Ned Soseman /
11.06.2011 03:13 PM
MPEG-2 basic training, part 2

Is MPEG compression your friend? Of course, the answer to this question is that MPEG compression is your friend, unless it’s not working properly. When that happens, it’s our job to make it friendly again. This “Transition to Digital” tutorial continues the discussion from the preceding mid-October “Transition to Digital” tutorial about monitoring and evaluating MPEG-2 streams.

Streams are made of packets with headers and are filled with metadata, compressed video or compressed audio. To reconstruct a program from a stream, all of its video, audio and table components, and the corresponding PID assignments, must be correct. Also, there must be consistency between PSI table contents and the associated video and audio streams. This is a good place to look for trouble in a suspicious MPEG-2 stream.

Program Specific Information
Program Specific Information (PSI) is part of the Transport Stream (TS). PSI is a set of tables needed to demultiplex and sort out PIDs that are tagged to programs. A Program Map Table (PMT) must be decoded to find the audio and video PIDs that identify the content of a particular program. Each program requires its own PMT with a unique PID value.

The master PSI table is the Program Association Table (PAT). If the PAT can’t be found and decoded in the transport stream, no programs can be found, decompressed or viewed.

PSI tables must be sent periodically and with a fast repetition rate so channel-surfers don’t feel that program selection takes too long. A critical aspect of MPEG testing is to check and verify the PSI tables for correct syntax and repetition rate.

Another PSI testing scenario is to determine the accuracy and consistency of PSI contents. As programs change or multiplexer provisioning is modified, errors may appear. One is an “Unreferenced PID,” where packets with a PID value are present in the TS that are not referred to in any table. Another would be a “Missing PID,” where no packets exist with the PID value referenced in the transport stream PSI table.

Good broadcast engineers never forget common sense. Just because there aren’t any unreferenced or missing PIDs doesn’t guarantee the viewer is necessarily receiving the correct program. There could be a mismatch of the audio content from one program being delivered with the video content from another.

Because MPEG-2 allows for multiple audio and video channels, a real-world “air check” is the most common-sense test to ensure that viewers are receiving the correct language and video. It’s possible to use a set-top box with a TV set to do the air check, but it’s preferable to use dedicated MPEG test gear that allows PSI table checks. It’s also handy if the test set includes a built-in decoder with picture and audio displays.

QoE
So, all the bits and bytes appear to be organized and in place. How do you evaluate the quality of an MPEG-2 stream? Most use the concept of QoE. Some engineers call QoE Perceived Quality of Service (PQoS), because QoE is the quality of service as it is actually perceived by the viewer. In this tutorial, we’ll call the measurement of viewer satisfaction QoE.

QoE methodology for the evaluation of audio and video content provides broadcasters with a variety of choices, covering low, medium or high levels of quality. The QoE evaluation allows operators to pre-determine a specific level of viewer satisfaction and then use it to minimize storage and network resources by allocating only the resources necessary to maintain that particular QoE level.

The most basic recognized method to measure video content QoE is known as referenceless analysis. Essentially, referenceless analysis is what everyone does subconsciously when they watch TV. Using this method of analysis, QoE is not measured by comparing the original video to what is delivered. Instead, the images are visually inspected for artifacts such as blockiness, blurred or jerky video, frame-by-frame if possible. The referenceless analysis approach is based on the theory that viewers don't know the quality of the original content.

These days, I wouldn’t be so certain. Bigger, brighter, undistorted plasma, LCD and LED screens make artifacts more difficult for even the most casual viewers to ignore. Funny thing about the new non-CRT screens: They don’t “Lie like a Trinitron.” That’s the good news and the bad news for engineers and others in the production and delivery chain.

More scientific evaluations of QoE consist of objective and subjective evaluation procedures, each one taking place after encoding. More subjective quality evaluation processes require more eyeballs, making the process more time-consuming with each viewer’s opinion.

Objective evaluation methods are based on and make use of multiple scientific metrics. Objective QoE evaluation methodology can provide results quicker, but it requires some physical resources and dedicated test gear.

One objective method of monitoring QoE is to use devices such as the one shown in our image. This device is an Ethernet video quality and service assurance monitoring and troubleshooting probe. Some products such as this provide analysis to the PID level, and may contain a hard drive for offline verification and inspection. Products like this are designed to monitor, analyze and possibly debug IP and MPEG transport quality issues at a problem viewer’s location, the receiving end of an STL, your home, your station’s maintenance shop or anywhere typically described as the video edge. It sure beats investigating problem locations with a portable TV and a 10ft mast.

QoS
Quality of Service is the ability to provide different priorities to different applications, users or data streams, or to guarantee a certain level of performance to a specific data stream. QoS may guarantee a required bit rate, delay, jitter, packet dropping probability and bit error rate. Quality of service guarantees are important if the network capacity has little headroom, especially for real-time MPEG-2 streaming, because it often requires a fixed bit rate and is delay-sensitive.

A network that supports QoS may agree on a traffic contract with the application software and reserve capacity in the network nodes, often during a session establishment phase. In computer networking and other packet-switched telecommunication networks, the term “traffic engineering” refers to resource reservation controls, not the achieved service quality.

During a session, QoS may monitor the achieved level of performance, such as the data rate and delay, and dynamically control scheduling priorities in the network nodes.

QoS is sometimes used as a quality measure, with many alternative definitions, rather than referring to the ability to reserve resources. Quality of service sometimes refers to a guaranteed level of quality of service. High QoS is often confused with a high level of performance or achieved service quality, such as a high bit rate, low latency and low bit error probability. A high level of performance is, in fact, a QoE factor.

Best-Effort non QoS
A so-called Best-Effort network or service does not fully support quality of service. It is also not all that unusual in broadcast facilities. Why? Because the technical foundations of most broadcast facilities are built on best-effort overprovisioning and redundancy. Many new devices such as routers and switches support QoS. Many older devices do not. As older devices are replaced within a station’s system, it will ultimately be capable of QoS monitoring and measurements.

A generously overprovisioned best-effort system shouldn’t need to rely on QoS, just as a well designed Master Control shouldn’t need a “Technical Difficulties” graphic. At least that’s the way some IT-centric people I’ve met seem to think. We broadcast engineers know it can’t hurt to have both readily available, just in case.

In the meantime, “Best Effort” can be a good substitute for complicated QoS control mechanisms. Your goal is to provide high-quality program content over a best-effort network by over-provisioning its capacity so that it has more than sufficient headroom for expected peak traffic loads. The resulting absence of network congestion eliminates the need for QoS mechanisms.

What is most interesting about MPEG-2 monitoring and evaluation is that there are more recognized methods worthy of discussion than space allows for now. The next “Transition to Digital” tutorial will address these methods to help you ensure your station’s MPEG streams meet viewer expectations.

The author would like to thank Les Zoltan at DVEO Pro Broadcast Division for his help in the preparation of this tutorial.



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