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With the wireless transmission of video compression technology, as well as the
rapid development of wireless networks through real-time video transmission has
become possible. The existing short-range communications technology transfer
Bluetooth, 802.11 and so on. Bluetooth technology, compared with other most
obvious advantage is low power consumption, small size and strong
anti-interference ability. This is in line with the advantages of the current
mainstream in portable video transmission system. However, Bluetooth's bandwidth
is quite narrow, if you want to meet real-time performance and image quality
requirements of both sides, we must choose a high compression efficiency of the
image encoding standard, MPEG4 encoding technology to meet this requirement.
This article is based on the above factors, the use of embedded
systems technology and Bluetooth technology MPEG4 codec combines a video image
transmission to the program, and established a whole system of system
performance testing, reached a real-time and high image quality of the transfer
request.
Key technologies and core chips, introduced
Bluetooth technology
"Bluetooth" is a short-range wireless connectivity
technology standard in lieu of that, the substance of Bluetooth is to establish
a common air interface and radio control software for open standards.
"Bluetooth" plan for all types of network data and voice equipment, the use of
wireless microwave way for them to go into a slight net more than can be piconet
interconnection between, thus quickly and easily between a wide variety of
equipment communications. Bluetooth to adopt decentralized network structure, as
well as fast frequency hopping and short-packet technology to support
point-to-point and point-to-multipoint communication, common in the global
2.4GHz ISM (ie industry, scientific, medical) band using time division duplex
transmission scheme to achieve full-duplex transmission.
Bluetooth 1.0
standard by the two documents. One is the core of the (Foundation Core), he
provides the design standards. Subset of the other is part of the agreement
(Foundation Profile), he is the operation of the provisions of the guidelines.
Bluetooth protocol can be divided into 4 layers, that is the core protocol
layer, protocol layer to replace the cable, telephone control protocol layer and
other protocol layer adopted.
The core of the agreement, including Bluetooth
baseband (baseband), LMP (LMP), Logical Link Control and Adaptation Protocol
(SDP) 4 parts.
Link Management (LMP) is responsible for the Bluetooth
component of the establishment of inter-connected. Initiated through the
connection, exchange, verification, identity authentication and encryption and
other security tasks; through consultation to determine the size of base-band
data packet; he also controls the power mode of the wireless unit and the work
cycle, as well as a Bluetooth piconet with components the connection status.
Logical Link Control and Adaptation Protocol (L2CAP) is located above
the base-band protocol layer, data link layer is a high-level transmission and
application layer protocol agreements shielding the base of the adapter with the
agreement.
Business search protocol (SDP) is of paramount importance,
he is the basis of usage patterns. Through the SDP, can query device
information, business and operational characteristics, and in the query after
the establishment of two or more connections between Bluetooth devices.
Bluetooth as a global open standards for wireless applications,
through a variety of voice and data devices with wireless links to connect
together so that people can at any time for data exchange and transmission of
information. There is no doubt that he will be in people's daily lives and work
plays an important role in the 21st century is becoming a hot spot for
investment.
MPEG4 encoding
MPEG4 officially in early
1999 as an international standard, is a low transmission rate for the
application of the program, he is not only for a certain bit-rate video coding,
more emphasis on interactive multimedia systems and flexibility.
MPEG4
standard is mainly used in video calls, video e-mail and electronic media, he
requested a lower transmission rate, MPEG4 very narrow bandwidth use, through
the frame reconstruction, compression and transmission of data in order to use
the least data get the best image quality.
MPEG4 technology includes
two main parts: the coding of audio-visual object description coding tools, as
well as tools and syntax language object code. MPEG4 coding standard is based on
the object, so easy to operate and control, while the traditional compression
method is based on the frame. Due to transmission bandwidth constraints, the
need for compressed bit-stream control, which directly affect the image quality.
Low bit rate in the past, the entire frame image quality is not affected at all
flexibility; and MPEG4 in control of the bit-rate object-based, even in
low-bandwidth, they can use the method of bit allocation for the user a sense of
the object of interest can be allocated with more bit-rate, and for the target
user is not interested in a smaller number of bit-rate allocation, so that the
subjective image quality can be guaranteed. A video object can contain a layer
or multi-layer scalable coding to achieve. Scalable syntax can be expressed in
the form of video layer reconstruction, the basic layer video encoding from the
beginning, together with a certain number of enhancement layers, so that
bandwidth or computing power in accordance with the different options to
generate and MPEG4 video streaming. An MPEG4 video scene contains one or more
video object. Each video object are time-domain characterization and spatial
information, these information objects shape, movement and texture to express
the form. At present, MPEG4 encoding technology is already quite mature, and
because of its excellent multi-media performance is receiving increasing
attention.
