Energy efficiency of mobile video decoding Tero Rintaluoma Olli Silven Hantro Products Oy Department of Electrical and Information Engineering Kiviharjunlenkki 1 P.O.B. 4500 FI-90220 Oulu, Finland FI-90014 University of Oulu, Finland Email: [email protected] Email: [email protected] TABLE I Abstract-In this paper, we consider the energy efficiency of CHARACTERISTICS OF TYPICAL PORTABLE MULTIMEDIA DEVICES. implementations of video codecs for mobile devices in a top-down manner. We start from typical applications and analyse device Portable Handheld Typical ratio architectures, codec implementations, and software platforms. Laptop PC Multimedia Terminal The physical size of mobile devices limits their heat dissipation, Display size (inches) 12-15 2-4 5x (area 20x) while the battery capacity needs to be used conservingly to Display resolution (pixels) 1024x768- 176x208- 15x provide for satisfactory untethered active use time. Together 1600x1200 640x240 with the required versatile capabilities of the devices, these Processor DRAM (MB) 256-1024 16-64 16x Processor clock (GHz) 1-3 0.1-0.3 lOx are essential constraints that must be taken into account from Max. power dissipation (W) 60 3 20x hardware to application software design. In video decoding Surface area (cm2) 1500 150 lOx additional constraints come from the need to support multiple Heat dissipation (mW/cm2) 40 20 2x digital video coding standards, and the platform oriented design Video resolution 720x576/25Hz 640x480/30Hz lx regimes of the device manufacturers. Battery capacity 4000mAh/14.4V 10OOmAh/3.6V 15x I. INTRODUCTION Wireless multimedia applications typically use content pro- vided via the web or broadcast services such as DVB-H [1], or the larger display of the laptop explains less than IOW of the play back locally stored music and movies. In addition, users difference. From the usability point of view the user interface can create content and stream it to the network for redistribu- is the biggest difference between these categories of devices. tion or make video calls that require real-time streaming. The The usability of the handheld devices is critically dependent popularity of laptop PCs as DVD players and as a means to on their active use times; these in turn depend on the energy access multimedia content via public WiFi networks can be efficiency. Table II presents the power consumption breakdown a prediction of the future uses of wireless terminals. We may of an early 3G phone in 384kbit/s video streaming mode [2], suspect that if the uses are similar, the same may apply to the clearly showing the limitations of the multimedia implemen- technical solutions. tation. Only 600mW is available for application processing, in The requirements for wireless mobile terminals are tough, this case decoding of video bit stream into sequences of image especially when considered from the energy efficiency point frames. This is a very hard requirement for software solutions. of view. At the same time high demands are placed on the With a 1000mAh battery the active time is limited to around an usability that includes not only the intuitiveness of the user hour. The application power needs of the PDA device [3] are interface, but also the length of active usage time between in the same region, while the larger display and frame buffer charging the batteries. memory explain most of the higher power consumption. The A typical laptop PC user carries a charger and connects to hypothetical power budget for a future device that provides for the mains whenever possible, and uses the device while sitting three hours of active use time has been estimated based on the down. In contrast, a hand held device is expected to provide data of the most power efficient system components available for a longer active use time as they are used anywhere in an today. untethered manner, and are charged only at night. Another energy efficiency related aspect is heat dissipation. This is The increasing bandwidth needs and the increasing com- mostly the concern of the handhelds, as most of the time the plexity of air interfaces make it very difficult to save in laptop devices are desktop operated. RF and baseband signal processing, while essential efficiency The characteristics of typical wireless handheld and laptop improvements can be expected from the display technologies, multimedia devices are compared in Table I. The application e.g., by switching from TFT LCDs to OLEDs. However, requirements are almost the same, but the handheld devices application processing is usually the first target when looking provide the services using around l/10th of the size, energy, for ways to cut down power. and processor speed. The maximum heat dissipation via sur- In the following we consider the energy efficiency issues faces can be only half of the laptop level to prevent the devices from the point of view of mobile video decoders. Comparative from becoming too hot to handle. The power consumption of evaluations are presented when data is available. 