Design of media storage box system based on PR818S

The timing of launching the Media Storage Box (MSB) is a digital mobile multimedia device that is recommended based on the current development limitations, application shortcomings and marketing difficulties of PMP (media player) products. However, the commercial mobile devices with this type of function can integrate a variety of technologies, and there is also a lot of room for change in development and positioning. Take the MSB product as an example. The original design concept was to decouple from the PMP marketing positioning and position itself as a substitute derivative product of PMP, that is, the so-called "multimedia hard disk external box" for sale. However, in terms of the functions and specifications of the MSB product, it is almost a replica of the PMP design, and it can also be said to be a front-end technology development platform for products such as PMP, personal video player (PVP) or MP4 player. This is the most important technical challenge and thinking basis for launching hard disk-type mobile devices such as MSB.

PR818S development platform and related important components

MSB chose Sigmatel's single-chip processor (SoC) PR818S because of its powerful technical advantages:

Figure 1: a. The three-state mode in which one party is the subject. b. The mode in which both parties are the subject.

c. Power switching method. d. Use a switch buffer to isolate the two.

1. Powerful computing power

PR818S is packaged in 380LTFBGA (small outline fine pitch ball grid array) with a package size of 16.5mil (W) × 1

6.5mil (L), ball grid pitch is 0.65mil, using 0.13 micron CMOS process, built-in ARM922T 220MHz processor. Video codecs including MPEG-1, MPEG-2, and MPEG-4 are all hardwired, so when it is purely used for video, the ARM922T processor still has 90% of computing power surplus. This computing power meets the needs of a wide range of embedded products, such as V2oIP phones (visual IP phones), media gateways, IP Cams, etc. Compared with the above, MSB can be regarded as a platform for the development of such embedded products. The main operating system for PR818S is the Blue Cat Embedded 4.0 tool provided by Linux Works, and its Linux Code kernel is Ver.2.4.18.

2. Superior video quality

PR818S uses MPEG4 ASP compression technology, and its video quality is close to that of H.264. The resolution can support D1, NTSC 720×480, PAL 720×576 up to 30fps (frames per second). It also has full-duplex synchronous decoding/encoding capabilities, which is essential for realizing scheduled recording (time-shift function). Another feature of PR818S is that it can support JPEG, which can be as high as 16M pixel resolution as the input of the image head end.

Figure 2: PR818S chip-level system structure functional block diagram

3. Low power consumption

The core operating voltage of PR818S is 1.2V, the input/output is 3.3V, the chip itself consumes only 350mW, and the MSB system power consumption is also less than 1W, which is an important element in the development of portable embedded products.

4. Multiple external functions

I2S interface - used to connect the audio decoder. The audio decoder here is Wolfson's WM8978, which has a built-in PLL and can generate an audio clock to the PR818S.

CCIR656/601 interface—used to connect the video decoder and the video encoder. Here, TI's TVP5150 and ADI's ADV7179 are used respectively. Both are low-power designs. ADI7179 supports 3 DACs internally and can be used as 3 different signal outputs.

IDE interface - used to connect the internal hard disk. Its 2.5-inch compatibility has been tested well on PR818S.

MCU interface - used to connect to Ethernet. The Ethernet control chip here is ASIX's AX88796L. In addition to USB connection, it seeks storage applications on the network (Network Attached Storage).

Figure 3: MSB functional block diagram based on PR818S

SD interface - SD memory card is used to download digital camera content to PR818S.

USB2.0 interface - used to connect to the PC host, here we use the GL811E from Gensys. GL811E is a high-speed USB2.0 to IDE control chip, which has a considerable market share and is compatible with 2.5-inch hard drives.

