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Video output enhances mobile viewing

Posted: 16 Feb 2007 ?? ?Print Version ?Bookmark and Share

Keywords:video design? video output for cellphones? video output for portables? Jeremy Tole? Fairchild Semiconductor?

The convergence of technology in cellphones and other ultraportable devices has rapidly increased the use of video in applications requiring extremely small size and low power. A new, emerging feature is the ability to drive a video signal from a cellphone to view that image on a conventional TV set. Sending video signals to different applications is useful in many ways, since it can be used for videoconferencing, photo viewing, movie streaming, video phone, Internet gaming and other applications that have not yet been dreamed of.

To enable ultraportable 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. 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 TV sets.

Composite video output
A mobile device's TV-out function outputs composite video, which is the most common video signal in use today and is readily available on any TV set. On a high level, a portable device (e.g. cellphone 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 TV. Additionally, the analog signal needs to be amplified and impedance-matched to the characteristic 75-ohm cable.

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.

Since the standard composite video connector is fairly large for portable devices, there is a modified connector called a mini A/V connector. The mini A/V connector 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.

Video encoder
To create a composite video signal, encoding must be implemented. It 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 IC, 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 8bit parallel CCIR-601/656 or ANSI/SMPTE 125M format) into a standard analog baseband TV 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.

Reconstruction filtering
Following digital video encoding, the signals are typically converted back into the analog domain by a DAC in a process called reconstruction. 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 the picture's small detail elementssuch as highlights or fine pattern detailsas 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 that an integrated video filter/driver such as Fairchild Semiconductor's FMS6151 be used. 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 op amps, capacitors, resistors and inductors. The FMS6151 is a fifth-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.

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.

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 2.5-5.5V. The fifth-order filter provides better image quality compared to typical second- and third-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 eliminates 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 220-1,000F). 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.

- Jeremy Tole
Technical Marketing Manager, Signal Path Analog Product Line
Fairchild Semiconductor Corp.

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