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
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
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