1-4244-1058-4/07/$25.00 C 2007 IEEE 103 TABLE II TABLE III POWER CONSUMPTION BREAKDOWN EXAMPLES OF POCKET SIZED MULTIMEDIA USE CASES FOR A PROCESSOR PLATFORM ON 3.6V 800MAH DEVICES. BATTERY. Use case Target Power Usage time System component Power consumption (mW) bitrate (Mb/s) consumption (mW) (h) 3G phone in PDA device Expected future capture 1-4 video streaming in MPEG-4 playback mobile devices Video 350 8 mode [2] [3] Movie playback I 500 6 Application processor 600 833 100 and memories Display, audio, keyboard 1000 2441 400 and backlights (UI) Misc. memories 200 754 100 provided for encoding, while decoding is performed in soft- RF and cellular modem 1200 N/A 1200 Total 3000 4028 1800 ware. We may conclude that hardware for video decoding Battery capacity 1000/1h N/A 1500/3h mAh/usage time would cut the consumption figures of playback below the currently estimated ones. The assumed processing platform implementation technology in this case is 90nm CMOS. II. POWER CONSUMPTION AND BATTERY LIFE III. MOBILE VIDEO APPLICATIONS A standard way to estimate the battery life time at a The multimedia applications of mobile devices include the given rate of discharge is the well-known Peukert's law [4], uses as camcorders, video phones or mobile digital TVs, and although it can be somewhat inaccurate for mobile devices. have an impact on system designs and power budgets. The This is explained by the dependency of the battery capacity more versatile devices tend to be less energy efficient due to on temperature that in the absence of active cooling strongly the added software and hardware complexity, and platform depends on the load current [5]. Figure 1 below shows the technologies needed to support rapid development. actual behaviour of a 64OmAh LiON battery in a PDA device Camcorder use requires real-time encoding and preview under constant load based on the experiments in [4]. As capabilities, while during playback decoding and display are the battery life is a non-linear function of the load current, needed as illustrated in Figure 2. Encoding in consumer use is improved power efficiency in the knee region of the curve mostly limited to short DI (720*576@25frames/s) sequences will result in super-linear improvements. within the memory capacity of the device, but requires signif- icantly more processing and power than decoding. Typical consumer camcorders need around 8-9W of power 1200 in encoding mode, while their displays are in the same size 1000 I class as multimedia capable mobile phones. Approximately 1- 2W [8] of the disparity is explained by the electro-mechanics 800 of the DVD drive and additional electrical interfaces of the camcorders, but a significant portion, 6-7W comes from the 600 computing platform and display interface. 400 Decoding flow 200 Mass Post Display Memory Processor Device 0 0 100 200 300 400 500 600 700 Encoding flow Discharge current (mA) Camera Pre- -0- 0 Display Interface Processor Device Fig. 1. Discharge time of 64OmAh LiON battery u]nder constant load. Encoder ; Mass Table III shows estimates provided by a multimedia pro- lviciiiory cessor supplier [6] for a hypothetical mobile device with a smart energy efficient display. The video standard in question Fig. 2. Decoding and encoding data flow. is MPEG-4 SP [7] at 30 frames/s that is close to DVD quality. The power results are for a full system including all compo- The mobile TV is about to create a demand for terminals nents such as camera, display, speakers, etc. However, this that support several simultaneously decoded program streams device lacks wireless connectivity. For comparison, Microsoft to provide for living thumbnails, as shown in Figure 3. This Zune player is based on the same processor technology and feature comes from the expectations to seamless channel achieves 4h video playback in QVGA format on 800mAh 3.7V surfing despite the energy saving time-slicing technique used battery. in the air-interface of DVB-H. However, thumbnails and split In the case of Table III video encoding consumes less power displays effectively multiply the power and memory bandwidth than decoding. This is explained by hardware acceleration needs of the decoding task. In practice, either at least two 104 TABLE IV decoders, as in some digital TV set-top boxes, or the shared PROCESSOR CYCLES/S AND POWER NEEDS OF MPEG-4 AND H.264 use of the decoding resources are needed. DECODERS (VGA 30 FRAMES/S, 47OKBIT/S) ON THREE INTEL PROCESSORS. Processor EPI (nJ) MPEG-4 H.264 Cycle rate Power needs Cycle Rate Power needs (MHz) (mW) (MHz) (mW) Pentium 4 48 273 5060 725 13440 (Cedar Mill) Pentium M 15 400 2320 1060 6140 (Dothan) Core Duo 11 280 1190 744 3160 (Yonah) TABLE V POWER NEEDS OF H.264 DECODERS ON ARM PROCESSORS (VGA 30 FRAMES/S, 512 KBIT/S).