Virgin Mobile & BT to Trial Ultra- Low- Quality Mobile TV via "DAB"

3rd June 2005

Virgin Mobile and BT are to trial ultra-low picture quality mobile TV probably using the DMB system (which can be transmitted on a standard DAB system). BT purchased 20% of the capacity of the Digital One multiplex last year, and plan to launch 3 mobile TV channels in this space.

20% of the capacity of a DAB multiplex only amounts to 237 kbps, so each channel will only have a bit rate of 79 kbps, and this 79 kbps has to encode both the video stream and the audio stream!

To give an idea of how poor the quality of these channels will be, DVB-H intends to use 384 kbps for its mobile TV channels, so 79 kbps is only a fifth of the bit rate that DVB-H intend to use. The DVB-H streams would use the CIF resolution of 366 x 288 pixels with a frame rate of 15 frames/s. Presumably, the Virgin Mobile channels will use a far lower frame rate than DVB-H and the picture quality will also have to be significantly lower as well -- unless they used 3 frames/s, which would be ridiculous, and shouldn't really be termed video at all. Frame rate is the number of times the screen is updated with a new image per second, and your TV at home updates the screen 50 times per second, so although small screens can get away with lower frame rates than 50 per second if these mobile TV channels use ridiculously low frame rates such as 7 frames/s or less then anything moving on screen will appear extremely jerky.

Presumably, these channels will either be using the DMB specification, which can be transmitted over the DAB system, or they will use DAB's forthcoming addition of an outer layer of FEC (forward error correction) coding (which no DAB receivers in the UK can ever take advantage of), because video streams require a far lower BER (bit error rate) than audio streams, which either requires very high transmitter powers or very high error protection code rates, which requires high amounts of redundant data to be added, thus leaving little useful bit rate for the audio or video. And because the bit rate per channel of these streams is already only 79 kbps without taking into consideration any additional redundancy added for the higher error correction required, then it is virtually certain that they cannot use DAB's weak, single-layer of FEC coding alone.

Also, it sounds like the headphone lead will have to be used as the antenna, because with DAB using Band III frequencies of around 200 MHz then the very short antennas that are integrated into the mobile phone itself incur an antenna loss of around 17 dB at such frequencies, which would require the transmitter power being increased by a factor of 50! The drawback of using the headphone lead as an antenna is that as the headphone lead moves, for example while walking, the capacitance between the headphone lead and the body varies considerably, and so the antenna loss also varies considerably -- although probably not by as much as 17 dB, but this will still give rise to variable reception quality.

Orange have also recently announced that they are planning to launch mobile TV services, but Orange are planning to transmit the channels over their 3G network. The problem with this is that each mobile phone cell (i.e. area served by a single base station) has a fixed upper limit of capacity, and all users must share this capacity, so if a lot of people want to watch these mobile TV channels then the reception quality for both phone calls and the mobile TV channels will suffer, and only a very small number of people within a cell can watch mobile TV at any one time. This is the reason why DVB-H was designed, because DVB-H uses a separate network to the 3G network, so it frees-up the 3G network for voice traffic and downloading data, and DVB-H can be received by as many people that want to receive it because it is a broadcast system, whereas with Orange using their 3G network, each user that wants to view a mobile TV channel consumes bandwidth equal to the bandwidth of the mobile TV channel. Interestingly, the peak times at which mobile TV will probably be viewed will be during the evening rush-hour, which is also one of the busiest times of the day for voice traffic, so if Orange's mobile TV proves to be popular then this could seriously overload cells leading to both people that want to make a call and people that want to view mobile TV being unable to.

Unfortunately, as has been mentioned in the technical press recently, these ultra-low-quality mobile TV offerings risk giving mobile TV a bad name in terms of picture and audio quality before the higher quality systems that have been designed specifically for mobile TV, such as DVB-H, have even had a chance to launch, and this risks turning what could be a very successful format into a format that is viewed as being of very low quality and of limited usefulness, especially given Virgin Mobile's offering being for only 3 channels.