All,

i've here a Mitsumi MDF8-UE3681 RF modulator that with some help from
http://ask.metafilter.com/56826/ now connects an old DVD player to an
even older TV set.

But the picture is negative.

The modulator has 4 pins, 3 potmeters, a switch, coax in/out, and a
cinch connector at the right.

The pins are marked: BS, V, A and B.

A and V is connected to Audio and Video signal.
BS connects to +5v
The a/v shielding and chassis go to GND.

(1 potmeter controls the channel, 1 ferrite core does something with
audio, and the last is a dragging one that does some w/the video. The
switch seems to be for a testpattern with two vertical bars?)


Berend

On Dec 4, 3:42*pm, Berend wrote:
All,

i've here a Mitsumi MDF8-UE3681 RF modulator that with some help
fromhttp://ask.metafilter.com/56826/now connects an old DVD player to
an
even older TV set.

But the picture is negative.

The modulator has 4 pins, 3 potmeters, a switch, coax in/out, and a
cinch connector at the right.

The pins are marked: BS, V, A and B.

A and V is connected to Audio and Video signal.
BS connects to +5v
The a/v shielding and chassis go to GND.

(1 potmeter controls the channel, 1 ferrite core does something
with
audio, and the last is a dragging one that does some w/the video.
The
switch seems to be for a testpattern with two vertical bars?)

Berend

Are you _certain_ that the modulator requires sync negative? I've
never seen one but it's conceivable to build a modulator that requires
sync positive. It's not like audio where the absolute phase doesn't
matter. (OK some audio nuts swear they can tell the difference but
most of us can't).

G²

writes:

On Dec 4, 3:42*pm, Berend wrote:
All,

i've here a Mitsumi MDF8-UE3681 RF modulator that with some help
fromhttp://ask.metafilter.com/56826/now connects an old DVD player to
an
even older TV set.

But the picture is negative.

The modulator has 4 pins, 3 potmeters, a switch, coax in/out, and a
cinch connector at the right.

The pins are marked: BS, V, A and B.

A and V is connected to Audio and Video signal.
BS connects to +5v
The a/v shielding and chassis go to GND.

(1 potmeter controls the channel, 1 ferrite core does something
with
audio, and the last is a dragging one that does some w/the video.
The
switch seems to be for a testpattern with two vertical bars?)

Berend

Are you _certain_ that the modulator requires sync negative? I've
never seen one but it's conceivable to build a modulator that requires
sync positive. It's not like audio where the absolute phase doesn't
matter. (OK some audio nuts swear they can tell the difference but
most of us can't).

Composite video has negative sync.

Are you sure the channel tuning is correct?

--
sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

In article ,
(Samuel M. Goldwasser) wrote:

writes:

On Dec 4, 3:42*pm, Berend wrote:
All,

i've here a Mitsumi MDF8-UE3681 RF modulator that with some help
fromhttp://ask.metafilter.com/56826/now connects an old DVD player to
an
even older TV set.

But the picture is negative.

The modulator has 4 pins, 3 potmeters, a switch, coax in/out, and a
cinch connector at the right.

The pins are marked: BS, V, A and B.

A and V is connected to Audio and Video signal.
BS connects to +5v
The a/v shielding and chassis go to GND.

(1 potmeter controls the channel, 1 ferrite core does something
with
audio, and the last is a dragging one that does some w/the video.
The
switch seems to be for a testpattern with two vertical bars?)

Berend

Are you _certain_ that the modulator requires sync negative? I've
never seen one but it's conceivable to build a modulator that requires
sync positive. It's not like audio where the absolute phase doesn't
matter. (OK some audio nuts swear they can tell the difference but
most of us can't).

Composite video has negative sync.

Not as transmitted; tip of sync is peak output power.

Isaac

On Dec 5, 5:25*am, (Samuel M. Goldwasser) wrote:
writes:

Are you _certain_ that the modulator requires sync negative? I've
never seen one but it's conceivable to build a modulator that
requires
sync positive. It's not like audio where the absolute phase
doesn't
matter. (OK some audio nuts swear they can tell the difference
but
most of us can't).

