Do you know for sure if your DTV transport stream is performing the way it should? If it isn't, it can cause all kinds of problems. Not all stations have the budget for a cache of equipment to test and analyze every nuance of their transport stream. The same is true of measuring your frequency. Most stations have a service that does that, so why not a service that checks your station for transport stream errors and compliance?
Probably the biggest transport stream issue is with PSIP. (For a complete tutorial on PSIP, see BE June, p.140.) PSIP has gotten off to a rocky start, but if implemented properly at the television station and addressed in the digital television receiver, it's a valuable tool.
Although PSIP may contain a station's program guide, that's only a small part of what it can do. Art Allison, director of advanced engineering for the NAB, uses a cake as an analogy: “In NTSC we send the entire finished cake, but with PSIP, we send a digital recipe, along with the ingredients and then give the DTV receiver the directions on how to bake it. In addition to this, there are capabilities for branding of virtual channels. Here's a case where the engineers provide the tools and then get out of the way and let the marketing folks use them as they see fit,”
Unfortunately it hasn't gone all that smoothly. The ATSC has a complete set of specifications for PSIP and transport stream compliance. The FCC does not require stations to use PSIP, so some digital stations have chosen not to implement it as part of their DTV transition. But what if you have PSIP and it or any other part of your transport stream isn't error free?
If a set is expecting to get the PSIP information and the station isn't sending it or isn't sending an error-free transport stream, you can rest assured, no matter how good the received signal is, it isn't going to get past the decoder and there will be no picture. Some early DTV sets didn't make accommodations for PSIP, so what happens to those stations that want to do multicasting? The sets don't know what to do with the additional information and, again, probably no picture. The bottom line is that there is nothing wrong with PSIP or the other ATSC standards, if they're implemented properly.
As the result of field tests done from Nov. 1, 2000, through April 30, 2001, Triveni Digital found that many DTV transport streams do not comply with the ATSC standards, confirming the suspicion that equipment problems and/or configuration errors abound. This accounts for many of the tuning problems with digital television receivers, among other problems.
The company's technology permits the DTV stream tests to be recorded either by a DTV station, system integrator or the employees of Triveni Digital itself.
When station personnel record their own transport streams, the data is stored on commonly available digital media such as a CD, Zip disk or Org disk and then sent to Triveni for analysis. The typical duration for recorded streams ranges from 60 to 65 seconds (approximately 150MB) of transport stream sampling.
Once the tests had been conducted, the findings are analyzed for compliance with ATSC standards. The net result of all this testing and analyzing is a confidential report delivered to the client DTV station or system integrator informing them of any problems.
The test focuses primarily on the following areas:
- ATSC Program and System Information Protocol (PSIP) standard (A/65): PSIP problems can interfere with DTV receivers tuning to channels or displaying them in electronic program guides.
- Audio and video buffer usage: Buffer underflow or overflow can cause audio or video to break up or be presented with incorrect synchronization (lip sync issues).
- Program Clock Reference (PCR) frequency and jitter: PCR problems can cause synchronization problems with video and audio. In extreme cases they may even result in glitches in the audio or video due to lost data.
PROBLEM AREA POSSIBLE IMPACTS % OF ANALYZED STREAMS HAVING THIS PROBLEM PSIP/PSI table transmission interval Receivers may take longer to tune 70% Audio buffer usage Degraded sound quality or lip sync problem 48% Missing PSIP tables or syntax errors Receivers may not be able to navigate or access EPG 39% No EITs/Blank EITs Blank lines in EPG for that channel 35% PCR jitters and frequency deviation Receivers may not be able to synchronize with encoders 17% No PSIP metadata Receivers may not be able to navigate, access EPG, update the clock, etc 13% Video buffer usage Degraded video quality or lip sync problem 9%
Table 1 tells a revealing story and summarizes tests taken of 23 streams at 21 different DTV stations over the test period mentioned earlier.
DTV stations may have some degree of these problem errors and don't even know it. A simple test, similar to that performed by Triveni, could put this question to rest.
For additional information, visit Triveni Digital's website: www.TriveniDigital.com
Send questions and comments to:larry_bloomfield@intertec.com
