digital to analog

[DaveWM]

I know when I tune thru the uhf and some vhf bands on my old sets I get what I presume is a digital signal (noise both audio and video).

I wonder if a digital to analog converter could be built that would look a the IF signal and convert it back to analog installed in the tuner link (tuner to IF connection).


I guess this is a kind of silly proposition since it assumes NTSC, so the conversion would have to be more complicated. Not enough coffee this morning.

[jr_tech]

I would guess that poor stability and the poor noise figure of vintage TV front ends would make this a very "iffy" situation.
A few years ago, I tried to use a 50s UHF converter as the "front end" for a digital converter, thinking that I might possibly use the vintage converter to tune the DTV signal for a vintage set, and place the DTV box out of sight. Results were *not* good.
jr

[Jon A.]

You mean a device that could block-convert the digital channel spectrum and allow full use of an analog tuner? That I would like to see. I have thought about the possibility myself.

[DaveWM]

Originally Posted by jr_tech

I would guess that poor stability and the poor noise figure of vintage TV front ends would make this a very "iffy" situation.
A few years ago, I tried to use a 50s UHF converter as the "front end" for a digital converter, thinking that I might possibly use the vintage converter to tune the DTV signal for a vintage set, and place the DTV box out of sight. Results were *not* good.
jr

this is what I was thinking.

[Username1]

I wanted to do that as well... My thoughts were several of today's Insignia or similar converters, each tuned to a different local channel (ATSC), then feeding the output to rf modulators, each tuned to transmit on a different old analog channel, mixing it all to a single cable to run to the tv... Then most likely another box locally that I could just set the tv to, and have the local converter work as normal with the remote.... So I could use the tv tuner for 4 or 5 channels, then a 6th that would be remote-able, in the room. Other D to A boxes out of remote control range.

[jr_tech]

Indeed , a rack of digital tuners and B-T modulators combined into an "in home cable system" would be fun to set up and allow our vintage TVs to tune as they were intended to. When I loose the last analog channel in the area, I plan to set up at least one channel into a B-T modulator so my little vintage portables will have something to receive. But really, I have little desire to use vintage sets as daily watchers, since HD sets are good and pretty cheap these days.
jr

[zenith2134]

Intriguing premise, to convert the digital information to analog and view the resulting pattern/scope curves taken from the IF strip. I have here a bunch of monochrome analog TVs which, when tuned to UHF 58 or UHF 82, display sporadic flashes of digital data in the form of lines and dots.
These must be cellular or perhaps experimental WiMax type networks. Not sure how far the WiFI-blanket networks have progressed since their inception. I do know, of course, that the old UHF analog TV spectrum has been sold in part to the cellular companies & to government entities using it for first-responders.

UHF 58, by the way, is held in high esteem by paranormal investigators. Evidently, Albert Einstein was quoted as saying that it was possible to slightly de-tune a television receiver backwards from the center of 58 to conduct investigations with e.v.p. A germanium detector diode would be connected to the UHF tuner via a whip-type antenna 18" in length.

Taken with a grain of salt, this experiment would be largely impossible nowadays with digital garbage being beamed at these frequencies.

[dtvmcdonald]

As to actually converting the digital signal to a form that is equivalent to
a Windows .bmp file for each frame (assuming a progressive format) ... its
not easy! In fact, its probably impossible since there are "gotchas" in
the public description of the signal format intentionally
put there to make it impossible without paying tribute.

I'm talking ATSC signals here.

There are two parts: first you have to digitize the analog signal.
This is easy. Its normally done at the IF frequency with subsampling
(i.e. if the IF is 44 mHz you digitize at say 15 or 16 MHz sample rate).
This is done in quadrature so the resulting "positive" and "negative"
frequencies can be distinguished. Note that this is a heterodyne that
leaves the carrier (ATSC has a carrier!) at a couple of MHz.

Everything else is done in software.

Next, and hardest, you have to take the signal
and de-ghost it. The descriptions of how the signal works in the
public documents are good enough for this. What's hard is that
mathematically it is a difficult problem. Normally its done using
adaptive Weiner filters but each implementation is highly proprietary
and there is "magic" involved. I own a couple of patents that
show how to do it using Fourier transforms and adaption in the frequency domain.

Next you have to demodulate (the signal is digital, but its not yet baseband).
To do this you have to phase lock the carrier. This usually is trivial ...
BUT ... though the carrier was transmitted IT MAY NO LONGER BE THERE ...
there could be a ghost at exactly 0dB level and exactly 180 degrees phase
which cancels. This is guaranteed to happen sometimes, it called Murphy's Law.
Fortunately there is a trivial method that regenerates the carrier even if missing!
(I should add this ATSC is vestigial sideband AM ... if it were quadrature AM
as used by cable this would be no easier ... in fact the difference, to the software,
is trivial). Now you have baseband.

Conceptually these two steps are separate and done in this order ... but in
practice they are inside a feedback loop that makes the de-ghosting easier.

Once you do that, you simply take the (already digital probably 32 bit floating point) signal and
"quantize" it into the 8 levels of ATSC (well, its a bit more complicated
because of sync pulses that are not at those levels.) You then have an
error-corrected MP3 with meta-data (e.g. frequency mapping and program guide).
The rest, using to error correction to get
plain MP3 ... is straightforward ... but this is where
there are ambiguities in the spec.

As to using a commercial box at a fixed frequency with a pre-converter
this simply cannot be done, as the boxes presuppose regular frequencies
and use digital information in the signals to "remap" channels, and this
remapping can't be turned off. Try it and the box gets confused. That
is true for all the boxes I'm familiar with ... there is no reason at all a box can't
be built that does not do it.

[dtvmcdonald]

Originally Posted by zenith2134

Intriguing premise, to convert the digital information to analog and view the resulting pattern/scope curves taken from the IF strip.:

One more thing: without an excellent de-ghost/amplitude and phase correct,
you will never see the actual digital waveform. Even with a perfect
signal, no ghosts, arriving at the antenna, phase errors alone
in the antenna will render the waveform useless. You can't look at
the waveform (QAM) from cable either as channels are all filled in and
without DSP there is no way to get a perfect frequency separation
with no phase errors.

Its raining newts and salamanders here for the first time in months!

[jr_tech]

Originally Posted by dtvmcdonald

As to using a commercial box at a fixed frequency with a pre-converter
this simply cannot be done, as the boxes presuppose regular frequencies
and use digital information in the signals to "remap" channels, and this
remapping can't be turned off. Try it and the box gets confused. That
is true for all the boxes I'm familiar with ... there is no reason at all a box can't
be built that does not do it.

So my experiment with the vintage UHF converter ahead of the DTV box was doomed to failure before it was even started.
Oh well... it was a fun afternoon anyway!

I have noticed a carrier about 0.310 mHz up from the low end of the DTV channels ... is this the carrier that you mentioned? This carrier is useful for indication of DX channels that a TV or converter can't receive.

Thanks for posting all the great info on the DTV process!

jr

[dtvmcdonald]

Yes, that's the carrier.

I just realized that I had tested what I said with all boxes I have seen ...
but not my Sony Tv, so I just tried it. Simply tuning
to a channel does work if the logical and physical
channel are the same, but not if they are different. This has to be tried
after "channel scanning" the TV with the antenna
removed.