Composite video has negative sync.

Are you sure the channel tuning is correct?


Yeah I know composite video is negative sync - to the outside world -
but it _is_ conceivable that a manufacturer for some reason uses
inveted video internally. I know I've designed some gear that has
inverted video in some stages but of course is back to sync negative
when interfacing to other gear.

G²

On Dec 5, 9:20*pm, isw wrote:
In article ,
(Samuel M. Goldwasser) wrote:
Composite video has negative sync.

Not as transmitted; tip of sync is peak output power.

Isaac

Also not necessarily true.

http://en.wikipedia.org/wiki/Video_modulation

G²

wrote:


Yeah I know composite video is negative sync - to the outside world -
but it _is_ conceivable that a manufacturer for some reason uses
inveted video internally. I know I've designed some gear that has
inverted video in some stages but of course is back to sync negative
when interfacing to other gear.

May be worth the OP obtaining the circuit diagram of the video recorder
he pinched the module from, and checking this (or a data sheet for the
module if Mitsumi or Samsung ever published it)

--
Adrian C

Hello
In the other thread you mention you get a snowy picture, how did you
fix that?
As for the negative picture it may be an impedance problem, these
modulators do not put the required 75 ohm load to the video source and
the DVD may be overdriving the modulator without that load. Measure
the resistance of modulator video input to ground with an ohmmeter and
the DVD disconnected and if it reads high values or infinite, place a
75 ohm resistor from video input to ground. I did the same as you did,
also with a VCR modulator and had a similar problem.

With another modulator I was having problems because I didn't connect
an antenna to the antenna input.

BS may stand for Bypass supply, as it supplies a small bypass
amplifier that allows the incoming antenna signal to pass to the RF
output.
CONV+ is the converter (modulator) supply. For a proper operation you
should supply both inputs with 5V.

On 5 Des, 00:42, Berend wrote:
All,

i've here a Mitsumi MDF8-UE3681 RF modulator that with some help fromhttp://ask.metafilter.com/56826/now connects an old DVD player to an
even older TV set.

But the picture is negative.

The modulator has 4 pins, 3 potmeters, a switch, coax in/out, and a
cinch connector at the right.

The pins are marked: BS, V, A and B.

A and V is connected to Audio and Video signal.
BS connects to +5v
The a/v shielding and chassis go to GND.

(1 potmeter controls the channel, 1 ferrite core does something with
audio, and the last is a dragging one that does some w/the video. The
switch seems to be for a testpattern with two vertical bars?)

Berend

In article
,
wrote:

On Dec 5, 9:20*pm, isw wrote:
In article ,
(Samuel M. Goldwasser) wrote:
Composite video has negative sync.

Not as transmitted; tip of sync is peak output power.

Isaac

Also not necessarily true.

http://en.wikipedia.org/wiki/Video_modulation

Let's just say that the odds of running into anything but sync-up is
really, really, low. Especially in the US.

Isaac

isw wrote:

In article ,
(Samuel M. Goldwasser) wrote:

writes:

On Dec 4, 3:42 pm, Berend wrote:
All,

i've here a Mitsumi MDF8-UE3681 RF modulator that with some help
fromhttp://ask.metafilter.com/56826/now connects an old DVD player to
an
even older TV set.

But the picture is negative.

The modulator has 4 pins, 3 potmeters, a switch, coax in/out, and a
cinch connector at the right.

The pins are marked: BS, V, A and B.

A and V is connected to Audio and Video signal.
BS connects to +5v
The a/v shielding and chassis go to GND.

(1 potmeter controls the channel, 1 ferrite core does something
with
audio, and the last is a dragging one that does some w/the video.
The
switch seems to be for a testpattern with two vertical bars?)

