At present, there are still many consumers who have a vague understanding of MP4. They simply think that MP4 is the next generation of MP3 and a player that can play MPEG-4. Later, with the accumulation of knowledge, MP4 Have a relatively systematic understanding. It is difficult to describe the exact concept of MP4 in one word, because no matter in terms of MP4 brand, market, product specifications, configuration standards, etc., it can all be described with one word - "chaos". I don’t know if it’s a hype or a trend, but in the eyes of current businesses, any player that can play video is called MP4. This statement is simple and intuitive, and seems to be basically consistent with the author’s vague understanding at the beginning.
There are different opinions on the concept of MP4. The picture below is the author’s understanding of the concept of MP4 based on the opinions of hundreds of experts. MP4 is both “soft” and “hard”, and it is complicated.
Audio MP4 format: AAC
MP4 was originally an audio format and has little to do with MPEG-4, just like MP3 has nothing to do with MPEG-3. MP3 is the abbreviation of MPEG-1 Audio Layer 3; and MP4 is MPEG-2 AAC, which is completely an audio compression format, adding new features such as perfect reproduction of stereo sound, multimedia control, noise reduction, etc., and the most important thing is Yes, MP4 achieves digital copyright protection through special technology, which is incomparable to MP3.
Video MP4 format: MPEG-4
Most of the MP4s on the market now tend to be multimedia players, and those that can play AAC can be said to be rare. Some media even referred to MP4 as the abbreviation of MPEG4. This seemed to be a fallacy in the past, but after the continuous hype of merchants, this fallacy has become the truth. Now if you go to a computer store and hear the word MP4, it is definitely a concept of a multimedia player that can play video formats, not a Walkman that can play audio MP4 AAC. This strange phenomenon is not without reason. As mentioned above, AAC has a copyright protection function. This is also the reason why many record companies support AAC. If you want your player to support AAC, you must pay a certain copyright fee or patent. In addition, the source of AAC is also a problem. It is not as open as MP3, and there are very few online sources. Therefore, the development of audio MP4 players is not yet mature. In view of the above situation, many merchants simply "resurrection" and equate MP4 with MPEG-4 In terms of abbreviations, this also coincides with the rule that MP4 is the next generation of MP3. In addition to supporting the music playback function of MP3, it also has powerful MPEG-4 video playback capabilities. In addition, the "4" happens to be in the "3" "Later, from this point of view, it is reasonable to equate MP4 with MPEG-4.
MP3 that can play videos
This kind of player is not actually MP4, it is essentially MP3. Video playback is just its accessory function. It is called MP4 and is purely commercial. Hype. This kind of player has great limitations. The screen is small (0.8~1.8 inches), the flash memory capacity is small, and it supports specific formats (MTV, MP4, MPV and DMV, etc.), and most of them use low-end screens such as OLED and CSTN. So to be precise, this can only be called an MP3 that can play videos. Representatives include: DEC F12R, Jinbang Colorful King, 3E E1000 and PISA Colorful Airship, etc.
Hard drive MP4
This type of MP4 is the mainstream of development at this stage, and the number of products also accounts for an absolute advantage. The concept of hard drive MP4 simply uses a hard drive as the medium. Watch it anywhere. A big belly can accommodate the world. With a hard drive, you no longer have to worry about insufficient space. Generally speaking, hard disk MP4 also integrates many other functions, such as digital cameras, camcorders, voice recorders, digital companions... There are 11 hard disk MP4 models in this Hengping review.
Flash-memory MP4
Compared with hard-disk MP4, flash-memory MP4 is a portable device that uses flash memory as the storage medium. This kind of MP4 generally supports internal flash memory card expansion. It's an SD card. There are two MP4s of this type in Hengping this time, namely Govideo PMP-120 and JXD-680. Flash MP4 is much smaller and lighter, and its price is several times cheaper.
MP4 without display screen
Some manufacturers believe that the 3.5-inch screen of MP4 is too small to play high-quality videos. In addition, the size of 128~256M of flash memory MP4 also makes it difficult to play high-quality videos. People cannot appreciate blockbusters, so these manufacturers have designed a hard drive MP4 without a screen. This MP4 can be output to a TV screen through an output terminal such as AV-OUT, and uses a 2.5-inch hard drive, which is relatively large in size, but for This kind of MP4, which does not emphasize mobility, is much smaller than traditional DVD players. There are only a few models on the market. This kind of MP4 has a capacity of tens of gigabytes, but its price is as high as that of flash memory. Similar to MP4. For example, BoVis MP4.
