William Sommerwerck wrote:

Many modern /high-performance/ receivers omit the RF stage (such as the Sony
DSP FM tuner), or allow you to bypass it (many ham receivers).

Ham receivers omit it because most hams don't need or want it. Since you are
going to be transmitting into a ham antenna, it needs to be resonant or appear
to be resonant.

A resonant antenna, unless it is 100% a resistor, is not resonant on frequencies
you don't want, so they are signifcantly reduced in strength anyway.

If the antenna is not resonant, a device called a "tuner" is used to make it
appear resonant to the transmitter (it does not affect the actual antenna).
In the process of "tuning the antenna", as it were, it detune signals away from
the frequency desired, so in practice it acts as a TRF stage.

For reception only the device is called a pre-selector, although sometimes they
are also called tuners, because they contain the same circuity.

Early (pre WW-II and soon after) TV sets used TRF receivers because of both
the lack of competing signals and the very wide bandwidth needed. As the bands
became more crowded (signals receivable 1), more services used nearby
frequencies and receiver sophistication increased they moved to superhets.

Eventualy they went to digital sythesizers with mixers, filters and decoders
on a chip. Digital TV receivers are the same thing, except instead of the
"receiver" chip outputing a video signal and an audio signal, it outputs a
bit stream to a decoder chip.

Going back to the other discussion (ducks) that's the irony of DVB-T versus
ATSC. The RF part of the receiver is so generic and adjustable "on the fly",
that it can tune almost anything, the video stream for both is MPEG TS
(transport stream) data, it's the encoding of the bit stream in between.

ATSC was chosen specificaly NOT to be the same as DVB-T.

Someone already sells a chipset to laptop manufacturers to give them ATSC/DVB-T
reception capability, which gives you a TV set that will work almost anywhere.
The TS decoding is done by program in the CPU, so it can support any changes
that come down the line as it where.

Geoff.


--
Geoffrey S. Mendelson N3OWJ/4X1GM
Those who cannot remember the past are condemned to misquote it.