HDD (Hard Disk Drive) Embedded DSR (Digital Satellite Receiver)

ABSTRACT

This paper presents a new design and implementation of HDD (Hard Disk Drive) embedded DSR (Digital Satellite Receiver). The digital broadcasting technology enables multimedia access via broadcasting system. The amount of digital data to be processed is increased as compared to the previous broadcasting environment. In this paper’ a method is proposed to provide storage ability of a DSR to cope with these changes. Efficient digital data storage and management technology is also discussed. The real data recording and playing back are the most important function of the proposed system. The trick modes such as fast forward and fast backward support are the other important factors of the proposed system as compared to the previous analog recorders. The above-mentioned functions are the main evolution criteria for the system performance. The DSR uses new file system that is designed by considering disk cluster size and limited memory in the system, which is more appropriate than that of the general personal computer. This system enables us to watch broadcasting and to manage multimedia data efficiently.

INTRODUCTION

As the digital satellite broadcasting is developing, the needs for efficient management of various multimedia dates have been increased. The increase in the number channel makes it difficult to select channel that they want. Efficient digital data storage and management are the indispensable as the demand for multimedia increases. This paper presents a system, which embeds the HDD to record the broadcasting data and play them back whenever the user wants. 

Nowadays, efficient storage and management of enormous broadcasting data are necessary, as the digital broadcasting is getting popular rapidly. According to the current situation, the international standards such as MPEG (moving picture experts group) –2, MPEG-3, and MPEG-7, have established efficient data storage and transmission. The standards such as DVB (digital data broadcasting) and ATSC (Advanced Television Committee) are established in order to apply these standards to the real broadcasting system. The characteristic of the digital broadcasting is multi-program emission through only one single physical channel so that the number of logical channels will be much more than that of analog TV channels. It is not easy to find the beloved program among a large number of programs because the number logical channels increases in geometric progress as the broadcasting environment changes from analog to digital. Therefore, the probabilities that the program the user wants can be broadcasted at the same time are increased.

The emergence of the digital broadcasting technology enables multimedia data access besides TV programs via broadcasting. As a result, the multimedia data will be increased remarkably. With increasing multimedia data, the needs for managing these data efficiently have been increased rapidly. Therefore, the combination of the digital data storage device and DSR is essential to store and process such a large amount of data. The only recording device used in the past years was VCR (video cassette recorder). This was suitable for the analog-broadcasting environment. However, as the broadcasting environment is changed to digital, the alternative solution for digital recording is needed, and HDD is regarded as the best solution for storage of digital data. The ATA (AT Attachment) is the HDD interface standard between host system and storage device. Almost all HDD used nowadays are IDE (Intelligent drive Electronics) or AT bus interface type, which implement the ATA standard. This type of HDD used because it is most prevailing and cheaper than any other HDD type. The HDD can satisfy these needs by its speed, accuracy, random accessibility, and low price. 

Multimedia information requires a new physical data structure for its efficient storage and retrieval. This data structure must meet the real-time dead line required for replaying the media. We develop an audio video stream recording system with efficient file management for the intelligent DSR. Time-shift viewing, which means that a program is recorded at the same time while another program saved previously being viewed, is one of the important functions of the proposed system.  This paper present design and implementation of DSR, which can store, retrieve and classify the broadcast data by implementing an interface between the conventional DSR and digital storage media.

 SYSTEM OVERVIEW

This system contains the HDD interface for data transmission in addition to the basic DSR. Figure 2.1 shows the block diagram of the HDD embedded DSR. The received data from the satellite is demodulated and changed into the digital signal. Then it goes through the NIM (Network Interface Module) block. These digital signals contain multiple programs that are mixed into one transport stream. The single program that is selected by the user is filtered at the DEMUX (Demultiplexer) part and sent to the DSR inner buffer. If the user wants to record the program, the data in the buffer will be stored in the HDD, otherwise the data will be directly sent to the MPEG decoders to decompress. When the user wants to play the recorded program, the data is read from the HDD and sent to the MPEG decoders. The MPEG decoders decompress the MPEG PES (Packetized Elementary Stream), and then send them to the TV screen and the speakers through the video encoder and the PCM (Pulse Code Modulation) respectively.  