Berend

Are you _certain_ that the modulator requires sync negative? I've
never seen one but it's conceivable to build a modulator that requires
sync positive. It's not like audio where the absolute phase doesn't
matter. (OK some audio nuts swear they can tell the difference but
most of us can't).

Composite video has negative sync.

Not as transmitted; tip of sync is peak output power.


That was done so that any 'snow' in a weak picture was white, instead
of black dots.




--
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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

Berend wrote:

All,

i've here a Mitsumi MDF8-UE3681 RF modulator that with some help from
http://ask.metafilter.com/56826/ now connects an old DVD player to an
even older TV set.

But the picture is negative.


If you over modulate the video modulator, or overload the receiver you
will have inverted and distorted video.



The modulator has 4 pins, 3 potmeters, a switch, coax in/out, and a
cinch connector at the right.

The pins are marked: BS, V, A and B.

A and V is connected to Audio and Video signal.
BS connects to +5v
The a/v shielding and chassis go to GND.

(1 potmeter controls the channel, 1 ferrite core does something with
audio, and the last is a dragging one that does some w/the video. The
switch seems to be for a testpattern with two vertical bars?)

Berend


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listed, or I will not see your messages.

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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

On Fri, 5 Dec 2008 21:28:22 -0800 (PST), put
finger to keyboard and composed:

On Dec 5, 5:25*am, (Samuel M. Goldwasser) wrote:
writes:

Are you _certain_ that the modulator requires sync negative? I've
never seen one but it's conceivable to build a modulator that
requires
sync positive. It's not like audio where the absolute phase
doesn't
matter. (OK some audio nuts swear they can tell the difference
but
most of us can't).

Composite video has negative sync.

Are you sure the channel tuning is correct?


Yeah I know composite video is negative sync - to the outside world -
but it _is_ conceivable that a manufacturer for some reason uses
inveted video internally. I know I've designed some gear that has
inverted video in some stages but of course is back to sync negative
when interfacing to other gear.

G²

But would inverted video produce the OP's symptom?

___ ___
| | | | ____ Black level A
__| |__ _______| |_ ____ Black level B
| |
| _| ---- Black level C
| _|
|_|

Assuming the TV's sync separator expects negative sync, then where
would the black level be?

AFAICS, if the black level is at B or C, then the TV wouldn't see the
sync pulses. If at A, then the TV wouldn't see any video information.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Franc Zabkar wrote:

On Fri, 5 Dec 2008 21:28:22 -0800 (PST), put
finger to keyboard and composed:

On Dec 5, 5:25 am, (Samuel M. Goldwasser) wrote:
writes:

Are you _certain_ that the modulator requires sync negative? I've
never seen one but it's conceivable to build a modulator that
requires
sync positive. It's not like audio where the absolute phase
doesn't
matter. (OK some audio nuts swear they can tell the difference
but
most of us can't).

Composite video has negative sync.

Are you sure the channel tuning is correct?


Yeah I know composite video is negative sync - to the outside world -
but it _is_ conceivable that a manufacturer for some reason uses
inveted video internally. I know I've designed some gear that has
inverted video in some stages but of course is back to sync negative
when interfacing to other gear.

G²

But would inverted video produce the OP's symptom?

___ ___
| | | | ____ Black level A
__| |__ _______| |_ ____ Black level B
| |
| _| ---- Black level C
| _|
|_|

Assuming the TV's sync separator expects negative sync, then where
would the black level be?

AFAICS, if the black level is at B or C, then the TV wouldn't see the
sync pulses. If at A, then the TV wouldn't see any video information.


If you over modulate the video, you compress, or clip the sync, and
destroy the DC restoration in NTSC video. The distorted signal affects
the AGC system, compounding the problems. That modulator may be
expecting a terminator, or pot t the input to set level, and provide
termination. It was no fun to find that some idiot had swiped the
terminators from the modulators in a CATV headend.


