Managing broadcast data
Feb 1, 2010 12:00 PM, By Ian Hudson
Table 1. The use and amount of data is growing in line with changes in the broadcast industry and will continue to grow with developments like IPTV and with the changes from linear to file-based broadcasting.
Select table to enlarge.
Broadcast is shifting rapidly from its traditional linear production model to file-based systems. This gives rise to tremendous flexibility, instant delivery and huge opportunities for cost reduction primarily through improved efficiency.
Those efficiencies can only be realized, however, if those files are both accessible and usable. Access relies on accurate information, much of which regards how the file is cataloged and archived. Usability relies on the format of all aspects of the content, the obvious picture resolution and audio format ,but also the data carried both within the VBI, VANC and HANC.
Indeed, the flexibility and efficiency improvements can be fully realized only if the original content (picture + audio + data) can be read, managed, manipulated and converted to meet the final output criteria. Modern broadcast facilities rely on information: Equipment is triggered by cue tones, switched by active format descriptors (AFDs), aligned to time code and managed by automation. (See Table 1.)
Data, embedded within the content, is not only there for the benefit of the broadcaster, but also it is there for the viewer in the form of captions, teletext, program guides, red-button interactivity, parental control and much more. Regardless of whom the intended user may be, there are only three reasons for inserting data: to enhance, to control or simply to meet regional legal requirements. Whoever makes use of the data, for whatever reason that data was created and whatever the intended use, the importance of inserting, reading, managing and monitoring data efficiently and effectively throughout the broadcast chain is essential. (See Figure 1 on page 60.)
Data insertion is commonplace throughout the broadcast industry and has been for the past 30 years. Initially, data was inserted into the VBI portion of the program stream as analog waveforms. With the advent of SDI, these waveforms were digitized. Indeed, much data today is still in analog format and simply converted to a digital waveform to be inserted into the SDI stream.
Figure 1. Data is used at every stage of the broadcast chain. Throughout that chain, data needs to be inserted, read, monitored and manipulated. The equipment to perform those tasks must be flexible, future proofed and capable of performing multiple functions in a multitude of formats.
Select figure to enlarge.
With the advent of HD, VBI has been superseded by VANC. In terms of the physical insertion of data, there is no real difference. The primary difference is HD carries only packetized data (i.e. not digitized waveforms). These are now formatted as digital ancillary data packets (with 000 3FF 3FF prefix), which carry parallel data rather than analog signals that have been digitized or serial digital data (e.g. video index).
This allows considerably more data per line than previously possible. For instance, one VBI line of teletext contains 40 displayed characters (+5 characters of run-in and framing codes). One line in 1080i video contains 1920 Y and C words in the active video portion (i.e. that used for VANC). One OP47 (see the teletext bullet point on page 65) multipacket can contain up to five analog teletext lines and is about 255 words long. It is theoretically possible to fit seven of these multipackets onto a single HD line (equivalent to 35 old teletext lines), or more than a full page on a single line. Again, it is theoretically possible to fit the same amount of data into both Y and C, which would give approximately 70 analog SD teletext lines on a single HD line.
Ultimately there are questions regarding bandwidth in the ASI transmission stream to put all this data in, but it proves the vast potential for data to be used in HD transmissions. A suitably equipped set-top box is able to read this data and processes it as required. The increase in volume of data will inevitably lead to increased interaction between the viewer and the broadcaster, making true interactive TV a certainty in the coming years.
Formats
Figure 2. Shown here is the format of type 2 ancillary data in a VANC multipacket. Type 1 ancillary data has a similar format, with the secondary identifier (SDID) replaced by a data block number (DBN). Type 1 is more commonly used for audio data.
Select figure to enlarge.
Within the broadcast facility, there is no difference with regard to the transport or use of data in one format or another. Regardless of whether the data started life as an analog waveform and was subsequently digitized or is inserted as SD or HD-SDI, it is carried with the picture and audio as a single digital stream.
As discussed, the primary difference for the broadcaster is not the format of the data but the amount of data and the uses it is put to. For instance, widescreen signaling (WSS) on line 23 was primarily a European initiative. It was first introduced to account for the consumer take-up of 16:9 TVs, which was quicker and more widespread than in the rest of the world. WSS was introduced specifically to address the consumer requirements to switch formats between programs transmitted in 4:3 and those transmitted as 16:9. AFDs are in principle the same thing as WSS. However, AFDs are used extensively throughout the broadcast facility to switch, for instance, aspect ratio converters.
Similarly, cue tones or triggers can be in the form of a line 23 waveform (analog or digitized) or as a data string, e.g. a packet 31 teletext stream. There are always advantages and disadvantages to whichever is chosen. As an example, a line 23 waveform is a robust signal. While in use, it is present on every frame. Because line 23 is considered part of the picture, it is effectively bandwidth-free. The primary disadvantage with line 23 is it can, when passing through the MPEG encoder, become corrupted by stray motion vectors unless care is taken in the encoding process.
Packet 31, however, is a teletext packet and is, therefore, passed without incidence by all encoders. Packet 31 is normally only transmitted with a single frame, so a dropped frame will result in a dropped cue tone and, therefore, a missed action somewhere downstream. If it were a trigger for an ad server, it could be a very expensive dropped frame indeed. Packet 31 could be pulsed with each frame, but this is not bandwidth-free. So Packet 31 is a simple solution and easy to work through encoders and decoders. A WSS waveform cue tone, on the other hand, is bandwidth-free and robust but not as simple to get through the entire signal chain.
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