When Samsung introduced the idea of what it called a Vestigial Side Band approach to mobile TV at the CES earlier this year, everyone got very excited. Now Harris and Samsung’s Korean rival LG Electronics have suggested a variation to the ATSC encoding method using 8VSB modulation, to send both digital TV and mobile TV.
Harris is suggesting, in the same way that Samsung has suggested before, that local US TV stations can send a digital TV signal and a mobile version of the same signal in the same spectrum. This is the approach pursued by ISDB-T in Japan, also to be adopted by Brazil, as well as by China in its DMB T/H system.
Most other mobile TV systems separately put all mobile TV channels into a single piece of spectrum, rather than each of them sharing spectrum with another broadcast of the same channel in full SD or HD resolution. This can be easily achieved by using the H.264 encoding since an ATSC 8VSB signal has an aggregate data throughout of over 19 Mbps.
VSB is really the use of Nyquist filters to give a very clean edge to a transmitter signal and get rid of unwanted sidebands that leak into neighboring spectrum or other parts of the signal. It is an alternative to Coded OFDM, but since the US selected it as a transmission modulation for digital TV in the late 90s, COFDM has improved radically and offers similar range at similar power levels, but is also far more resilient against multiple path effects (bouncing signals from the sides of buildings) and Doppler.
As a result any mobile TV system built using this 8VSB technique has some specific advantages and disadvantages. It can be delivered with new modulation equipment at existing ATSC transmitters very cheaply, but needs a new type of mobile tuner for handsets. These handsets are unlikely to have good reception at high speeds in cars and trains, and may have poorer in-building penetration than COFDM based systems, such as DVB-H and MediaFLO. Even ISDB-T uses an OFDM variant.
It is likely that LG Electronics has solved some of the problems at the handheld receiver end of the equation, which requires a lot of precision in its sampling interval and which has to add buffers to manage time interleaving and forward error correction to preserve battery life in a handset and improve the signal.
It’s difficult to see how the entire eco-system in the US will come together using 8VSB, because it will require new chips in handsets and the major US cellcos in the form of AT&T an Verizon have signed up with MediaFLO and in Sprint’s case with Motorola and Samsung to use WiMAX for mobile TV.
Perhaps Harris hopes that local mobile operators will work alongside local TV stations, or that the big cellcos might add 8VSB as well as MediaFLO to handsets.
The deal with LG Electronics is through its US research subsidiary, Zenith Electronics, and the development is being called MPH in-band mobile DTV system, with the MPH standing for “Mobile-Pedestrian-Handheld”.
Tim Thorsteinson, President, Harris Broadcast Communications, said: “The results of our joint development with LG Electronics are in response to our customers’ interest in a compelling, in-band mobile solution. The versatile, robust MPH system is the first that strikes the right balance for broadcasters looking to deliver both high-definition programming to fixed receivers and standard-definition programming and datacasting to mobile and handheld devices.”
The MPH system is a multiple-stream approach, with the main service stream for existing DTV and HDTV services, and the MPH stream for one or more mobile services.
The system sits on the existing ATSC 8-VSB transmission and receiving equipment and the partners say it will work at high speed with a single antenna, which is either a breakthrough or is simply not true.
Of course local TV stations might work alongside digital TV manufacturers to produce devices which are not handsets for mobile TV. Harris continually refers to devices as mobile or handheld, implying they might not also be mobile phones.
Zenith Electronics claims that it invented the “VSB” digital transmission system at the heart of the ATSC Standard.