Getting the Most out of Your Video Gadget Storage - Brief Theory
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The video formats that are supported by mobile phones center around 3GP and MPEG4. Sure, newer cell phones and the latest and greatest portable media players may have AVI, DivX, FLV (flash video), XviD, and so forth, but we are going to focus on the basics. All of the latest cell phones support 3GP with MPEG4.
First of all, let's not confuse 3GPP and 3GPP2. The 3GP is the format extension set by the standard of 3GPP, which was designed for GSM-based mobile phones. The latter is known as 3G2 (extension); it was designed for CDMA2000-based phones and set by the 3GPP2 (3rd Generation Partnership Project 2).
MPEG4 is the collection of methods and standards that are used to transcode video files. MPEG4 was designed for low bit-rate communications, but later on its target base was expanded and it became a standard for high definition content. MPEG-4 part 10 contains the standards set for next-gen DVD formats, such as the Blu-ray Disc.
AMR stands for Adaptive Multi-Rate; it is a compression scheme optimized and designed for speech coding. It was adopted as the standard for speech recording by 3GPP. That's why your cell phone saves the output sound file in AMR when you use the recorder. Due to its architecture, AMR works best only for speech. Sure, you can use transcoding utilities to convert music and/or music with AMR as audio codec, but the results won't be impressive at all.
By its very design, it contains some elements of voice activity detection to eliminate the unnecessary "spaces" during natural speech and, therefore, improves robustness, voice clarity (by also reducing the overall noise of the environment, thanks to Comfort Noise Generation), and ultimately reduces the resulting file size.
For more details, just read Wikipedia's entry on AMR here. Contrary to the fact that each of the transcoding software suites support AMR (-WB or NB) along with AAC (-LC) as audio codecs, we are going to skip AMR settings. Unless, of course, you are planning to transcode speech, such as interviews or reports, then by all means, use AMR.
Finally, please refer back to the first half of this series where we also explained the AAC format. In this article we are going to find out what MPEG4-AAC and MPEG2-AAC are all about. These are the formats that we're going to use, since the video codec we'll choose to use is MPEG-2 or -4, while the audio should be AAC.
Check out this page for a brief, but comprehensive list of the differences between these two. Basically, AAC gives a lot of advantages when compared to the traditional MP3. AAC was designed to improve quality on lower bit-rates, while reducing file size (a win-win situation!), requiring less processing power to decode (thus, it works best for embedded chip-based players), and having the sampling rates go up to 48 kHz.
I don't want to bore you anymore with theory, so let's keep moving. Should you prefer to read more about the aforementioned topics, then just check out the following hand-picked links: here, here, and here. These are some of the best resources.
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