Electronic Design

Design Efficiency Will Keep Your Product From BOMing Out

Measured by part count and cost, design efficiency often is the most significant factor in determining the bill of materials (BOM) of a piece of electronic equipment. Design efficiency can be equally or more important than the additional expenses incurred by adding new features to a product. As a platform evolves by adding more capabilities, the importance of design efficiency rises.

The digital satellite set-top box (STB) serves as an excellent example of the ramifications of efficient and inefficient designs. A new wave of STBs with integrated personal video recorders (PVRs), which let consumers record digital video on a hard drive, illustrates how design efficiency often can have just as great an impact on a box's BOM as the addition of new hardware, according to an analysis of multiple designs conducted by iSuppli's Teardown Analysis Service.

There are variations, but satellite, cable, and terrestrial STB designs from various suppliers typically share the same basic design, noted Andrew Rassweiler, who manages the Teardown Analysis service for iSuppli.

A digital STB consists of seven major components: the tuner, the demodulator/forward error correction, the transport demultiplexer, the MPEG-2 decoder, the CPU, the NTSC/PAL encoder, and the memory (see the figure)

. The MPEG-2 video decompression function lies at the center of a digital STB's functionality. Thus, integration in digital STBs has focused on the MPEG-2 chip, with suppliers combining functions—including the CPU, the transport demultiplexer, and even the NTSC/PAL encoder—to form a single back-end device.

THINKING INSIDE THE BOX
Recently, iSuppli tore down 10 low-end digital STBs that do not integrate PVRs, plus five mid-range to higher-end models that do. Using an index that fixes the BOM of the least expensive PVR-enabled STB at a baseline value equal to 1, the non-PVR STBs had an average value of 0.56, while the PVR boxes averaged 1.35. The average component count of the non-PVR boxes was 960, compared to 1280 for the PVR boxes (Table 1).

The major design difference between the non-PVR and PVR boxes is the addition of the hard drive for storing video. Furthermore, the PVR boxes employ back-end chips that include additional logic for compressing digital video and interfacing with hard drives. Some PVR STBs also include dual tuners for watching and recording separate video streams simultaneously. These factors help drive up the BOM cost of the PVR STBs compared to the non-PVR boxes.

In many cases, the PVR and non-PVR boxes iSuppli dissected were made by the same manufacturer and identical in many regards. However, the teardowns revealed a wide variance in the PVR STBs' total BOMs (Tables 2 and 3). While there are significant variances between the BOMs and features of the boxes, the major difference between them is the capacity of their hard drives and the usage of either single or dual tuners.

The boxes with dual tuners tended to have higher BOMs than those without, as illustrated in Table 3. Yet the choice and capacity of the hard drive is a larger factor in determining the BOMs of the various STBs than the use of single or dual tuners.

In all the PVR STBs iSuppli examined, except for box B, the hard drive was the second most expensive segment of the product after the ICs. In box B, the hard drive was the most expensive item (Table 4). For comparison, iSuppli set the least expensive drive at a baseline value of 1. The drives, which are all 3.5 in., range from 40 Gbytes with a indexed value of 1 to 120 Gbytes with a value of 1.56.

The hard drives used in PVRs like these are generally the same as those found in PCs. But they are specified to be quieter and have a rotation speed of 5400 rpm, as opposed to 7200 rpm in PCs, according to Denise DeLeon, senior analyst, storage for iSuppli. Pricing of the STB PVR drives also is similar to those used in PCs.

Beyond the cost variations related to the use of different capacities of the hard drives and single/dual tuner configurations, the key to reducing the number of components in STBs is picking highly integrated back-end ICs that combine large numbers of functions. By using these effectively, designers can reduce the need for additional devices, said Shyam Nagrani, principal analyst, consumer electronics for iSuppli.

For instance, STB B uses 791 components, far fewer than the 1601 in the costlier STB D, Nagrani noted. This is because STB B uses a newer, more highly integrated back-end IC than STB D does. STB D is an older design based on more mature chips that has been shipping in high volume for some time, Nagrani said.

Another example of design efficiency can be seen by comparing STB A and STB B. While STB A is the least expensive of the PVR STBs examined here, it is relatively inefficient in its design, with 1249 components. STB A's use of a relatively low-capacity hard drive and a single-tuner configuration keeps it inexpensive.

In comparison, STB B's BOM is only 4% more expensive than STB A's, but it sports a hard drive with double the capacity. STB B's larger hard drive offers users a considerably enhanced experience by enabling them to record roughly twice as much video as they could with STB A. Therefore, a more efficient design can deliver an improved experience to users at only a marginally higher cost.

More features will be added to mid-range and high-end STBs in the coming years. To accommodate these new features while restraining STB BOMs, designers must continue to strive for maximum efficiency.

iSuppli Corp.
www.isuppli.com

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