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The convergence of technology in cell phones and other ultra-portable devices such as media players has rapidly increased the use of video in applications requiring extremely small size and low power. One new emerging feature is the ability to drive a video signal from a cell phone to view that image on a conventional television set (Figure 1). Sending video signals to different applications is useful in many ways since it can be used for video conferencing, photo viewing, movie streaming, video phone, Internet gaming and other applications that have not yet been dreamed of.
Figure 1 – Video signals can be sent from cell phones to TVs
In order to enable ultra-portable video technology, semiconductor manufacturers are developing devices such as video encoders and integrated video filter/drivers to drive the 75 ohm cable directly. The encoder, which is implemented after the main controller chip, includes the NTSC or PAL formatting and it has a combination of integrated video DACs, depending on whether only composite video is used or if S-video is added. The filter/driver is added after the DAC to reconstruct the signal and remove the high-frequency artifacts, which results in a higher quality image. In addition, it provides 75 ohm cable drivers to directly drive cables into television sets.
Composite Video Output
The TV out function of a mobile device outputs composite video, the most common video signal in use today, and which is readily available on any television set. On a high level, a portable device such as a cell phone or a portable media player needs a means to convert the digital video signal to analog and format this into an NTSC or PAL composite video. This allows the signal to be viewed on an external television. Additionally, the analog signal needs to be amplified and impedance-matched to the characteristic 75ohm cable. This implementation is shown in Figure 2.
Figure 2 – Video encoder and video filter/driver in a portable device
Composite video is expected to remain as a legacy signal and will be available for the near future as a means to display analog video. The anatomy of the video signal includes all of the information required to recover video at the receiving end, including horizontal and vertical synchronization, and luminance and chrominance signals (Figure 3).
Figure 3 – Composite video signal displaying color bars.
Since the standard composite video connector is fairly large for portable devices, there is a modified connector called a mini A/V connector that is more appropriate for portable video and has the added space-saving benefit of transporting the left and right audio signals on the same cable. Typically, the mini A/V is on one end of the connector and the larger RCA composite video and Left/Right audio jacks are on the other end (Figure 4).
Figure 4 – Mini A/V to RCA cable
Video Encoder
In order to create a composite video signal, a process called encoding needs to be implemented. This entails taking a formatted digital signal and converting it into a formatted NTSC or PAL analog composite video signal. The video encoder can either be integrated into a larger digital integrated circuit, or it can be a standalone device depending on how the partitioning is done.
From the main system processor (i.e., baseband chip), the standalone video encoder converts digital component video (in 8-bit parallel CCIR-601/656 or ANSI/SMPTE 125M format) into a standard analog baseband television composite video signal (NTSC or PAL standard) with a modulated color subcarrier. This is then fed into an integrated DAC and to the device’s output (Figure 5).
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Figure 5 - NTSC / PAL video encoder
Video Reconstruction Filtering
Following digital video encoding, the signals are typically converted back into the analog domain by a digital to analog converter (DAC). This process is called reconstruction (Figure 6). High-band spectral artifacts are introduced during this process and can distort picture quality. Reconstruction filters remove these artifacts. The filter’s reconstruction performance is based on how well the high-band spectral artifacts are removed without distorting the valid signal within the passband. Video signals are affected by these artifacts through a variation of the amplitude of small detail elements in the picture, such as highlights or fine pattern details, as the elements move relative to the sampling clock. The result is similar to the problem of aliasing and causes a distortion of details as they move within the picture.
To implement filtering, it is recommended to use an integrated video filter/driver such as Fairchild Semiconductor’s FMS6151. With such devices, integrated active filters replace several discrete components. Generally, the filters that are used in video multimedia applications are low pass active filters. The main components in these filters are operational amplifiers, capacitors, resistors, and inductors. The FMS6151 is a 5th order Butterworth filter and tends to be a good choice for the filtering of consumer video due to its overall performance such as low phase error, high stability, low parts count, and effective filtering characteristics. Due to their increased reliability and guaranteed specifications, these integrated active filters generally have more consistent filtering characteristics than discrete active and passive filters (Figure 7).