Packetized elementary system (PES) is the Result of the packetization process. The payload is the data bytes taken sequentially from the original elementary stream. No specific format for forming the PES packet. For example, an entire video frame in one PES packet but it requires variable size frames. Note that the packets are of fixed size. PES headers are used to distinguish PES packets of various streams and also contain timestamp information. Using MPEG-2 systems layer which provides us with a standardized method of providing integrated video, audio and data services. Currently programs consist of one video channel and multiple audio channels. The data streams are used only to broadcast program related data, like close captioning. Multiple video streaming is possible for instance when we using multiple cameras for a particular shot.

Transport Stream Multiplexes various PES into one stream along with the information for the synchronizing between them. They are Short and packets of fixed length of 188bytes which includes a 4 byte header and the rest being the packet payload. There are constraints for forming transport packets under which each transport packet must contain data from only one PES packet and the first byte in the packet must be of the transport packet payload.

A/V Stream Save

The audio and video streams coming from the DEMUX are stored in the pre-buffer in the form of 184 bytes PES that excludes the 4 byte header information from 188 bytes TS (Transport stream). The stored size in the buffer can be less than 184 bytes as per PES contents. The buffers are needed because the HDD needs more access time than RAM. The audio and video streams in the pre-buffer are transferred to the HDD when the packet buffer level is larger than the size of the HDD writing unit, and then we can start storing additional header information along with the audio and video stream in the HDD. As the audio and video streams should be saved at the same time, it is necessary to separate buffering task and writing task for the real time processing. The GOP (Group of pictures) header location is also saved separately for random access and trick mode play.

A/V Play

For the play back, the audio and video streams saved on the HDD are read and classified using header information, and are transferred to the bit-buffers using the DMA (Direct Memory Access) respectively. The streams in the bit-buffer are decoded and played by the audio and video drivers. The basic idea of the architecture is to build flow control to avoid underflow and overflow by controlling delay time. Bit-buffer level should be checked before reading the audio and video streams from the HDD. To avoid overflow, few delay times are given to the tasks, if the buffer level is greater than the threshold. Here, we have assumed that underflow does not occur because the HDD access speed is much faster than the bit rate of the streams. While playing a saved file, for the ‘pause’ and ‘resume’ like a conventional VCR, we stop the task and restart it again respectively.

Random Access and Forward/Backward Skip Play

Instead of playing the whole file consecutively for a long time, user wants to play specific random position where the users want to play using the random access ability of the HDD, which is the most distinguishable ability as compared to tape recorders. If users select the play position, the nearest GOP (Group of Pictures) header position is found at that point.  The audio and video data are read at the GOP header position and transmitted to the decoder. The decoder searches a nearest intra-picture header and plays it at that point. Skip play is also done by moving the playing position forward and backward from the current position.                             

Trick Mode Play                             

To implement trick mode play such as fast-forward, fast  -backward and slow play, packs that include intra-pictures, are marked and saved in the table. Controlling decoder’s flow and using the characteristics of the MPEG video can implement fast and slow play. The decoder flow control method is as follows. For the normal speed play, the decoder decodes the frame at every vertical sync interrupt and wait until the next interrupt. For the fast play, this wait is removed and next interrupt is made instantly and then the decoding process speed is increased. For the slow play, if this interrupt accumulates up to the fixed time, the picture is decoded. The other method using the characteristics of MPEG video is that only marked packs are read from a disk and delivered to the decoder. The frames to skip decoding depend on the picture type. Fast play, less than 12 times speed, uses skipping method. Fast backward play can be implemented by searching the GOP header position in the reverse order, and then only intra-picture is decoded. To play all frames, memory is needed to store more than12-decoded frames.

FILE MANAGEMENT OF HDD

We design and implement the file system for efficient management of the saved data. This file system offers direct file access service to the application program.

As an HDD device is primarily designed to record burst data, it is suited for audio and video signals and other continuous data. So there are few problem associated with recording of high bit rate audio and video signals on a conventional file system designed for storing the computer data. To increase the disk access efficiency, a disk should be accessed at the optimal size that minimizes the rate of the overhead, such as head-seek time and latency of the occupation time. We designed efficient file management system, which is called ‘Simplified FAT Method’, when we store audio and video streams. We read and write 8 sectors at one time. Audio and video have there own header information per writing unit. One cluster consists of multiple of HDD writing units. File system finds the nearest cluster from the current writing cluster. By using the proposed file system, file copy, delete, move and directory management operations are also possible. There are three parts in the file system: disk management, file management and directory management. For the disk management, we have to design logical format of disk like PT (Partition table), BR (Boot Record), FAT (File Allocation Table) and directory entry. 

