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Trident Signal Generation
Introduction
Introduction
[Back to Top]
Standards
Encoding/Transcoding
Packaging
DVG's Transport Stream Support
Channel Coding and Modulation
DVG's Channel Coding and Modulation Support
Signal Generation
Trident's Digital Video Player
Summary
Digital television (DTV) is a modern form of broadcast technology that includes all topics involved with digitally transmitting video, audio, and data (collectively termed content) from a broadcast station to an end device. These end devices can be anything capable of displaying video; some examples include televisions, set-top boxes, and mobile phones. This document discusses the key steps involved in transmitting content from the supplier to the end user (consumer).
Standards [Back to Top]
A broadcast standard, which defines a common, inter-operable way of providing content to the consumer, is the core of digital television. However multiples standards exist, each defined by a governing body that maintains and revises the standard, typically by committee. The four major standards for digital television include:
For each main standards category there are often multiple sub-standards that are based on how the signal is transmitted. Possible transmission methods are:
Combining the above categories with the transmission method can provide an extensive list of available standards. Some of these standards are:
Encoding/Transcoding [Back to Top]
In order to maximize available bandwidth, audio and video data must be compressed. Compression techniques can be classified as either 'lossy' or 'non-lossy,' based on whether or not the exact original signal can recovered from the compressed data. A lossy algorithm can reduce the required bitrate, but does so at the expense of signal quality.
Audio and video compression utilizes the fact that human eyes and ears are more sensitive to certain types of information (such as luminance or an audio frequency range). Information that humans do not perceive as well is aggressively compressed. Less compression is used for information that humans perceive well. Compressing digital and audio content greatly increases transmission quantity and speed. For example, a high definition video stream is typically compressed to around 20 Mbits/second. At 30 frames per second, 1920x1080 pixels per frame, and 3 bytes per pixel, the same video requires 1.5 Gbits/second if not compressed. The compressed video fits in 1/75th of the original space. Similar improvement in the required audio bitrate can also be achieved. All of this compression comes at a small, essentially unnoticeable, degradation in quality.
For video, there are two main compression standards: MPEG-2 and H.264. H.264 is also known as AVC or MPEG-4 part 10. For audio, there are many more supported standards. The mains ones include: MPEG-1/2, AC-3, and AAC. The required video and audio encoder and decoder ('codec' - from "enCOder/DECoder") for each standard varies as shown in the list below:
Packaging [Back to Top]
The encoded audio and video files are packaged in a standard way for transmission. Data is sent from transmitter to receiver as a bitstream, and several requirements (listed below) influence the composition of this bitstream.
To solve these issues, the audio and video data is packaged in a format known as an MPEG-2 Transport Stream. A transport stream consists of 188-byte packets transmitted one after another in a continuous fashion. The first byte is a sync byte used to align the receiver with the transport packets. The next two bytes contain an identifier that labels that packet's content (such as audio or video) as well as describing to which channel it belongs. The transmitter breaks the audio and video frames into these 188-byte packets along with other general system information before sending data to the receiver which will rebuild the audio and video frames.
DVG's Transport Stream Support [Back to Top]
The Transport Stream Generator ("TSG") application in the Trident Suite takes existing audio and video content, and rearranges it into a proper transport stream. Additionally, it injects required system information in that stream, based on the currently selected standard.
Beginning with version 2.0, the Transport Stream Generator will be able to convert video from one compression format to another, as well as change the existing settings of the audio and video data (such as frame rate or resolution). This conversion process is called transcoding. For example, transcoding can be used to create test pattern videos from BMP images, and to convert audio data from .wav files into a compression format such as AAC. This enables users to take advantage of picture and sound controls inside of LabVIEW to create customized test patterns and sounds.
Channel Coding and Modulation [Back to Top]
In order to send a transport stream from transmitter to receiver, it is converted into a form that is suitable for broadcast. This process is called channel coding and modulation. Demodulation is the process of converting the transmitted data back into processable data. Channel coding and modulation provides a precise compromise between error protection capability, bitrate/bandwidth, and complexity. Each standard handles this process differently, although each is based on a common set of algorithms. Some of the most common algorithms are:
DVG's Channel Coding and Modulation Support [Back to Top]
The DAQTron Video Generator takes existing transport streams and performs the desired channel coding and modulation. It supports multiple standards. It produces a binary format data file ("BIN file"). This file contains I & Q data ready to be played on NI's RFSG.
Signal Generation [Back to Top]
The final step in creating a broadcast signal is to generate the desired signal. Generation can be as simple as combining the I & Q data, setting the proper power level, and selecting the desired frequency.
However, in order to simulate real-world transmissions, impairments need to be added. Impairments modify the I & Q data to a point where the receiver is forced to use its error correction techniques in an effort to recover the original transport stream. A good measure of receiver performance is based on the level of impairments that causes the receiver to fail. Impairments can be anything from additive white Gaussian noise (AWGN) to phase noise or fading.
It can be difficult to look at the final video and tell when certain errors have occurred in the transport stream. In order to reliably measure them, a bit error ratio (BER) test can be performed. A bit error ratio test compares the original transport stream with the received transport stream. The number of bits in error are counted and analyzed. Because of the complexities of aligning transmitted and received transport streams, BER tests are usually performed using a PRBS which simplifies the verification.
Trident's Digital Video Player [Back to Top]
The Trident Digital Video Player ("DVP") uses the BIN file created by the Digital Video Generator in conjunction with the RFSG to generate broadcast signals. These files take advantage of the PXI-5441's Onboard Signal Processing (OSP) abilities in order to reduce the required calculations and symbol rates. As a result, the sample rate is limited to that allowed by the OSP (8.333 MSamples/sec on the PXI-5441).
The onboard memory of the PXI-5441 is limited to 32, 256, or 512 MBs which limits playback to approximately 1, 8, or 16 second loops, respectively. To overcome this limitation, Trident DVP supports streaming from a RAID hard-drive. With this setup, much larger files with longer playback times are read from the RAID drive and streamed to the PXI-5441 in real-time.
Impairments can be added with Trident DVP. These impairments are implemented by applying adjustments to the I & Q data before downloading. Once downloaded, the values cannot be changed without re-downloading. The current player application supports AWGN while future versions will support phase noise and fading.
Trident DVP can also perform BER testing on the transmission signal. In order to perform this testing, a properly formatted transport stream (one that was created with the Trident TSG) must be generated and modulated into a BIN File. At that point, Trident DVP begins signal generation through the RFSG and then interfaces with the device being tested to receive the recovered bitstream. The bitstream can be captured from the device in numerous ways, such as using High Speed Digital I/O (HSDIO).
Summary [Back to Top]
The list below summarizes the steps involved in taking existing content and creating a real-world broadcast television signal, and notes the Trident applications that perform those tasks.