Application Needs: Mobile
The New Market for Video
In an increasingly mobile society, the fastest growing video market is video on mobile devices.
The latest generation of mobile phones, featuring bigger screens with higher resolutions, is more clearly targeted at video than any previous generation. While the mobile phone dominates the market for pocket-size handhelds, for image-oriented devices, there is also a rapidly developing demand for larger mobile screens. Apple’s iPad, an inexpensive, video-oriented tablet computer, has been received with overwhelming enthusiasm, spawning a host of competing products. Intermediate-sized hybrid or cross-over devices, typically featuring screens in the 5-7 inch range, also command their share of attention. These systems typically combine aspects of a mobile phone with a tablet computer and a netbook.
Even in the older personal computer market, the fastest growing segments are the most highly mobile segments, namely small Netbooks, with 7-10 inch screens, and Ultra-portables, with 11-13 inch screens. Among larger, full-featured systems, portable notebooks with screens in the 14-17 inch range outsell traditional desktop computers (and larger “luggable” notebooks) by a wide margin.
Requirements for Mobile Encoding
Encoding for a full-featured notebook computer may be indistinguishable from HD broadcast encoding. However, as screen sizes shrink and functional capabilities dwindle, moving down along the path from a 7 pound notebook computer with a 17 inch screen to a 1 ounce Video Watch with a 1.5 inch screen, there are a number of considerations to take into account.
Screen Resolution: Large notebooks, with1920 x 1200 displays (16:10 WUXGA) or above, offer full 1920 x 1080 (16:9) HD resolution. From WUXGA, mobile resolutions range across a truly formidable spectrum of possibilities. The following list is a sample of some of the more common resolutions encountered on screens of diminishing size.
- 1536 x 2048 on the iPad Retina
- 768 x 1024 on older iPads & the iPad Mini
- 720 x 1280 on the Galaxy SIII
- 480 x 800/854 on Nokia Lumia phone
- 312 x 390 on the 42mm Apple Watch
- 272 x 340 on the 38mm Apple Watch
- 320 x 320 on Galaxy Gear & Android Wear
- 144 x 168 on the Pebble watch
A mobile video encoder must have the flexibility to accommodate any of these and other target resolutions, depending on the requirements of each customer.
Encoding Profile: The capabilities of a mobile encoder must be matched to the capabilities of mobile decoders. Some devices support Main profile, with both progressive and backward references, plus advanced CABAC entropy encoding. Other devices support the simpler Baseline profile, with progressive references only and less aggressive CAVLC entropy encoding. Again, a mobile encoder must have the flexibility to adjust to the capabilities of its target decoder.
Audio Codec: A mobile decoder also may support only a specific audio codec, for example MPEG-1, Layer 2 or AAC. Again, a mobile encoder must have the flexibility to provide audio encoding compatible with the audio decoding capabilities of the mobile device.
Bit Rate: In addition to restricted decode capabilities, mobile devices generally have limited bandwidth and storage space. For video on demand, highly compressed files download more quickly and take up less storage space. For live streaming over a wireless network, the lower the bandwidth, the less likely video is to stutter, skip or freeze during play-out. For stored video, the preferred solution these days is as simple as ABC, or Adaptive Bitrate Coding, where devices dynamically measure available bandwidth and switch from higher to lower bandwidth playouts as required by fluctuations in streaming rates. Similarly, the ability of an encoder to support highly compressed audio codecs, like AAC-LC or HE-AAC, can be a significant bit rate savings for mobile devices that can decode these formats.
Frame Rate: One way to effectively improve live streaming to mobile devices is to use a reduced frame rate. Although NTSC SD and 1080i HD television both use rates of 30 frames per second (fps) for interlaced video, film is traditionally shot at a lower 24fps rate—and even 24fps is higher than really necessary. The human eye will transform a succession of still images into the perception of smooth motion at any rate down to about 10fps. The eye’s willingness to accept low frame rates means the ability of an encoder to output reduced frame rates (compared to normal TV standards) can be an effective way to support live streaming of quality video on bandwidth-limited mobile devices. For 30fpps NTSC inputs, fps reductions of one-third (20fps), one-half (15fps), and even two-thirds (10fps) are useful. For 25fps PAL inputs, the corresponding reductions are one-fifth (20fps), two-fifths (15fps), and three-fifths (10fps).
Telairity Mobile Encoders
4:2:0 distribution mode encoding on Telairity encoders is optimized for the low bitrates typically required for streaming to mobile devices. Since all Telairity encoders support 4:2:0 mode, any model can be adopted for use with mobile devices, depending on channel count needs, starting with the single-channel BE8600 contribution encoder (which also supports high-bandwidth 4;2:2 mode). For streaming up to 4 channels, the multi-channel BE8700 is the best choioce. For higher channel counts, the SES3200 blade-based integrated encoder-transcoder-multiplexer, which scales as high as 32 channels in a 14RU telco-grade ATCA chassis, should be considered.
Since popular mobile targets are often incompatible in their resolutions, encoding profiles, audio codecs, and useable bit rates, multi-channel encoders that can accept a single input and simultaneously output multiple custom streams are often the best choice for live mobile transmissions.