Converting Video to Run on Your Cellphone

February 13, 2007 4:07 pm

Converting Video to Run on Your Cell Phone

Philip Shaddock, Pocketcine.com

In our previous article we told you how to get video on to your phone. In this article I am going to show you how to prepare the video for your phone. There is a lot a theory in the first part of the article and step-by-step instructions in the latter part.

Part of the problem of distributing to phones is that there are many different screen resolutions and three different video formats. Fortunately when video is distributed from a remote server you don’t really have to think about that, since carriers should convert video into the right format for the particular phone requesting the file, using transcoding software.

However if you want to test a video to your own phone, you will have to use software to save it to a format your phone can play. That information can be found in your phone’s manual or by doing a search on the Internet using your phone’s model number.

This article and tutorial will walk you through the major video formats and screen resolutions, and provide step-by-step instructions for preparing videos for your phone.

Cell Phone Video Formats

There are two file formats supported by cell phones, 3GP and MP4. 3GP itself comes in two flavors.

3GP and 3GPP

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3GPP is an acronym for 3rd Generation Partnership Project, a telecom industry partnership formed to promote collaboration around communication standards. A sister project is 3GPP2, the International Telecommunication Union's (ITU) International Mobile Telecommunications "IMT-2000" initiative. Both standards try to establish uniform delivery of rich media (like mobile videos) over 3rd generation broadband cellular networks. Both standards are based on MPEG-4.

3GPP standards have been developed for GSM (Global System for Mobile Communication networks, whereas 3GPP2 standards were developed for CDMA (Code Division Multiple Access) 2000 networks. The respective standards committees have worked closely to ensure interoperability. Both support MPEG-4 and H.263 video, AAC and AMC audio, and 3G text.

3GP is a file format defined by 3GPP for multimedia on mobile phones, based on MPEG-4. 3GP stores video streams as MPEG-4, and audio streams as AMR-NB or AAC-LC formats. 3GP also defines image sizes and bandwidth, so content is optimized for mobile display screens.

3GP files can be viewed on PCs with the latest RealPlayer or QuickTime players.

Extensions
.3gp 3GPP standard, GSM Network, Video: MPEG-4, H.263, Audio: AAC, AMR
.3g2 3GPP2 standard, CDMA2000 Network, Video: MPEG-4, H.263, Audio: AAC, AMR, QCELP.

More information about 3GPP can be found at www.3gpp.org. More information about 3GPP2 can be found at www.3gpp2.org.

MP4

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MPEG-4 is an international standard to define, encode, and play back time-based media, like video. It is from the same committee, the Moving Pictures Expert Group (MPEG), which established MPEG-1 and MPEG-2 as standards for digital video.

The MPEG-4 file format, called MP4, is based on Apple QuickTime. When you see the extension .mp4 it is referencing the file format. 

As MPEG standards have evolved they have got better and better at compressing video to smaller and smaller sizes. The first standard, MPEG-1, made video on interactive CD-ROMs possible, and the second standard, MPEG-2, made digital television possible. MPEG-4 was designed to be more flexible and scalable than previous standards. It can be used for all kinds of applications, from two-way video conferences to controlling animated 3D models. What's important to mobile video producers is that it makes mobile video possible because it scales to small power devices and tiny file sizes.

For mobile applications it features a well-developed server spec and supports streaming media types, not only audio and video but synthetic content as well, such as face and body animation, streaming 3D textures, and more.

Because MPEG-4 is an international standard created by committee, it has developed very slowly when compared to its proprietary rivals, notably those promoted by Real Networks and Microsoft. But because it is an international standard, it provides a way for different programs and devices to achieve interoperability. The programs can encode and decode video streams in whatever way they choose. They just have to create a standardized bit stream so that other players or decoders can interpret it properly. MPEG-4 absorbs many of the features of earlier standards and adds new ones. What features are implemented are left to the discretion of individual developers, so there is no such thing as a generic MPEG-4 player. That’s because MPEG-4 supports so many devices, including mobile phones with limited computing power and relatively small memory resources, to HDTV as well! MPEG-4 is many-flavored.

