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Automotive applications require fast analog video switching

Tue, 12/21/2010 - 7:54am
Fionn Hurley, Analog Devices

Fionn HurleyDespite the move to digital video for home entertainment, analog video still has a significant presence in applications such as the automotive industry. Analog video is a mature, proven technology which can be extended to meet the requirements of automotive applications. The reason for the continued use of analog video includes easy access to low cost circuit designs, use of inexpensive copper cabling, and analog video degrades “gracefully” in noisy environments. Cars, SUVs and trucks are now available with improved features such as surround view video cameras, consumer entertainment units, and navigation systems; all producing analog video content. A typical vehicle containing these features is shown in Figure 1.

Due to the growing demand for this “video in car”, the fast switching of analog video in automotive infotainment applications becomes a key criteria. Consumers expect to switch seemlessly between analog video sources in the blink of an eye (200 msec or less). This is especially significant when switching to video sources such as rear view cameras, where it is critical to view an image instantaneously for safety reasons. The requirement for fast switching, while maintaining high quality video, “raises the bar” for the performance of video decoders used in a vehicle display unit. In comparison, most video decoder products were designed for the TV market where the key performance criteria were to provide high video quality and robustness to nonstandard and poor timebase video signals (such as weak RF and VCR sources). These video decoders were designed to maintain constant output timing, even when the input timebase was corrupted. The locking and synchronization requirements for these applications is the inverse to the requirement for fast switching between video sources. This article outlines the challenges facing video designers to implement systems with fast switching and proposes realistic solutions. 

Figure 1. Typical vehicle video system

Fast switching challenges
Much research and design have been invested in developing video decoders to meet the challenging demands of non-standard and poor time base sources. The fast switching applications discussed here present new challenges. However when we try to envisage a solution that can be applicable to both TV and automotive applications, it becomes a near impossible task. The design techniques that will strengthen dealing with one signal type would weaken it with the other and vice versa. Analog Devices video decoder designers have worked a long time to achieve fast switching performance without compromising compatibility and image quality.
TV video decoders are designed to “flywheel” through interruptions on the input video stream and, where necessary, to regenerate the nominal synchronization signals. Most decoder algorithms ignore interruptions on the input timing, as shown in Figure 2. The design challenges to implement video decoder designs that can tolerate interruptions of the video stream without impacting image quality are numerous.

Figure 2. Typical TV video decoder application

The fast switching of analog video in an automotive application is based on the assurance that the video sources for these applications will maintain a constant and correct timebase. Genlocking (synchronizing) of the video sources is expensive in terms of the added cabling cost, board area and control processing requirements, which are all key considerations when designing for the automotive industry. High performance video decoders ideally have to minimize the overall system costs.

Figure 3 provides a representation of a typical automotive application where fast switching is required.

Figure 3. Automotive video decoder application

Selecting the correct Video decoder
When choosing a video decoder for a fast switching appication it is often difficult to determine the decoders fast switching performance from the specifications in the datasheet. Autodetection switch speed, color lock-in time and vertical lock time are all indicators of the decoders fast switching performance but do not tell the full story. System designers should ensure that the decoder they choose can be further optimised for fast switching. Some of the key product features a system designer should look for are:

- Ability to disable Hsync and Vsync processing blocks
- Control to force the standard or, at least, reduce the number of input standards allowed to be auto detected
- Controls to reduce the number of Hsync counts for determining the lock and unlock status
- Ability to force the timing reacquire of analog input circuitry
- Ability to generate interrupt when decoder locked or unlocked

Disabling Hsync and Vsync processing blocks
The Hsync and Vsync processing blocks allow high performance decoders to lock to signals with poor time bases. If the input sources applied to the decoders have good time bases, then the chosen decoder should have the controls to disable these blocks, thus reducing the lock time of the decoder.

Forcing standard
If the input source standard is known during the time of design, the decoder should be programmed to lock to this standard, thus reducing the lock time as the decoder does not require time to use the autodetection algorithm to determine the input video standard.