Figure 6 –Ultra-portable video reconstruction filter/driver
Reconstruction filters and cable drivers are typically left external to the encoder due to the voltage swing requirements and the need for higher ESD protection levels.
Figure 7 - An output reconstruction (image rejection) filter removes the clock and sideband components that are present from the sampling and analog reconstruction process
Video Filter/Drivers
Beyond the reconstruction filter, a video driver is required to amplify the video signal and drive the 75 ohm coax cable. The amplifiers need to have 6dB of gain to accommodate doubly terminated loads. The FMS6151 integrated video filter/driver solution combines the reconstruction filter with a low impedance video driver. The device will operate in applications with a Vcc ranging from 2.5V to 5.5V. The 5th order filter provides better image quality compared to typical 2nd and 3rd order passive solutions.
This filter/driver is intended to be directly driven by a DC-coupled DAC output, but can also operate with an AC-coupled input. The input common mode range of the device is 1.2Vpp, ground referenced. The output can drive an AC or DC-coupled single 75 ohm coax cable (150 ohm) load. DC-coupling the output removes the need for expensive output coupling capacitors. If the output is AC-coupled, the SAG correction circuit can be used to reduce the value and the physical size of the AC output coupling capacitors and still produce acceptable field tilt.
SAG Correction
Traditionally, if a video application is AC coupled, it will require a very large output coupling capacitor (between 220F and 1000F). SAG correction provides excellent performance with a small output coupling capacitor, which eliminates the need for a large coupling capacitor. The typical output circuit (220F into a 150 ohm load) creates a single pole (-3dB) at 5Hz. Reducing this capacitor causes excessive phase shift, resulting in video field tilt which can prevent proper recovery of the synchronization signals.
The SAG correction circuit in the FMS6151 provides a small amount of peaking, which in turn provides phase response compensation that significantly reduces video field tilt. This compensation enables the designer to decrease the large 220F output coupling capacitor. A 22F capacitor is used for SAG correction and a 47F is used for the output coupling capacitor, both of which are much smaller and less expensive than the alternative circuit requirements (Figure 8).
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Figure 8 – Video filter/driver with SAG correction
Enable/Shutdown
The FMS6151 has a shutdown feature that disables the output and reduces the quiescent current to less than 25nA, thus reducing power consumption and prolonging battery life. This feature is especially important in portable applications such as cellular phones, hand held gaming devices, and video cameras requiring video filtering and drive capability. Additional features include 12kV of ESD protection.
A Small Driver for The Big Screen
The implementation of the composite video TV out function on a portable device has many facets to take into consideration. The partitioning of devices, keeping power low, and the picture quality high present many design challenges. Video encoders are readily available to perform the task of conversion from digital to NTSC and PAL analog composite video.
The FMS6151 in a Micropak™ package is so small that it can be poured from a salt shaker, so it will have no problem fitting into a crowded handset and will provide quality video for TV viewing. The robust 12kV ESD protection provided in this device ensures the mobile device will be safe from harm. The 5th order low pass reconstruction filter smoothes the output video to prevent unwanted distortions. The device amplifies, drives, and matches the impedance of the 75-ohm coax cable. The board space is further reduced by offering a choice of output coupling modes, with the smallest configuration being direct DC coupling. Finally, this technology conserves battery power with a low current draw when the coax driver is enabled as well as less than 25nA of current when the device is disabled.
Figure 9 – FMS6151 in Micropak™ package – so small that it can be poured from a salt shaker
About the author
Jeremy Tole is a Technical Marketing Manager for the Signal Path Analog Product Line of Fairchild Semiconductor, where he has been developing the broadcast and consumer video business since joining in 1998. His current responsibilities include systems definition of video and signal path products and the business development of consumer and ultra-portable markets for signal path analog products in the Americas and European regions. He holds a Bachelors of Science degree in Electrical Engineering, with a concentration in Applied Electrophysics and Computer Engineering from the University of Virginia. He is a member of the IEEE, SMPTE, and Trigon Engineering Society. He can be reached at Jeremy.tole@fairchildsemi.com
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