The bottom line is that most 3GP (3GPP) and MP4 encoding software should produce a file that plays on most mobile devices. However, you should be prepared to try different encoding solutions if your particular cell phone refuses to play a video file.

Audio Compression

Video compression software often asks for the type of audio compression you want to use.

Audio codecs used in video compression software vary according to the purpose for which they were designed. Some are designed to compress voice without music, others are designed for music. The best that a music-oriented CD-quality algorithm can achieve is to create a file 25% of the original size.

The most common lossy codec is MP3 (MPEG-1 Audio Layer 3), widely used to store audio on portable audio players.  It is considered to be a first and second generation MPEG standard. MP3 encodes pulse-code modulation-encoded (PCM) audio data to a smaller size by discarding audio that is difficult for people to hear.

The Ogg Vorbis codec is an open source codec developed when Fraunhofer Gesellschaft decided to start charging licensing fees for MP3. It not supported as well in the market as MP3.

AAC (Advanced Audio Coding), is also known as MPEG-2 Part 7, and in a slightly different form, MPEG-4 Part 3. AAC is regarded as the successor to MP3 because it improves on the compression ratio (1:16 versus 1:10) and has better audio quality at a lower bit rate. Among different algorithms it uses is one called modified discrete cosine transform (MDCT) to more efficiently compress certain types of complex audio waveforms. MPEG-4 AAC codecs offer good audio quality at bit rates down to 24 Kbps.

AAC includes DRM (Digital Rights Management), unlike MP3, and supports multiple profiles for application to different environments, from constrained mobile devices to home theatre systems.

AAC has been widely supported by mobile phone manufacturers, as well as DVB, XM satellite,  iTunes, the iPod and PlayStation Portable. It is found in both MPEG-4 and 3GPP / 3GPP2 mobile video formats.

AACPlus (also known as AAC SBR) is an improved version of AAC. It uses an algorithm to improve reproduction of high end frequencies. It is also more efficient at compressing stereo audio.

Mobile Device Screen Resolutions 

Resolution describes the amount of pixels (picture elements) on a mobile screen, measured as a grid of pixels. The 176x144 pixel resolution of many 2 inch cell phone screen means that the screen is 176 pixels by 144 pixels. While most mobile phones are portrait-oriented, cell phone videos usually play in landscape mode. The new iPhone senses whether you are holding the phone in portrait or landscape mode.

Higher resolution screens do not mean they are bigger, it means they have more pixels per square inch or centimeter. A 176x144 two inch screen has 25,344 pixels. A 352x416 two inch screen has 146,432 pixels, an increase in density of pixels of nearly three times.

What resolution really tells you about a mobile device screen is the sharpness or clarity of the image. The same 176x144 image can be spread out over a 17" monitor or a 2" cell phone screen. On the cell phone screen the image will look sharp, on the computer monitor very blocky and blurry. Smart phones with 3" screens can have the same pixel resolution (1280x1024) as the standard resolution of 17" computer LCD monitors.

Why is the sharpness and detail of the image important for mobile video? The smaller the screen the more important detail is to the eye. There is more information traveling to the brain to help it put together the image. People are often surprised at how good a video image looks on a small screen displaying a high resolution image.

It is an important rule in video and graphics production to shoot and edit in the highest resolution possible, and then sample down to the target resolution of your playback device. With the rapid evolution of mobile devices, this advice is more important then ever, since screens are not only getting bigger, they are displaying more pixels per square inch for clearer videos.

You never know when your hit viral video may become a real property, worthy of distribution in other media, including HDTV, so it is not a bad plan to generate your assets (computer graphics or video raw footage) in high resolution and then down sample to the target resolution. You should still shoot for the small screen though. A landscape panoramic shot does not fit on tiny portrait-oriented mobile screen. 

The high end of cell phones is at the time of this writing (2007) is QVGA (Quarter Video Graphics Array liquid crystal diode) which is 320x240, or a quarter of the pixel resolution of the old analog TV resolution. Smart phones with 3 inch screens can have resolutions up to 1280x1024 pixels (SXGA). The standard for cell phones will become 320x240 in the near future.

