[opendtv] Re: Microtune MT2130 Tuner

  • From: "John Shutt" <shuttj@xxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Wed, 6 Dec 2006 16:55:10 -0500

Speaking of theoretical difference in digital tuners, I found this interesting tidbit in my mailbox today. From the letters section of this week's TV Technology, page 4:

"Dear Charlie Rhodes:

"I ran across your article entitled "The Superhetrodyne Concept and Reception" on www.tvtechnology.com. In the article you said that the (sic) direct conversion cannot be used in 8-VSB due to the problem of lower adjacent channel.

"How then can it be used in DVB-T while they have the same adjacent channels; is it because 8-VSB is not symmetrical at baseband?"

Richard Morrison,
San Diego, Calif.

Charlie responds:

"You are correct. Our 8-VSB modulation scheme places zero baseband frequency near one channel edge, while all QAM modulation schemes, being double sideband, place zero baseband frequency in the center of their channel.

"Now consider an undesired signal two MHz below the 8-VSB modulated channel. Upon demodulation, the undesired signal will appear at 2.31 MHz in the baseband signal, more or less in the center of our baseband which extends to 5.38 MHz. Thus this interference cannot be filtered out.

"Now in the case of any QAM modulated signal, and staying in a 6 MHz channel, this undesired signal turns out to be at 5.0 MHz which is well above the baseband bandwidth of any double sideband signal in a 6 MHz channel. Being outside the baseband, it can readily be removed by a low pass filter. You are the first of my readers to raise this question, and you've also correctly answered it.

"Stay tuned."

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