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Possible Vizio DTV Tuner Problem Reported

I received an interesting e-mail from a cable operator doing the right thing and trying to make it easier for subscribers with ClearQAM DTV sets to receive unencrypted programming without a cable box by grouping clear channels together and providing useful PSIP data.

In the communication, he mentioned that he found that the Vizio DTV sets with ClearQAM tuners they tested can't tune in QAM signals lower than 135 MHz or higher than 615 MHz. Analog reception was reported as fine over the entire channel range.

It seems very odd to me that a popular DTV set manufacturer like Vizio would limit their QAM tuning range to such a narrow band, when there is no obvious technical reason. The cable engineer discovered the problem after getting calls that some subscribers who were not seeing all the unencrypted ClearQAM channels. Sets with ClearQAM capability from all other vendors had no problem decoding all the channels being sent in the clear.

I've seen cases where small PSIP errors such as inconsistent data between tables, can cause problems on some DTV converter boxes, but not others. However, I've never heard of a case where PSIP restricted the frequency span of the tuners.

I'm asking readers who have Vizio DTV sets and cable TV (a digital subscription should not be required) to e-mail ( me if they can receive ClearQAM channels outside the 135 – 615 MHz band. As many cable operators put unencrypted signals at the upper end of the band--above 700 MHz--so it should be easy to see if this is truly a problem with Vizio's tuners.

Please note the tuners don't have a problem receiving all analog and 8-VSB channels. The reported problem is with QAM modulation only. I'll report the results next week.

Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.