow for any of the inputs to be switched to any of the outputs, and accept the most common input voltage levels: 3.3 V and 5 V.
The present invention achieves technical advantages as an audio bus switch with simple logic control. The switch is a monolithic solution that may be controlled and operate from multiple common input voltage levels.
Two preferred embodiments of the invention. The first preferred embodiment is shown as 3:1 3-bit bus switch with simple logic control. The second preferred embodiment is slightly more complex, shown as 4:2 bus switch with a I2C interface that can output any of four inputs on either of the two outputs. The invention is extremely useful in present & future audio applications.
The advantages and differentiating features that make these switches of the present invention better than today's existing solutions are:
1) A monolithic integrated circuit targeted at switching 3-bit buses, such as digital audio IIS. In one embodiment, the 3:1 bus switch 10 selects one of three inputs; in the other embodiment, the 4:2 bus switch 20 can output any of four inputs on either of the two outputs.
2) Reduction in number of control pins needed by the host controller.
3) I2C interface and registers to simplify control by the host controller (on 4:2 bus switch).
4) Reduced routing of clocks and signals on PCB, which is huge improvement over discrete logic, or “glue logic” solution.
5) Optimized use of input and output pins for the application, unlike existing discrete logic solutions.
6) Ability to tri-state any of the individual bus output signals that are redundant or aren't necessary, in order to reduce power to IC, and reduce EMC and switching noise (on 4:2 bus switch 20).
7) Eliminates need for multiple discrete logic devices.
8) Available at lower cost than programmable logic.
Though the invention has been described with respect to a specific preferred embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present application. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.
