In a relatively short time, MPEG encoding technology has completely changed the landscape of television content delivery to consumers. Whether through satellite, cable, or over-the-air, the efficiencies of MPEG-2 compression have enabled a veritable revolution within these industries, leading to set-top box development.
These set-top box designs go through a rigorous process to verify that individual parts of the system, such as RF receivers, demodulators, MPEG-2 processors, video and audio outputs, and analog RF modulators, work as one unit. However, testing during the mass manufacturing of set-top boxes is relegated to approximately 10 seconds per required test.
Within this short time span, a complex system goes through testing and evaluation to deliver a working unit to consumers. With production volumes of up to 10,000 units per day, the individual tests designed to be performed in the allotted 10 or so seconds must verify the integrity of the set-top box.
While MPEG-2 compression technology has changed the content delivery landscape to the consumer, the viewing experience is technologically the same analog experience as it has been since the dawn of the television era. Today's television sets are analog devices that require traditional signal sources derived within the set-top box from the MPEG-2 transport stream. Computer monitors and professional video monitors have digital video, and possibly digital audio, inputs. But, analog consumer television sets will dominate the living room for the foreseeable future.
Although this article focuses on MPEG-2 decoder problems that can be detected during manufacturing, remember that the set-top box system is symbiotic. Both the digital and analog sections rely on each other to produce acceptable pictures and sound. Because the system is interrelated, recognizing whether a problem is due to an error in the MPEG-2 system or in the analog system, and the correction of such errors, is the goal of the production line test.
Testing the system: An MPEG-2 set-top box will often be segmented into three separate sections or boards. Figure 1 shows a detailed example of the inputs, outputs, and each component within the set-top box. After assembly of the set-top box chassis (without its cover), several inputs and outputs are connected to verify its operation (Fig. 2).
Production test begins with a power-up test, which is as simple as it sounds. The power button is engaged and the unit comes to life. It typically has LED indicators on the front panel, and a manufacturer's logo, stored in the set-top box's memory, displayed on a monitor or television screen. While this might be the first indication of possible problems with the video section (if the LEDs light up but the logo doesn't appear or has chroma problems), this test is just for power-up. Regardless of what does or doesn't appear on the screen, if the unit powers up, it passes the test.
Once the set-top box is powered up, several independent tests are performed, taking about 10 seconds per test. The set-top box will be connected to various sources and analyzers to conduct and verify each test (Fig. 2). These sources include:
- At least two analog programs, audio and video, modulated to over-the-air channels (typically channels 2 and 13, as they're at opposite ends of the VHF band);
- At least two multiprogram digital MPEG-2 transport streams (with one program set for conditional access), modulated and upconverted to UHF L-band frequencies.
Both the analog and digital RF signals are combined to feed the RF input of the set-top box. The analog outputs of the set-top box are connected to separate video and audio analyzers, as well as to a video monitor and speakers (via an audio amplifier). If the set-top box contains a channel 3-4 modulator, then a frequency analyzer and a television set will also be connected to it.
Although the analyzers look at the technical characteristics of the audio and video signal, the manufacturing-line technician must watch the monitor and listen to the speakers. A variety of errors can "look" to an analyzer as though no error exists. For example, macroblocking (discussed below) technically conforms to all analog video requirements, but it provides an unacceptable image to the viewer.
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