10 January 2005
Is there a bug in your box?
The digital broadcast market is undergoing dramatic change. But can designers keep up at the debug stage? By Vanessa Knivett.
One of the most active areas within electronics is digital broadcast, with the consumer market especially lively. Within this area, there are new sources of digital video and audio and more efficient ways of delivering information – Mpeg4, for example.
Pace Micro Technology’s digital broadcast product manager Gary Stephenson explains the market forces behind these changes: “The first is high definition tv, which though already quite mature in the US and Japan, is the next big thing for Europe. Then there is the emergence of PVR/DVR technology and convergence.”
When Pace first entered the set top box (stb) market, all digital tv products were based on Mpeg2. To meet demand for more sophisticated interactive graphics applications and interactive multimedia, Pace, like other companies, is now handling the move to Mpeg4 (also known as H.264).
Though high definition services in the US have been established on Mpeg2, there is general consensus in Europe that high definition stbs will employ H.264. Stephenson is now seeing ‘huge momentum’ behind H.264 and Microsoft’s recently announced VC1 standard, both of which provide Mpeg2 type quality, but at half the bit rate.
And, with a race underway between operators to be first to market in Europe with a high definition service, stb vendors are experiencing extreme time to market pressures, as well as the technical challenges associated with major design changes. “A completely new stb design, with new silicon is required,” confirms Stephenson.
In addition, the satellite front end standard is changing. DVB-S2 will replace DVB-S as the Digital Video Broadcast (DVB) group’s digital video satellite broadcast standard. DVB-S2 provides a 30% increase in capacity over DVB-S under the same transmission conditions, boosting the number of channels that can be broadcast on a conventional transponder.
Looking to the horizon, there is the shift from static tvs to converged mobile television services. After the DVB Forum finalised the specification in April 2004, Europe’s DVB-H handheld transmission standard has gained in standing. The European Telecommunications Standards Institute (ETSI) approved it in December 2004 and, with field trials in progress, DVB-H looks set to go live in 2005. Philips is developing next generation DVB-T/H decoder chipsets for handheld tv applications and system designers, including Pace, are well underway with initial DVB-H investigations.
The debug challenge
The first step in the debug process involving a signal source generator; an stb design, for example, typically needs an Mpeg2 transport stream generator. The next step requires signal modulation, with a test modulator capable of emulating the QPSK modulation scheme, either used with DVB-S, or 8PSK for DVB-S2.
But to capitalise on new compression standards, engineers not only need a way of verifying standards compliance, but also of analysing the performance of different components. For example, most next generation stbs would employ either H.264 and/or VC1 codecs. It is important to be able to compare the interoperability of different video codecs or different versions of a codec. This requires an elementary stream analyser – a software package that supports the latest video streams.
Whilst analysing the different design components requires the use of very accurate standard signals, signal sources are also required to take components to their design limits. Paul Robinson, senior manager of Tektronix’s Video Product Line, explains: “Stbs are more closely related to computers than tvs in terms of their user paradigm, so parametrical stress testing is an important part of the design process.”
Essentially, this means throwing as many nasty signals as possible at the stb to see if it can cope. This can be done using poor quality elementary streams to check whether the encoder locks up or not, and transport streams with lots of errors to give an insight into whether the Mpeg demodulator can interpret the information without locking up. Meanwhile, sending poor rf signals can create transport stream errors which themselves create elementary stream errors. “You are winding down the stb until the point at which it stops working – you want it to mute the signal, to say that it can’t connect.”
“Where things can get difficult from a hardware point of view is in supporting high definition, where the bandwidth used is much wider,” says Robinson. One of the complicating factors is the processing stage. In HDTV, video component signals, such as luminance and chrominance, are maintained as discrete components. This compares to the NTSC and PAL systems, where luminance and chrominance might be combined to form a single composite video signal. The implication for the debug stage is that there is a need to look at parameters like frequency response and channel delay between the signals – a long and laborious process. With this in mind, Tektronix has introduced the VM5000 – an automated test system that has been positioned as ‘the ultimate solution for testing component analogue video signals’. Measurement parameters include sync amplitude and timing, levels, noise, frequency response, non linearity, transient response and inter channel timing.
Similarly, other test vendors have used the ‘one box’ test concept to help designers through the debug process. Rohde & Schwarz, for example, has recently launched its SFU, a one box test platform designed to take the pain out of digital transmission test. Supporting the latest standards (DVB-H, DVB-S2 and DMB-T) and with a software upgrade path for future standards changes, the SFU combines a test transmitter, a signal generator for Mpeg2, a channel simulator (for simulating fading), digital AWGN noise source, ARB generator (for simulating interference) and RF modulator.
Important capabilities include being able to modify standard parameters, simulate signal impairments and dynamic fading, says Ward. The latter is important for testing mobile and multipath tranmission (for DVB-H), as in the real world, transmitted signals reach receivers over different signal paths. “You need to know how your chipset deals with unwanted signals,” says Rohde & Schwarz’ Nick Ward.
Another feature is the ability to do bit error real time (BER) measurement. BER shows how many bits difference there is between the signal broadcast and the reference stream, providing data on how efficient your chip is at a recorded broadcast. Other features include forward buffers, being able to simulate different kinds of noise, and the capability to do multiple transmissions so that a designer can test adjacent channels e.g. an analogue and a DVB-T signal together.
But one of the biggest challenges is that, unlike standard definition analogue equipment, high definition expertise is in short supply. Bearing this in mind, Tektronix has produced a dedicated site(www.tek.com/mpeg), with various white papers and tutorials, to guide designers through the broadcast maze.
Author
Graham Pitcher
Supporting Information
Websites
http://www.pace.co.uk
http://www.rsuk.rohde-schwarz.com
http://www.tektronix.com
Companies
Pace Micro Technology Plc
Rohde & Schwarz (UK) Ltd
Tektronix UK
This material is protected by Findlay Media copyright
See Terms
and Conditions.
One-off usage is permitted but bulk copying is not.
For multiple copies contact the
sales team.