About the various names of MP4
Although the concept of MP4 can be divided into soft and hard, but in terms of hardware MP4, it differs according to the design standards adopted or the functions. They have different focuses and different names, for example, MP4, PMP, PMC, PVP, PVR, PMA, etc.
MP4
A general concept without a unified definition. What we are talking about here is the concept of hardware MP4. MP4 can be watched on the go, it can be the next generation of MP3, and it can be used for everything. PMP, PMC, PVP, PVR...the collective name for many such players.
PMP (Portable Media Player)
There is no unified standard, but it is produced completely according to the manufacturer's own requirements, such as iRiver PMP-120. The software system of PMP-120 is based on Linux. PMP systems are generally developed on Linux. There are no fixed software and hardware requirements and can be combined in any way, but this is also the reason for the diversification of MP4 products. In addition, the format compatibility and decoding capabilities of PMP are related to the hardware, so it is difficult to upgrade.
PMC (Portable Media Center)
The hardware and software of this platform are uniformly stipulated by Microsoft. The hardware is designated to use Intel's XScale processor, and the software system is Windows Portable Media Player. , this platform has an open architecture and is easy to expand in terms of software. PMC is a new standard developed by Microsoft to enter the mobile entertainment digital field. For example, YH-999 completely complies with PMC standards.
PVR (Personal Video Recorder)
The function of PVR focuses on video recording. It can be said that PVR has powerful video recording function. PVR generally has AV-IN/AV- OUT or video recording function. For example, iTouch, Mustek PVR-H140.
PVP (Personal Video Player)
Similar concept to PMP.
PMA (Pocket Media Assistant)
Simply put, it is a combination of PDA and hard drive MP4. It is a relatively trendy digital product. The Archos PMA 4XX series is the world's first and only one today. A PMA that integrates personal entertainment, business applications and wireless Internet access.
2. Hardware architecture of MP4
The "core" of MP4 is dirty
In principle, there is not much difference between MP4 and MP3, but in terms of hardware performance, Said that the two are far apart, mainly because of the video playback function. The playback of MPEG-4 such as Divx and Xvid requires higher processing power of the central processor and DSP, and a certain amount of system memory. At the beginning, encoder developers used computers with a main frequency of 400MHz or above to complete decoding. It can be seen that MP4 requires the chip to have high computing performance. Many MP4 gorgeous operating interfaces will also consume a lot of system resources. MP4 is not just for video. A processor of data and image data. Today's MP4 is still the unity of many digital functions and multimedia functions. It needs to realize various functions, such as digital companion, video capture, DC, FM, Game... and even Some MP4 also support multi-line work. Therefore, the MP4 chip not only requires high computing power, but also integrates multiple functions.
Since the production of MP4 core chips requires too high a level of craftsmanship, general MP4 manufacturers are unable to do anything. These cores are generally made by companies with traditional chips, such as Intel, TI, Sigma Designs, etc., and even AMD. Launched MP4 dedicated processing chip - AU1200. At this stage, MP4 mainly uses TI's solutions and Wintel's complete software and hardware solutions. Among them, TI (Texas Instruments) is a chip giant for mobile entertainment devices, and Intel is a chip giant for desktop platforms. Since TI started early, most MP4s now use TI's chip solutions to occupy the majority of the market share. Although there are certain standards for hardware, there is no unified standard for software systems. Instead, merchants develop their own software (mostly Linux), which is also a big reason for "chaos". Nowadays, most manufacturers use TI's solution, the main ones include Archos, as well as Sony and Toshiba from European, American and Japanese digital manufacturers.
The Wintel alliance has launched a new PMC standard. The central processor uses Intel's XScale series, and the software system is Microsoft's Windows Portable Media Center system. Both software and hardware have unified standards, fully embodying Microsoft's It is ambitious to enter the portable media market. Since the system is Windows Portable Media Center and the overall architecture is open, application software can be expanded on the basis of the operating system. Wintel's dislike unified the industry's standards and to some extent pointed the direction for the development of MP4. Currently, this program is supported by iRiver, MSI, Samsung and Innovation.
TMS320DM270 chip:
TMS320DM270 internal structure:
From the core of TMS320DM270, it can be seen that this chip is an extremely powerful chip, mainly composed of It consists of three cores: ARM (80MHz), C54xDSP (90MHZ) and iMX (180MHz). The ARM7 RISC microprocessor is mainly responsible for the management of external interfaces, the C54X DSP digital signal processor is responsible for various audio processing, and the iMX engine processes video data and image data. In addition, the integrated various functional modules and controllers enable it to have other powerful functions, such as photography, card reader, recording, AV-IN/AV-OUT, etc.
