The ATSC Standard allows for 18 different video formats. These range from 480 to 1080 horizontal lines, at different frame rates, in either progressive or interlace, and in some cases in two different aspect ratios.
This poses two challenges to the broadcaster. The first is which format to choose for emission, and second, especially important to Canadian Broadcasters, is understanding the impact of format conversion. Broadcasters in Canada rely on programming from all the US networks and program syndicators, as opposed to the US affiliate that may only receive programming from one network.
The choice of the emission format depends on many factors. The first being what is the main source and type of programming. In the US, an affiliate typically has chosen the same format that the Network has chosen. Some Networks have made their choices based on the type of programming. For example, sports, in most users view, is better reproduced in a progressive scan format. Whereas, 1080I provides better spatial resolution which is better suited to dramas and documentaries. (1080 P60 is acknowledged by many to be the ultimate but is still two years away.)
A second factor is bandwidth efficiency. Progressive formats are said to stand up better to compression and hence can be transmitted at a lower bit rate, leaving room in the channel for other services, such as a second video signal at standard definition channel or data. Other factors are whether the broadcaster wishes to broadcast a high definition signal or multiple standard definition signals and lastly, availability of equipment. Until recently 720P equipment has been less prevalent than 1080I.
Display technology also continues to evolve and it is helpful to understand what goes on in the home set. Set manufacturers have chosen native formats for their displays for reasons of cost and market distinction. There are some reports that current technology cannot reproduce the 1920 horizontal pixels of the 1080I format, so why bother? As well, decoders must be capable of decoding any format but they all convert the signal to the native format of the display. So the signal undergoes yet another possible conversion.
Whatever the choice, it is clear that the broadcaster is going to have to operate in a multi-format environment for some time to come. It will be imperative to understand the trade-offs and capabilities of the technology. CDTV’s Technology Working Group (TWG) is developing a demonstration that will help start that understanding.
A plan is being put together by a small group of the TWG. The intention is demonstrate end-to-end performance of a video signal from capture, through encoding to display, with format conversion thrown in for certain cases.
With the help of the NFB six different scenes will be captured on six different video formats plus 35 mm film. These will be edited and assembled together to provide identical video sequences in 480 I30, 480 P60, 720 P60 and 1080 I30. The next step is to encode these sequences using SD and HD encoders and recording the ATSC transport stream onto a server for later play back. Prior to encoding, format conversion will be applied, in some cases, to simulate conditions that might occur. The CRC will be assisting CDTV with this work.
By recording the ATSC transport stream, we are able to simulate repeatable transmit and receiver conditions. These transport streams can be fed to different receiver/ decoders and displays in order to see the final result one might get in the home.
The target is to hold demonstrations at a CDTV meeting in the fall followed by more public demonstrations at other venues. We hope to have CDTV board approval for the plan at the May meeting, after which the NFB will start its work.
This work can only be made possible with the loan of equipment from manufacturers and for this CDTV would wish to thank, in advance, those manufacturers who have expressed an interest in participating.