What follows is a guide to acronyms used to describe the screen resolutions of mobile devices. We do not attempt to describe specific manufacturer devices. Some cell phone manufacturers, have screen dimensions and pixel ratios that are unique and do not fall within a "standard" pixel ratio.

VGA Resolutions
The resolutions in this category were developed to describe pixel resolutions of computer monitors made possible by succeeding generations of graphics cards. The VGA acronym refers to the grid of pixels on the computer monitor (Video Graphics Array).

QVGA: 320x240, quarter VGA

VGA: 640x480, comparable to the over-the-air analog TV standard

SXGA: 1280x960, Super Extended Graphics Array

QVGA: 320x240 is a common resolution for mobile TV and many portable media centers.

 

CIF Resolutions
CIF (Common Intermediate Format) is a standard that was traditionally used in teleconferencing. You will often see mobile phones screen resolutions described using the CIF teleconferencing naming convention, such as QCIF (Quarter CIF). This means that the target video system is a quarter of full screen resolution. Because there are two major broadcast systems in the world, NTSC and PAL, how CIF is measured in pixels is different for the two systems.

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CIF Resolutions

 

A table shows the lower resolutions in pixels:

 

Format

NTSC

PAL

SQCIF (Sub QCIF)

 

128 × 96

QCIF (Quarter QCIF)

176 × 120

176 × 144

QCIF+

176 × 220

176 × 220

CIF

352 × 240

352 × 288

4CIF

 

720 x 576

 

 

 

 

 

 

 

SIF Resolution

The Source Interchange Format is defined as 352x240 (NTSC) or 352x288 (PAL or SECAM). SIF is a standard used to move graphics between computer systems.

As you can see, there is a wide variety of screen resolutions for cell phones and smart phones. Normally when videos are delivered to your phone from a server that has video transcoding software installed, the server software looks at the header sent by your phone to see what kind of phone it is. It then looks into a database of phones to determine what the acceptable file formats and screen resolutions for your phone. In the next section we are going to show you how to perform that function manually.

Encoding Video for Your Phone: Tutorial

First you should determine the video specs for your phone. This will be in the phone’s manual. If you do not have the manual handy, a search on the Internet using your phone’s manufacturer name and model number and the search term “video specs” should yield up the key information (video format and screen resolution).

What software you use to encode the file will depend to some extent on the source video’s format. There are quite a few MP4 encoders on the market, some of them specialized to converting DVD file formats, others DivX and other formats. However in this article we are going to assume you are using content you have created in graphics and video software. This means you should save the video as an uncompressed file and use it in the compression software (the encoder). I tend to create the project at 640x480, twice the resolution of 320x240 mobile video phones.

There are two uncompressed formats that are common, uncompressed .mov (Apple QuickTime) and .avi (primarily a Windows format). Both the video and the audio should be uncompressed. The reason why? Compressed video and audio will have compression artifacts that will be faithfully reproduced and perhaps enhanced when the video is re-compressed.

There are two very good encoders that we recommend. One is provided as an upgrade to the Apple QuickTime player (Pro), and the other is compression software by Sorenson Media, called Sorensen Squeeze. QuickTime Pro costs $29 US and Sorenson Squeeze costs $499 US. What’s the difference? For mobile video application, the Sorenson Squeeze software offers more control over the compression process. There are a host of tweaks you can make to videos, but perhaps the most useful one is to set the maximum file size of the output video. We were able to reduce the Floyd Genome BC viral video from over 800 megabytes (640x480) to less than one-half a megabyte (320x240), a compression ratio of 1700 to 1, using Sorenson Squeeze.  While Apple QuickTime Pro allows you to set the maximum data rate (the maximum amount of data at any given moment), the Sorenson Media software allows you to set either the maximum file size or the maximum data rate. Apple QuickTime Pro was able to reduce the file size to less than one-half megabyte, but the visual quality of the video was noticeably poorer than that of the Sorenson Media software.

One important feature that is implemented in both cases is the ability to independently specify the amount of compression to apply to the audio or video channels, allowing you to produce a video with reduced audio quality for voice clips or optimal audio quality for music video clips. 