Reducing number of Hsync counts
When choosing the decoder for a fast swtiching application the system designer should ensure that the decoder has programmabilty which allows the user to reduce the number of consecutive and correct Hsync pulses that must be detected by the video decoder to determine that it is locked to the video signal. The Count out of Lock (COL) limit should also be reduced, so the video decoder requires fewer consecutive Hsync pulses to be absent from the input video signal before the decoder determines that it is out of lock and goes searching for a new input video type.

Forcing timing reacquire
The ability to force a reset of the front-end analog circuitry in the video decoder which reintialises the blocks used in acquiring a lock to the video signal is also key in a fast switching decoder choice. By forcing a timing reacquire, the front end block resets and “hunts” for a video signal, thus ensuring the speedy locking to a new video source.

Generating Interrupt when decoder locked or unlocked
The chosen video decoder should have a dedicated Interrupt pin which can flag when the decoder becomes locked or unlocked. This enables the system controller to react to these events without the need to be constantly monitoring the decoder via I2C.

Effects on output video
As the video sources are not genlocked, it is not possible to switch seamlessly from one input video to another. The output video stream is affected while the decoder switches from the timing on one input to the video timing on another input. As can be seen in Figure 4, the image becomes corrupted during the switching time. It is not possible to prevent this when switching between asynchronous video sources. 

Figure 4. Screen capture of output video images during swiching

Different systems implement different solutions to prevent the tearing of the video image being displayed on the screen. The system controller should have the ability to quickly react to the interrupt generated by the decoder and control the image displayed on the video screen during this switching time ensuring that the image displayed does not contain any of the tearing artifacts as shown in Figure 4.

Sequence of events during switching
This section describes the sequence of events which occur when switching to a new input video signal. This will aid a system designer in quantifying the fast switching performance of their chosen video decoder. 

Figure 5. Waveform of input and output video signals

The sequence of events followed when acquiring a video signal is numbered in Figure 5. These numbered events are described below.

1) I2C writes to decoder.
The decoder is programmed to switch into a new video input and is informed of the video standard. Writes to speed up the switching time are done at this point.

2) Analog clamps react to video input.
In a typical application, the video signal is capacitively coupled into the video decoder. The decoder must then ensure that the video signal is DC restored and the video blanking level is clamped so that the ADC outputs a specific code. There are several methods for achieving this; the sinking and sourcing of current onto the input node on the decoder is the most common. Figure 6 shows the typical clamping of a video signal by a video decoder. 

Figure 6. Typical clamping of video signal

3) Video fixed to correct voltage level and sync signal is extracted.
In high performance, video decoder products contain both coarse and fine clamp loops. Once the coarse clamps force the video signal to approximately the correct voltage, the fine clamps optimize fully the video signal level and maintain the video input at this correct DC level. The input video is now fixed at a stable DC level where the blank level is set to a known ADC code. The decoder extracts the sync signal from the video signal and monitors the Hsync, Vsync and Field sequence to determine the input video standard. If autodetection is used, the time to determine the video standard is increased.

4) Decoder outputs correct stable video timing.
The decoder locks to the input video signal, optimizes internal IP blocks and filters for this input video type and starts ouputting stable and correct video timing. This results in the displaying of a correct video image on the LCD panel or screen.

All of the above results are based on measurements carried out using ADI decoders such as the ADV7180 and ADV7181C which have been designed to meet fast switching requirements.

ADI decoders are used extensively throughout the automotive industry to meet the fast switching needs of many automotive system suppliers. The ADV7180 and ADV7181C contain all the fast switching features described in this article.

Summary
This article describes the reasons why analog video is still popular for automotive applications and the advantages that analog video maintains over other solutions. Car makers have seen how full-featured infotainment systems help sell their cars at higher margins and improve safety ratings. The trend for expanded “video in car” systems is now moving to where 5 LCD panels per vehicle is a likely near-term scenario.

The need for fast switching of analog video is outlined here along with the challenges associated with implementing fast switching in these applications. The key video decoder features that a system designer should look for are also outlined to aid them in their video decoder component selection process. Automotive qualified video decoders products such as the ADV7180 and ADV7181C are able to meet the demanding fast switching requirements found in automotive infotainment applications.

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