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your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

In article ,
"Michael A. Terrell" wrote:

isw wrote:

In article ,
(Samuel M. Goldwasser) wrote:

writes:

On Dec 4, 3:42 pm, Berend wrote:
All,

i've here a Mitsumi MDF8-UE3681 RF modulator that with some help
fromhttp://ask.metafilter.com/56826/now connects an old DVD player to
an
even older TV set.

But the picture is negative.

The modulator has 4 pins, 3 potmeters, a switch, coax in/out, and a
cinch connector at the right.

The pins are marked: BS, V, A and B.

A and V is connected to Audio and Video signal.
BS connects to +5v
The a/v shielding and chassis go to GND.

(1 potmeter controls the channel, 1 ferrite core does something
with
audio, and the last is a dragging one that does some w/the video.
The
switch seems to be for a testpattern with two vertical bars?)

Berend

Are you _certain_ that the modulator requires sync negative? I've
never seen one but it's conceivable to build a modulator that requires
sync positive. It's not like audio where the absolute phase doesn't
matter. (OK some audio nuts swear they can tell the difference but
most of us can't).

Composite video has negative sync.

Not as transmitted; tip of sync is peak output power.


That was done so that any 'snow' in a weak picture was white, instead
of black dots.

But mostly because sync circuits didn't work very well, and since video
is transmitted as AM (well, the sync frequencies are, anyhow), sync up
gave even a weak signal the best chance of getting a lock.

Isaac

In article ,
Michael A. Terrell wrote:
Composite video has negative sync.

Not as transmitted; tip of sync is peak output power.


That was done so that any 'snow' in a weak picture was white, instead
of black dots.

Quite the reverse, actually.

--
*A fool and his money can throw one hell of a party.

Dave Plowman London SW
To e-mail, change noise into sound.

forgot to mention, this should be a PAL signal (Europe/Netherlands)
i have this working now, reasonable picture quality (ok, colours,
sharp enough to use the menu/mp3 player which is what it wanted; no
snow really but does distort a bit)
encountered more of the same probs (bad colours, negative, no sync),
can't reproduce consistently
tried different video outputs on the player (beside 'video' it also
has 'coaxial' and Y, Pb/Cb, Pr/Cr outs) have a scart breakout, video
seems to be the composite signal.
also i wonder if the player was giving the wrong output (NTSC, PAL 60?
or something in the video mode..)

could be inferiour hardware, cables... the cinch cable i use prolly
has been discarded for a reason. or the channel was wrong and it was
tuned to some 'echo' (don't know how to call this)
i really don't have much to test it with (just the player, the tele
and some wires, breadboard, batteries and leds)
my first thoughts where i was overdriving something or that i should
not mix the shieldings/GNDs.. but i'm really just an amateurs that
figured the shiny box in the VCR did what it appeared to do, modulator
video to antenna, and it does

thanks for the comments. i don't think i can improve much on this
without getting measuring devices etc.

(note that the metafilter.com thread is from another guy, though the
modulator is somewhat alike...)

regards, Berend

isw wrote:

In article ,
"Michael A. Terrell" wrote:

isw wrote:

In article ,
(Samuel M. Goldwasser) wrote:

writes:

On Dec 4, 3:42 pm, Berend wrote:
All,

i've here a Mitsumi MDF8-UE3681 RF modulator that with some help
fromhttp://ask.metafilter.com/56826/now connects an old DVD player to
an
even older TV set.

But the picture is negative.

The modulator has 4 pins, 3 potmeters, a switch, coax in/out, and a
cinch connector at the right.

The pins are marked: BS, V, A and B.

A and V is connected to Audio and Video signal.
BS connects to +5v
The a/v shielding and chassis go to GND.

(1 potmeter controls the channel, 1 ferrite core does something
with
audio, and the last is a dragging one that does some w/the video.
The
switch seems to be for a testpattern with two vertical bars?)

Berend

Are you _certain_ that the modulator requires sync negative? I've
never seen one but it's conceivable to build a modulator that requires
sync positive. It's not like audio where the absolute phase doesn't
matter. (OK some audio nuts swear they can tell the difference but
most of us can't).

