Opencable™ digital television
OpenCable™ is a digital cable standard for the United States, designed to offer interoperability between different hardware and software suppliers. A subset of the standard is being incorporated inside digital televisions.
A summary of the OpenCable™ standard is shown in Table 8.3.
Table 8.3. Summary of the OpenCable™ Standard
| Parameter | OpenCable™ |
|---|---|
| Video compression | MPEG-2 |
| Audio compression | Dolby® Digital |
| Multiplexing | MPEG-2 transport stream |
| Modulation | QAM |
| Channel bandwidth | 6 MHz |
OpenCable™ receivers use the following four communications channels over the digital cable network:
6-MHz NTSC analog channels. They are typically located in the 54–450 MHz range. Each channel carries one program.
6-MHz Forward Application Transport (FAT) channels, which carry content via MPEG-2 transport streams. They use QAM encoding and are typically located in the 450–864 MHz range. Each channel can carry multiple programs.
Out-of-Band (OOB) Forward Data Channels (FDC). They use QPSK modulation and are typically located in the 70–130 MHz range, spaced between the 6 MHz NTSC analog and/ or FAT channels. SCTE 55-1 and SCTE 55-2 are two alternative implementations.
Out-of-Band (OOB) Reverse Data Channels (RDC). They use QPSK modulation and are typically located in the 5–42 MHz range. SCTE 55-1, SCTE 55-2, and DOCSIS® provide three alternative implementations.
OpenCable™ receivers obtain content by tuning to one of many 6-MHz channels available via the cable TV connection. When the selected channel is a legacy analog channel, the signal is processed using a NTSC audio/video/VBI decoder. When the selected channel is a digital channel, it is processed by a QAM demodulator and then a CableCARD™ for content descrambling (conditional access descrambling). The conditional access descrambling is specific to a given cable system and is usually proprietary. The CableCARD™ then rescrambles the content to a common algorithm and passes it on to the MPEG-2 decoder.
Insider Info
The multi-stream CableCARD™ is capable of handling up to six different channels simultaneously, enabling picture-in-picture and DVR (digital video recording) capabilities.
When the CableCARD™ is not inserted, the output of the digital tuner's QAM demodulator is routed directly to the MPEG-2 decoder. However, encrypted content will not be viewable.
OpenCable™ receivers also obtain control information and other data by tuning to the OOB FDC channel. Using a dedicated tuner, the receiver remains tuned to the OOB FDC to receive information continuously. This information is also passed to the CableCARD™ and MPEG-2 decoder for processing.
The bidirectional OpenCable™ receiver can also transmit data using the OOB RDC.
The OpenCable™ standard uses an MPEG-2 transport stream to convey compressed digital video, compressed digital audio, and ancillary data over a single 6-MHz FAT channel. Multiple video streams, multiple audio streams and/or data may be present in the MPEG-2 transport stream.
The MPEG-2 transport stream has a constant bit-rate of ∼27 Mbps (64-QAM modulation), ∼38.8 Mbps (256-QAM), or ∼44.3 Mbps (1024-QAM).
The available bit-rate can be used in a very flexible manner, trading off the number of programs offered versus video quality and resolution. For example, if MPEG-2 video, statistical multiplexing, and 256-QAM are used:
(2) HDTV programs+(6) SDTV programs+data
Video Capability
Digital video compression is implemented using MPEG-2 and has the same requirements as ATSC. There are some minor constraints on some of the MPEG-2 parameters, as discussed in Chapter 7. Support for using MPEG-4.10 (H.264) up to [email protected] is being added to the specifications.
Although any resolution may be used as long as the maximum bit-rate is not exceeded, there are several standardized resolutions. Both interlaced and progressive pictures are permitted for most of the resolutions.
Compliant receivers must also be capable of tuning to and decoding analog NTSC signals.
Audio Capability
Digital audio compression is implemented using Dolby® Digital and has the same requirements as ATSC. Compliant receivers must also be capable of decoding the audio portion of analog NTSC signals.
Application Block Diagrams
Figure 8.4 illustrates an OpenCable™ set-top box. Part of the requirements is the ability to output both high-definition and standard-definition versions of HD content simultaneously.
FIGURE 8.4. OpenCable™ Receiver Set-Top Box Block Diagram.