OMAP1510 and GO7007SB
OMAP1510 Dual-Core has a
unique structure, the processor includes an enhanced ARM processor (ARM925) and
the development of TI's new low-power fixed-point DSP (TMS320C55x). Design of
the dual-core components of the purpose is to effectively deal with the
application of multimedia applications and MMI. In these two core, ARM925 for
the implementation of the operating system (OS), and the DSP for the processing
of all multimedia applications. In addition to these two outside the core of a
powerful, OMAP components are also available in a variety of on-chip
peripherals, allowing users to an almost seamless manner with USB, UART,
Bluetooth and GSM modules and other components common component interface.
GO7007SB is a multimedia video compression chip, the use of a wide
range of algorithm and the compressed video data buffer, through the USB port
power supply and transmission of data, the output of the video stream can be
MPEG1, MPEG2, MPEG4, MJPEG, or H.263 format, the output images to distinguish
rate range 64 x 64 ~ 720 x 576, and can be modified according to the specific
needs of the corresponding registers and set encoding parameters. He comes from
one of the encoded data CMOS sensor.
System
Implementation of
the system are: system video data collection, encoding and the sending end by
the sensor, GO7007SB, OMAP1510 and Bluetooth CLASS2 (BC02) 4 parts; video data
reception, decoding and display client using CLASS2 (BC02) and OMAP1510 (DSP for
data decoding, LCD display) combination, which constitute the system as a whole.
The data in the system mainly refers to the collection and coding of video
image data, data transfer in hardware in the process of the arrow shown in
Figure 1.
System Software
Comprehensive GO7007SB the SDK, the
Bluetooth protocol stack and data flow in the system 3 factors, and ultimately
determined by the process shown in Figure 2 to achieve the video data
collection, encoding and sending functions, according to the process shown in
Figure 3 to video data reception, decoding and display.
The
experimental results and analysis
The eventual realization of a systematic
program to build:
The sending end
(1) CMOS sensor;
(2) GO7007SB coding;
(3) format: MPEG4 compression, NTSC
format, QVGA format (320 x 240);
(4) OMAP1510 control and operation;
(5) Bluetooth CLASS2 (BC02) sent.
The receiving end
(1) Bluetooth CLASS2 (BC02) to receive;
(2) OMAP1510
decodes the DSP;
(3) OMAP1510's LCD display.
Video image
transmission is most concerned about the issue of real-time performance and
image quality in video coding using the premise of GO7007SB, that is a
prerequisite for determining coding efficiency, the frame rate and bit rate on
the impact of these two very significant, and at the same time GO7007SB encoding
parameters and Bluetooth settings must match the transfer rate, the real-time
video transmission can be guaranteed.
BC02 Bluetooth module value of
the theory of transmission can be achieved 1Mb / s, author of the actual testing
of the system after the BC02 was found in the average transfer rate 700kb / s or
so. Although it can be carried out GO7007SB bit rate and frame rate settings,
but can not be strictly limited, a number of image data by the Statistics and
found that GO7007SB bit-rate than the actual value must be set the value of the
variable, when set up the encoding bit rate of 512kb / s, he may be the actual
value has reached 600kb / s and above.
Combination of these factors, taking
into account the receiving end DSP decoding and the efficiency of LCD display,
in order to achieve the best video transmission effect, decided to set up the
encoding bitrate GO7007SB for 512kb / s, frame rate to 15 frames / sec, image
format, QVGA ( 320 x 240). By the end of the test system, the data in Table 1.
Real-time video images to meet the basic needs of the requirements
of transmission, QVGA format image quality to meet the requirements. Overall
this experiment has reached the desired effect.
Conclusion
Experimental use of the OMAP1510, GO7007SB and Bluetooth CLASS2 (BC02) as
the sending end, OMAP1510, CLASS2 and OMAP1510 in DSP decoder as a receiver, and
ultimately the adoption of Bluetooth in real-time video encoding data
transmission channel of the system. For the conditions of this experiment is
concerned, has reached the transmission of video data of image quality and
real-time two requirements, while the system still can be improved to enhance
performance can be carried out in two ways:
(1) air transport parts:
the experimental use of BC02 module author, and his limited transfer rate, BC04
modules can achieve the theoretical transfer rate of 3 Mb / s, if the
replacement of BC04 to BC02 module module, then the improvement of system
performance can be like common sense;
(2) codec optimization: the use
of DSP to decode the OMAP1510, his decoding rate of 20 / s, if the decoder is
more efficient to upgrade the existing decoder or decoding efficiency, system
performance will be improved . Time and experimental conditions as a result of
the reasons, the author did not build, and test decoders BC04 and other
components of the system to achieve performance.
Generally speaking, the
design of the system, decoding and display on the back-end hardware and software
are more relaxed, at the same time be applied to monitor, multimedia
entertainment and many other occasions. In today's mainstream in the direction
of portable multimedia applications, after all, is a better choice.
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