Composite video has negative sync.

Not as transmitted; tip of sync is peak output power.


That was done so that any 'snow' in a weak picture was white, instead
of black dots.

But mostly because sync circuits didn't work very well, and since video
is transmitted as AM (well, the sync frequencies are, anyhow), sync up
gave even a weak signal the best chance of getting a lock.


Really? Sync doesn't work? Then why have we bothered to transmit TV
for over 60 years? In the US, analog TV visual is transmitted as
'Vestigial Sideband', with the sync tips 'Blacker than Black'. That
means it it is at 100% Visual modulation, with full brightness at the
lowest level. US TVs typically used 'keyed AGC', which measures the
received signal levels during the sync pulses. This prevents changes in
average video from affecting the overall system gain. If the visual
modulation is out of spec, the system doesn't work properly.

I was a RF broadcast engineer at three TV stations. I've built CATV
headends, mobile TV production vehicles, and worked with CARS & STL
systems before I became 100% disabled.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

"Dave Plowman (News)" wrote:

In article ,
Michael A. Terrell wrote:
Composite video has negative sync.

Not as transmitted; tip of sync is peak output power.

That was done so that any 'snow' in a weak picture was white, instead
of black dots.

Quite the reverse, actually.


Really? Show us a valid cite for NTSC Visual modulation that agrees
with you. (Type M) An old term in the US TV industry for sync was
'Blacker than Black'. If the modulation is inverted, any loss of signal
strength causes sync to be affected before the video. I've read the
NTSC documents and arguments that were published at the time the system
was created.

http://en.wikipedia.org/wiki/NTSC has a condensed descripton of various
TV transmission systems.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

In article ,
Michael A. Terrell wrote:
"Dave Plowman (News)" wrote:

In article ,
Michael A. Terrell wrote:
Composite video has negative sync.

Not as transmitted; tip of sync is peak output power.

That was done so that any 'snow' in a weak picture was white,
instead of black dots.

Quite the reverse, actually.


Really? Show us a valid cite for NTSC Visual modulation that agrees
with you. (Type M) An old term in the US TV industry for sync was
'Blacker than Black'. If the modulation is inverted, any loss of signal
strength causes sync to be affected before the video. I've read the
NTSC documents and arguments that were published at the time the system
was created.

PAL as used in most of Europe uses negative modulation - and one reason
was precisely that interference is less noticeable being black flecks
rather than peak white.

--
*Don't byte off more than you can view *

Dave Plowman London SW
To e-mail, change noise into sound.

PAL as used in most of Europe uses negative modulation -- and
one reason was precisely that interference is less noticeable,
being black flecks rather than peak white.

"Snow" and "interference" are not the same thing.

I was brought up being taught that interference -- such as spark-plug
pulses -- was more likely to increase the signal level than decrease it.
Therefore, it made sense for the sync pulses to be at peak modulation, so
that interference would not reduce or even erase them.

"Dave Plowman (News)" wrote:

In article ,
Michael A. Terrell wrote:
"Dave Plowman (News)" wrote:

In article ,
Michael A. Terrell wrote:
Composite video has negative sync.

Not as transmitted; tip of sync is peak output power.

That was done so that any 'snow' in a weak picture was white,
instead of black dots.

Quite the reverse, actually.

Really? Show us a valid cite for NTSC Visual modulation that agrees
with you. (Type M) An old term in the US TV industry for sync was
'Blacker than Black'. If the modulation is inverted, any loss of signal
strength causes sync to be affected before the video. I've read the
NTSC documents and arguments that were published at the time the system
was created.

PAL as used in most of Europe uses negative modulation - and one reason
was precisely that interference is less noticeable being black flecks
rather than peak white.


NTSC uses 'Negative Modulation' as well, and for the reasons I've
stated.

Did you even look at the URL I posted?

http://en.wikipedia.org/wiki/NTSC


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listed, or I will not see your messages.