Hardware design concepts and design breakthroughs

As far as the entire MSB hardware design is concerned, there are two major bottlenecks that need to be carefully considered:

1. IDE circuit design

Genesys GL811E chip and PR818S are independent entities, and only one can work at the same time. The two are connected in parallel to the HDD, so there are generally four ways to use them: a three-state method with one as the main body, pulling down the reset signal of the other to force it to be unable to access the HDD (Figure 1a); a three-state method with one as the main body, pulling down the reset signal of the other when one is working (Figure 1b); a power switching method, so that when one has power, the other has no power supply (Figure 1c); using a switch buffer to isolate the two (Figure 1d

Most of the hardware used in the market is the first mode, and this MSB hardware uses the third mode, but in terms of technical application, hardware engineers are willing to use the fourth mode. In short, each has its own advantages and disadvantages.

2. Line layout

The most difficult part of the entire circuit board is how to route the inner circle line of PR818S outside PR818S. The main reason is that the ball spacing (0.65mil) of PR818S is too small, the maximum line width is 5mil and the via must be in the center of the four balls, so no layout design engineer uses 5mil line to route the inner circle line of PR818S. When all the lines are pulled out, everything is easy.

Specifications of MSB System

Since MSB is positioned as a derivative product of PMP, its design ideal is to provide "Video-To-Go" like PMP, but it cannot allow consumers to enjoy videos "anytime, anywhere". Instead, the videos are downloaded from PVR or PC network and played on computers, monitors or TV screens (car screens, etc.). However, it does not only play videos, as mentioned earlier, it also has the functions of hard disk external box (U disk), photo browser and video recorder:).

Figure 4: MSB board layout

Hard disk capacity: 2.5-inch capacity up to 120G

Transmission interface: High-speed USB2.0×1

Fast Ethernet × 1

Video playback supports: MPEG1(.Dat), MPEG2(.MPG, .VOB), MPEG4(.AVI, Divx).

Video control: play, pause, stop, fast forward, fast forward and volume adjustment

Video recording format: MPEG4 ASP

Image file support: JPEG

Music playback support: MP3 WAV

External output: AV out/in outputs/imports videos to PAL or NTSC TV systems

Three languages: Simplified Chinese/Traditional Chinese/English

Power supply: External power supply

Size: 13cm(L)×7cm(W)

Figure 5: PCBA block diagram

Whether MSB products can be quickly accepted by the market, price is of course a key factor. At least, it solves the "four no's and one no" problem of PMP, namely, insufficient power, which means that the battery life may be insufficient, various decompressions may be difficult, and the LCD response is not smooth. Obviously, MSB does not have this problem; the picture is not clear, which means that the palm-sized panel, image quality, and resolution are limited, and the viewing experience will be a bit depressing and dull. Obviously, MSB does not have this problem either; it is not cheap enough. Currently, HDD-type PMPs are priced at US$300, which is close to PDAs, while MSB is sold in the retail market and the hard drive belongs to the after-sales market; it is not "smart" enough, which means that video format conversion is difficult (for example, DV-MPEG4), and only a few experts or players can edit and use it on the computer, while MSB has encoding function, which can directly record and compress analog video (such as DV/VCD/VHS) into files suitable for playback and store them; there is not enough and convenient video content, which means that it is generally difficult to prepare and arrange images suitable for small-size playback. Since MSB supports multiple video formats and the resolution can be as high as D1, it naturally enriches the source of content to support dynamic imaging functions.

Next-generation system specification considerations

As for the design development of the next generation MSB, it is considered to be lightweight and easy to carry. For example, the 2.5-inch HDD is changed to a 1.8-inch HDD, and the power supply is changed to a built-in one. The type and size of the LCD is a matter of opinion, but it should not be a key factor affecting the selling price.

The MSB function includes "direct recording" of analog video, and even "pre-set recording" for high-end models. The TV tuner function makes it even more valuable! In addition, since the power supply has built-in function to upgrade USB devices to OTG (On-The-Go), it is convenient to read memory cards (or USB flash drives) or connect digital cameras. It can be used as a database and photo browser to make it a super-functional mobile companion device. This is the reason for the various integrated technologies mentioned above.