Chroma: Because most digital set-top boxes drive analog outputs, there's room to introduce luma and chroma errors. If the analog outputs aren't calibrated using a reference pattern and a video analyzer, there might be a problem with the luma and chroma when viewed on a color-picture monitor.
Conditional access (Pay Per View): Conditional access to scrambled programming is available through a SmartCard interface on the set-top box. It's triggered by the "transport scrambling control" 2-bit parameter, which is the sixth parameter in Figure 3. One way to test this feature is to make one of the moving programs on the digital carrier a scrambled program. Tuning to this program and viewing a clean moving picture will verify that the SmartCard interface and the supporting circuitry work well. If the program isn't viewable, the error is typically in the SmartCard interface. Usually there's no time to test for unauthorized conditional access, where unauthorized users can receive a program.
Audio and video synchronization (lip-sync): If it's known that the audio and video in the transport stream are synchronized (a Tektronix VM700T provides an objective lip-sync measurement to quantify the delay), and there's an obvious lip-sync error from the set-top box output, the culprit is the MPEG-2 decoding section. This error can occur at random times. The simplest solution is to power down the unit and restart the test. This allows the set-top box to re-initialize. If the error continues, then the set-top box is faulty.
No video or no audio: This problem happens frequently in the design phase, but hopefully never within the production process. Assuming the analog output section is working properly, the cause is usually from the originating MPEG-2 multiplexer. It also occurs when the decoder fails to extract the video and audio elementary stream program identifier (PID) values. If the set-top box fails to find the program association table (PAT) and program map tables (PMTs), also known as the electronic program guide, the video or audio will usually be absent. Sometimes, missing video will cause the decoder to just repeat the frame from the last channel to which it was tuned. If the set-top box finds the tables and identifies the correct PID values, but a problem now exists within the demultiplexer or decode section, the video or audio will still be absent (see the demultiplexer and descrambler board in Fig. 2).
A number of factors cause these errors:
Solder bridges across MPEG decoder chips: Bridges and opens commonly cause circuit-board failures within production. All the above symptoms can be seen depending on which component is bridged or left open. This type of problem means the entire set-top box will be rejected and further tested and evaluated. High occurrences of this type of failure would hopefully result in a revisit to the solder and ECB process to eliminate as many failures as possible.
Firmware errors:
Problems with software, firmware, and hardware definitely happen in the design phase, but hopefully never on the production line. After manufacturing engineering has thoroughly tested the pilot runs of the first set-top boxes, the only types of software errors allowed are those derived from failures of EPROM, DRAM, and flash memory. Because the mean time between failures (MTBF) is never infinity, hopefully CRCs and checksums will catch most problems. If not, the fact that the production procedure needs changing carriers and programs, along with verification of test and moving patterns, will help catch any memory problems.
Bad decoder chips: Normally, bad decoder chips are weeded out during the design phase, and sometimes during the evaluation of the initial pilot builds of the decoders. If a problem occurs on the production line, it might happen when the original decoder part goes obsolete and is replaced by a nearly identical part. Once again, if the new part isn't perfect, it should be caught by manufacturing engineering, not by the production technician.
Given the goal of manufacturing is to produce huge quantities of set-top boxes in the shortest period of time, it's important to focus on the inputs and outputs, and just on a few internal tests. It's even better if the I/O test incorporates the testing of the embedded circuits. High-volume production with one minute per box, or 10 seconds per test, means that a lot of engineering testing must take place before starting production.
In-depth RF and MPEG testing is required by engineers to verify that all of the real-world combinations can be met. Obviously, different power and noise levels will occur, as well as a wide range of video bit rates from 2 to 18 Mbits/s. After verifying all in-depth tests, production can choose a small subset of tests to ensure that the set-top box will be usable by the consumer under a wide variety of conditions.