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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

In article ,
Michael A. Terrell wrote:

"Dave Plowman (News)" wrote:

In article ,
Michael A. Terrell wrote:
"Dave Plowman (News)" wrote:

In article ,
Michael A. Terrell wrote:
Composite video has negative sync.

Not as transmitted; tip of sync is peak output power.

That was done so that any 'snow' in a weak picture was white,
instead of black dots.

Quite the reverse, actually.

Really? Show us a valid cite for NTSC Visual modulation that
agrees with you. (Type M) An old term in the US TV industry for
sync was 'Blacker than Black'. If the modulation is inverted, any
loss of signal strength causes sync to be affected before the video.
I've read the NTSC documents and arguments that were published at
the time the system was created.

PAL as used in most of Europe uses negative modulation - and one
reason was precisely that interference is less noticeable being black
flecks rather than peak white.


NTSC uses 'Negative Modulation' as well, and for the reasons I've
stated.

'Snow' due to poor signal tends to be random noise - with the white parts
showing more. And of course with a weak signal interference is more
likely.

Did you even look at the URL I posted?

http://en.wikipedia.org/wiki/NTSC

No. Wiki is in no way a reliable reference source.

--
*Arkansas State Motto: Don't Ask, Don't Tell, Don't Laugh.

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" wrote:

In article ,
Michael A. Terrell wrote:

"Dave Plowman (News)" wrote:

In article ,
Michael A. Terrell wrote:
"Dave Plowman (News)" wrote:

In article ,
Michael A. Terrell wrote:
Composite video has negative sync.

Not as transmitted; tip of sync is peak output power.

That was done so that any 'snow' in a weak picture was white,
instead of black dots.

Quite the reverse, actually.

Really? Show us a valid cite for NTSC Visual modulation that
agrees with you. (Type M) An old term in the US TV industry for
sync was 'Blacker than Black'. If the modulation is inverted, any
loss of signal strength causes sync to be affected before the video.
I've read the NTSC documents and arguments that were published at
the time the system was created.

PAL as used in most of Europe uses negative modulation - and one
reason was precisely that interference is less noticeable being black
flecks rather than peak white.

NTSC uses 'Negative Modulation' as well, and for the reasons I've
stated.

'Snow' due to poor signal tends to be random noise - with the white parts
showing more. And of course with a weak signal interference is more
likely.

Did you even look at the URL I posted?

http://en.wikipedia.org/wiki/NTSC

No. Wiki is in no way a reliable reference source.


Fine you win. You know everything, and everyone else in the world
are morons.

--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

In article ,
"Michael A. Terrell" wrote:

isw wrote:

In article ,
"Michael A. Terrell" wrote:

isw wrote:

In article ,
(Samuel M. Goldwasser) wrote:

writes:

On Dec 4, 3:42 pm, Berend wrote:
All,

i've here a Mitsumi MDF8-UE3681 RF modulator that with some help
fromhttp://ask.metafilter.com/56826/now connects an old DVD player
to
an
even older TV set.

But the picture is negative.

The modulator has 4 pins, 3 potmeters, a switch, coax in/out,
and a
cinch connector at the right.

The pins are marked: BS, V, A and B.

A and V is connected to Audio and Video signal.
BS connects to +5v
The a/v shielding and chassis go to GND.

(1 potmeter controls the channel, 1 ferrite core does something
with
audio, and the last is a dragging one that does some w/the
video.
The
switch seems to be for a testpattern with two vertical bars?)

Berend

Are you _certain_ that the modulator requires sync negative? I've
never seen one but it's conceivable to build a modulator that
requires
sync positive. It's not like audio where the absolute phase doesn't
matter. (OK some audio nuts swear they can tell the difference but
most of us can't).

Composite video has negative sync.

Not as transmitted; tip of sync is peak output power.


That was done so that any 'snow' in a weak picture was white, instead
of black dots.

But mostly because sync circuits didn't work very well, and since video
is transmitted as AM (well, the sync frequencies are, anyhow), sync up
gave even a weak signal the best chance of getting a lock.


