Patent Application
20080204601
Video drivers, or video amplifiers, supply or deliver video signals carried over an AC (alternating current) signal. A video driver should be able to deliver the video signal with high linearity to a consuming or receiving component and to match an inherent or characteristic impedance of a connecting cable to the receiving component. The receiving component may be a video receiver or what is commonly termed a TV (i.e., television). Video receivers or TVs may be part of various systems and devices that present or display video. Systems and devices that may have video drivers and video include wireless communication devices such as mobile or cellular telephones.
In many instances, it may be desirable or necessary for video drivers to consume a very small amount of power, while operating on a limited voltage supply. For example, devices such as those described above, may only provide or use a relatively small or limited power supply. Because of the limited power supply constraints of such devices, video drivers of such devices typically are required to consume a small amount of power. In other words, the video driver may not be able to draw too much power from a limited or small power supply.
Such low-voltage or low-power video drivers may typically couple a DC (direct current) signal to ground, or alternatively couple an AC signal to ground. In particular, the low-power video driver may either be DC coupled or AC coupled, depending on a cable that connects the video driver to the video receiver. In other words, the connecting cable may be either DC coupled or AC coupled. It may be typical that different applications or markets dictate the use of a DC coupled or AC coupled cable. In situations where the low-power video driver supports a DC coupling application, such a low-power video driver has no control of the DC level if it is driving an AC coupling cable. This results in a video signal that is unacceptably distorted.
In an embodiment, a component, such as video driver, provides a video signal that includes a DC component and AC component. The video signal is provided to a receiving component, such as a video receiver. For DC coupling mode, the video driver relies on a receiving component to establish the DC component. For AC coupling mode, where a cable that connects component to the receiving component, a capacitor prevents the DC component from traveling to the receiving component. Without correction of the DC component, the AC component in video signal is distorted. An internal signal path in the video driver is provided to establish the DC component at video driver such that AC component of video signal can pass through without distortion.
This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
The system 100 includes one or more controllers or processor(s) 104. Furthermore, the system 100 may include various memory components as represented by memory 106. Memory 106 may store machine or computer readable instructions and is accessed or controlled by processor(s) 104. Memory 106 may include read only memory (ROM), random access memory (RAM), flash memory, and various media (e.g., compact disk, digital versatile disk, etc.).
Various analog and/or digital data inputs and outputs are represented by input/output 108 which may be coupled to or controlled by processor(s) 104. In particular, input/output 108, includes an output to a video display or screen (not shown) which the video driver 102 supports.
A digital to analog converter (DAC) 110 may be included in system 100. The DAC 110 converts a digital signal into an output current signal. The output current (i.e. DAC output) may then be converted into voltage by an I-V (current to voltage) converter (not shown), where the voltage is fed or input into video driver 102. DAC 110 may include a current steering DAC to support an exemplary input bandwidth of 6 MHz received by system 100. A current steering DAC provides an advantage of speed and good performance in a relatively small form factor (i.e., less area).
The video driver 102 is a relatively low-power video driver working in a low voltage supply. An example power supply value of video driver 102 is approximately 1.8 volts. An example of video driver is described in co-pending patent application incorporated by reference herein: U.S. patent application Ser. No. 11/642,133, entitled Integrated Analog Video System, filed on Dec. 20, 2006, and assigned to Texas Instruments Incorporated. The video driver 102 is connected by a connecting cable 112 to a receiving component as represented by video receiver or TV 114 that consumes video data provided in a video signal from the video driver 102. The video receiver or TV 114 supports one of the outputs of input/output 108 for video. The connecting cable 112 may be either an AC coupled cable or a DC coupled cable.
A low-pass circuit 116 provides an additional signal path to copy a DC signal to output (i.e., input to video receiver or TV 114) so that DC level at the output (i.e., input to video receiver or TV 114) is decisively defined. Although the example describes the use of a video driver (i.e., video driver 102), other components and applications may make use of the low-pass circuit 116 and AC coupling. Examples of AC coupling include video equipment and devices that AC couple the output of a video signal, since it is a safer way to cross voltage domains. While DC coupling is accepted as being a more power efficient solution, there is a potential risk as two different power sources are not connected through a load. Therefore, examples of AC coupling are also applicable to systems that involve crossing across voltage domains.
The receiving component (i.e., video receiver or TV 114) discussed above is represented by a resistor RL 224. The connecting cable 112 discussed above is represented by cable 226. In AC coupling mode, the cable 226 has a capacitance represented by Ccable 228, where Ccable 228 may be inherent to cable 226. In DC coupling mode, Ccable 228 is not present.
The low-pass circuit 116 of
In DC coupling mode, the amplifier 202 and transistor M1 212 realize or provide an I-V (current to voltage) conversion. A video signal which includes DC and AC information is established at the drain of transistor M1 212. The transistor M2 214 copies the current of M1 212 and sends it to cable 226. The resistor Rm 220 matches the characteristic impedance of the receiver cable 226. If the impedance is matched, no current flows through Rm 220.
In AC coupling mode, capacitor Ccable 228 is between cable 226 and output voltage Vout 222. Therefore, no path exists for a DC signal establishment at Vout. This may lead to a DC level at Vout 222 that is not well defined. Furthermore, transistor M2 214 may not be able to work at normal condition. This leads to a video signal going to cable 226 that may be totally distorted. The additional low-pass circuitry as described above is added to form a DC signal path and establish the DC condition at Vout 222. The DC level at Vout 222 is well controlled and the video signal can be transmitted to cable 226 with high integrity. In particular, the amplifier 230 provides a low bandwidth signal path for the DC signal to flow, where the DC signal is represented by IDC 236. IDC 236 is a controlled amount of DC current, as determined by input values provided by the video driver (i.e., current iin 206). Since amplifier 230 uses relatively low bandwidth, any additional power consumption of the DC coupling circuit is relatively negligible. Amplifier 230, capacitor 234, and transistor M3 232 form a closed loop to decisively control DC level at output equal to the DC component in video signal.
At block 302, a determination is made if a cable connecting the video driver or other component, has a capacitance that necessitates AC coupling mode as describe above. If DC coupling mode is implemented, the video signal received at the cable may be allowed to simply pass through. As discussed above, if the video signal includes a DC component or DC signal that cannot pass through the capacitor of the cable, the DC signal is established at output.
At block 304, for AC coupling mode, a determination is made as to a current input value received by the video driver or component. The current input value determines the DC signal value to be passed or coupled in AC coupling mode from the cable.
At block 306, a low bandwidth path is provided for the DC signal. This low bandwidth signal path is a relatively low power consuming circuit relative to the video driver or component. An AC component or AC signal that carries actual video information is allowed to pass to the cable and is consumed by a receiving component such as a video receiver or TV.
At bock 308, the DC signal is established through the low bandwidth signal path.
The above-described systems and methods describe supporting AC coupling mode and DC coupling mode for a video driver or other component that provides a video signal.
Although the invention has been described in language specific to structural features and/or methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as exemplary forms of implementing the claimed invention.