APPLICATION PROGRAM AND USER

INTERFACE

If users press the REC (Record) button on the RCU (Remote Control Unit) while watching the broadcasting, the recording process starts. A saved program file is named from the channel name concentrated with serial numbers. The user can also change file name. While recording, the small REC box appears at the left upper side on the screen to indicate the recording process. If the users want to change the channel in the same transponder, or to change the channel to a different transponder, a message is given to select whether to stop recording or not. The operating routines for recording are as follows,

1. If current status is recording, stop recoding.

2. Get information whether a current channel is TV or radio, and whether it is locked or not.

3. Do ‘file open’ to find the empty space in the HDD.

4. If the ‘file open’ operation is successful, extract single event from MPEG-2 system, i.e., to stop current watching streams and take new streams to record.

           If the users press the STOP button on the RCU, recording will be stopped and continue watching the current channel. Operating routine for stopping recoding as follows, 

1. Do ‘file close’ to close the saved file.

2. If the watching channel and recording channel are different, continue watching the current channel, else stop the recording stream in the buffer then send the stream directly to the decod            

FUNCTION IMPLEMENTATION AND

PERFORMANCE

Because OS (Operating system) in DSR does not support the HDD management service, low-level services for HDD management should be developed. ATA interface through the PIO (Programmed I/O) mode 4 which can support access speed up to 16.6Mbytes/sec is implemented. Storage capacity is 20Gbytes, and user can expand the HDD capacity. HDD can store over 10 hours of program for the stream with 4Mbyte/sec stream rates. In order to implement these system services, we will divide low-level services into five hierarchical layers and endow each layer with the independence of the others. It prevents bugs of one layer from spreading the others. The following classification is preferred for the layered system services design.

1. Physical interface layer

· It must be designed with compatible ATA specification. 

· A 40-pin connector will establish interface between HDD and DSR.

· There are two types of transition modes: DMA and PIO. PIO mode will be used.

2. Connection layer

· Extracting single TS, which is associated with a single program, from the multiplexed TS.

· Data linking and transmission to HDD.

3. Low-level service layer

· Translation between physical and logical address of the HDD.

· Low-level error handling.

4. File system management

· Present direct file access service to the application layer.

· Directory, file, disk managements.

5. Application layer

· User interface. 

· High-level file managements.

If the implementation system uses nBYTE=2, then maximum disk size is as follows. If the sectors per cluster are 1024, 1024sectors * 28*2 * 512bytes=32,768Mbytes. Figure and table shows the relation between cluster size and performance of the file system for the file saved for 1 minute if we suppose that the general stream rate is 500Kbytes/sec=30,000Kbytes/min. file read/write speed is not affected severely by the cluster size through the speed increases as the cluster size increases. The memory allocation size decreases as the cluster size increases in the fixed disk size. The maximum disk size also increases as the cluster size increases. Disk usage efficiency decreases as the cluster size increases. By considering the above four performance factors, it is best choice to use a cluster size of the maximum disk size for the fixed disk size, e.g., if the disk size is from 16,384Mbytes to 32,768mbytes, 1024 sectors are used.

CONCLUSION

We have discussed the design and implementation of HDD embedded DSR. ATA interfacing between high capacity storage media and the DSR with support PIO mode is implemented. A single PES (Packetized Element stream) is extracted from the multiplexed MPEG-2 TS (Transport stream). The proposed system can record program to the HDD for more than 10 hours duration. We also implement a user interface such as PIP view of the saved broadcasting data. File management function such as delete, move, modification of the saved file and the disk management functions using of the proposed system are also implemented. Digital video function such as record, play, pause, random access forward and backward skip, fast and slow play give users conventional VCR function.

Recording and playing the real time data are the most important function of the proposed system, and the trick mode such as fast forward and backward support are the other important factors of the proposed system. The above functions are the main evaluation criteria of the system performance. It is possible for the proposed system to record enormous received data in order to rewind missed scenes or watch recorded programs after relatively long time. In addition to this simple storage functionality, adding efficient video retrieval and processing functionalities, which are helpful for users to find a data efficiently, will be further studied.