TI's MP4 system solution:
TI's tms320dm270 solution is one of the most advanced MP4 solutions at present, with good image quality and good video format compatibility. This time Most of Hengping's MP4s use this solution. Most of the internal core architecture of MP4 uses the central processing unit to work in coordination with the DSP. Generally, the central processing unit and the DSP are integrated together.
DSP (Digit Signal Processor) is mainly responsible for video decoding and encoding; while the CPU is mainly responsible for file management, access, and control of user interfaces and peripheral components. In addition, special chips need to be added to some functional modules, such as DC control modules and display control modules.
As seen from the picture above, the solution mainly consists of audio processing, video display, access management, power management, video acquisition and TMS320DM270.
Audio processing
In terms of audio processing, TI mainly uses its own developed TLV320AIC23B encoder, which is a high-performance and low-power stereo audio Codec chip with a built-in headphone output amplifier that supports There are two input modes, MIC and LINE IN, and both input/output have programmable gain adjustment. AIC23 integrates analog-to-digital conversion (ADC) and digital-to-analog conversion (DAC) circuits. The output signal-to-noise ratio can reach 90dB and 100dB respectively, and can provide 16/20/24/32-bit sampling in the frequency range of 8K~96K. . The sound quality is relatively pure, with high fidelity, loud treble and solid and clean bass. The MSI MEGAWVIEW 561 reviewed this time uses this audio decoding chip.
Video display
Video display can be output to the outside through NTSC or PAL format, or it can be output to the built-in LCD screen. The screens currently used by MP4 mainly include: CSTN , TFT and LPTS LCD screens. For knowledge about the characteristics and effects of these three screens, please refer to Pacific's earlier article: "Comprehensive Explanation of Literacy Movement Portable Media Player MP4".
Access management
The current mainstream MP4 hard drives use 1.8-inch hard drives from HITACHI, FUJITSU and TOSHIBA, and occasionally 2.5-inch laptop hard drives are used; and for To save costs, there are also MP4s on the market that use small-capacity and reduced-volume flash memory. However, for MP4, flash memory is used as the medium. Judging from the current technical background and market price, although flash memory has advantages that hard disks cannot match (stability, energy consumption low, good shock resistance), but flash memory is not the right way yet. Since HITACHI released the 0.8-inch microhard drive last year, it has brought new hope to MP4 which is in an embarrassing situation.
Power management
Regarding the power supply problem, just like the film source, it has always been the Achilles heel of MP4. A general hard drive MP4 can only support the video playback life of a blockbuster movie. This is absolutely not enough, especially for MP4s with built-in lithium batteries. This is extremely inconvenient, because the battery life is too short and it loses the meaning of watching it with you. However, this time we saw new hope for the battery problem from the more than 10 MP4 models reviewed. Some MP4s use removable lithium batteries and some use AA dry batteries. What is even more gratifying is that some of them can play videos continuously for 8 hours. and 11 hours. If you want to know who is right and who is wrong, please read below.
The core used by Microsoft PMC - Intel XScale PXA255
The internal composition of PXA255
3. About MPEG-4 encoding and format
The encoding formats supported by MP4 players can be said to be "chaos within chaos". Except for PMC, since there is no unified standard for the hardware platform, the software system is also designed by the manufacturer itself. This results in the decoding compatibility of MP4 players and The fundamental reason for the uneven decoding stability; in addition, there are many formats derived from the MPEG-4 encoding system. In addition to MP4 other than PMC, the decoding compatibility is related to the hardware, which greatly increases the difficulty of decoding. Video formats There are many such as AVI, ASF, MPG, WMV, and there are also many encoding algorithms: MPEG-1, MPEG-2, MPEG-4, and the MPEG-4 encoding supported by most MP4 players has many derivatives. Encoding algorithms: DivX, XviD, H.263, MS MPEG-4 3688, Microsoft Video1, Microsoft RLE...
Among the streaming media formats, ASF and WMV developed by Microsoft are Using MPEG-4 encoding, some MP4s support the two streaming media formats ASF or WMV. However, RM and RMVB, two streaming media formats widely circulated on the Internet, are not supported by any MP4. The mpg format can be either MPEG-1 and MPEG-2 encoded or MPEG-4 encoded.
Another thing to note is that MPEG-4 does not determine what extension must be used. It is just an encoding method. Using avi as the extension is a habitual use, which is consistent with the standard. AVI is somewhat different. Nowadays, mainstream MP4 generally supports DivX or Xvid encoded avi format, but many other encoded avi formats may not be supported. Finally, format compatibility is also related to the resolution, frame rate and bit rate of the video format. If an MP4 player that supports DivX cannot play DivX avi, it may be related to the previous three factors.