Encoding with Apple QuickTime Pro

For this tutorial you will need a short clip of uncompressed video, including audio. You will also need Apple QuickTime Pro. You can buy QuickTime Pro directly from Apple. Basically you buy a serial number. Entering the serial number into the QuickTime player activates additional features in the software, including the ability to encode video to a variety of formats. See the “Buy QuickTime Pro…” option under the QuickTime player’s Help menu.

Once it is installed and running:

1.       Load the uncompressed video clip using the player File > Open File… menu option.

2.       Click on the File > Export… menu option.

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3.       Under the Export option choose “Movie to MPEG-4”, then click on the “Options…” button beside the Export drop down menu. (If your cell phone only plays .3gp video files, choose the 3G menu option instead of the MPEG-4 option.)

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The key options are “File Format” and “Image Size.”

4.       Choose MP4(ISMA) as the file format option. Although you probably will not know if your mobile phone is ISMA (a standards organization) compliant, try encoding with this option. If your device fails to play the file, try encoding with plain vanilla MP4.

5.       Set the image size to the video resolution of your cell phone. If you choose the wrong size, your video may play and you will hear sound but it will have no image.

6.       Select the Audio codec by clicking on the drop down menu at the top of the screen and choosing “Audio.”

7.       Set the audio options to the following:

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8.       Upload the file to your cell phone and try to play it.

If your phone chokes on the video file, load the uncompressed file back into QuickTime Pro and try different settings. The buttons with question marks on the interface will take you to an Apple web page where you can walk through what the different settings do.

Sorenson Squeeze

Sorenson Squeeze software has a number of presets that take the guess work out of compression. You can load a preset and tweak it to your needs. Let’s do that now.

1.       Load the uncompressed video clip into Squeeze by clicking on the “Import File” icon at the top left of the interface.

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In the window at the bottom of the interface, you will see a list of files and filters associated with the current compression project. At this point you just have the file that you loaded into Squeeze in the list.

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2.       Select  a preset from the “Format and Compression Settings Window.” We chose a setting we saved earlier, called “Mobile 320x240.”

3.       Drag the preset into the file list window. It should show up right under the name of the video you loaded into Squeeze. (You can load any preset and modify to the settings we show you below.)

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4.       Now open the preset by double-clicking on the preset name (in this case, “Mobile 320x240.”

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This opens up the preset to reveal the audio (aac) and video (MPEG-4 Pro) components of the preset.

5.       Double click on the video preset (MPEG-4 Pro in this case). This opens a dialog.

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6.       In the case of the Floyd Fruit Fly video animation, I used the settings you see above. Going into the details of the settings is beyond the scope of this tutorial. The important settings are the choice of the Stream Type (no streaming), the Frame Size (320 x 240) and the frame rate (15).

These are the most basic settings you need to get right to reduce the video to a size and resolution that will allow it to play on your phone. The Frame Rate (15 fps) was chosen because it matched the frame rate of the animation. It was chosen because the animation did not need to be 30 frames a second. Reducing the frame rate in half reduced the data in the file considerably. The “Sorenson 2-Pass VBR” setting means that the file will be more efficiently compressed. For more
information about two pass and variable bit rate processing see the MPEG-4 compression primers on the Pocketcine site (www.pocketcine.com).

7.       Save the preset under its own name, by clicking on the Export… button and saving it to the folder where presets are stored. Click on OK to exit the dialog.

8.       You can apply other filters to the file, including such fine tuning as normalizing the audio (to smooth out the valleys and peaks in loudness) or brightening or darkening the video. Leave those options alone for now. Click on the “Squeeze It!” button on the bottom right of the screen.

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Depending on the size and complexity of the raw video file, and the power of your computer, the software will take some time to compress the video file. It will save it in the same folder as the source video file.

9. Save the project after you have finished.

There you have it, a massive video file reduced to mere kilobytes, ready for downloading over the air or by cable to your phone. I strive to keep file sizes below 500 kilobytes if I am downloading over the air, to avoid costly data bills from my carrier, or to make the process of sending the file to another user over Bluetooth quick and painless.


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Here are the links to my previous tutorials:

February 13, 2007
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