Really? Sync doesn't work? Then why have we bothered to transmit TV
for over 60 years? In the US, analog TV visual is transmitted as
'Vestigial Sideband', with the sync tips 'Blacker than Black'. That
means it it is at 100% Visual modulation, with full brightness at the
lowest level. US TVs typically used 'keyed AGC', which measures the
received signal levels during the sync pulses. This prevents changes in
average video from affecting the overall system gain. If the visual
modulation is out of spec, the system doesn't work properly.

Think about how long ago all the specs for RS-170 were laid down. *In
the beginning* things were a bit more difficult, and synchronization was
a bit tricky, what with marginally stable multivibrator oscillators and
all. Using the highest-powered part of the signal for sync made things a
bit easier. "Vestigal sideband" was used instead of true single-sideband
because true SSB is more difficult to receive (since it needs a
synchronous carrier). An interesting benefit of VSB is that for
modulation frequencies near the carrier *where the sync signals are* the
signal is standard double-sideband AM, which *doubles the amplitude of
the detected signal* compared to the single-sideband part; this helps
sync performance. Most of the lower sideband is wiped off on the way to
the antenna, and the characteristics of the filter used must be
carefully specified because there is a complimentary filter in every
receiver.

I was a RF broadcast engineer at three TV stations. I've built CATV
headends, mobile TV production vehicles, and worked with CARS & STL
systems before I became 100% disabled.

In that case, you may have used some gear that I designed, or helped
design, or managed the design of.

Isaac

isw wrote:

In article ,
"Michael A. Terrell" wrote:

Really? Sync doesn't work? Then why have we bothered to transmit TV
for over 60 years? In the US, analog TV visual is transmitted as
'Vestigial Sideband', with the sync tips 'Blacker than Black'. That
means it it is at 100% Visual modulation, with full brightness at the
lowest level. US TVs typically used 'keyed AGC', which measures the
received signal levels during the sync pulses. This prevents changes in
average video from affecting the overall system gain. If the visual
modulation is out of spec, the system doesn't work properly.

Think about how long ago all the specs for RS-170 were laid down. *In
the beginning* things were a bit more difficult, and synchronization was
a bit tricky, what with marginally stable multivibrator oscillators and
all.


A TV station that I visited in Fairbanks in the early '70s still used
their vacuum tube RCA sync & chroma generator. The master monitor in
the control room was based on the RCA CTC4 TV chassis, and the GE film
chain was tube, as well. The equipment worked, but was very temperature
sensitive. Bad tubes were a problem, as well. Some stations had over
1000 vacuum tubes in the studio & control room. That was one reason TV
stations ran a test pattern for a half hour or more each morning.


Using the highest-powered part of the signal for sync made things a
bit easier. "Vestigal sideband" was used instead of true single-sideband
because true SSB is more difficult to receive (since it needs a
synchronous carrier). An interesting benefit of VSB is that for
modulation frequencies near the carrier *where the sync signals are* the
signal is standard double-sideband AM, which *doubles the amplitude of
the detected signal* compared to the single-sideband part; this helps
sync performance. Most of the lower sideband is wiped off on the way to
the antenna, and the characteristics of the filter used must be
carefully specified because there is a complimentary filter in every
receiver.


Not to mention the careful alignment of the diplexer to allow the use
of a single antenna.


I was a RF broadcast engineer at three TV stations. I've built CATV
headends, mobile TV production vehicles, and worked with CARS & STL
systems before I became 100% disabled.

In that case, you may have used some gear that I designed, or helped
design, or managed the design of.


I've worked with a Gates 500 W on Ch 8, a 130 KW Comark UHF on Ch 55,
and a 25 KW RCA TTU-25B that I moved & rebuilt. It had been on Ch 55,
and had to be converted for Ch 58. The RCA was built in 1952.

I've designed a few things for broadcast use, as well. We got a
notice from the FAA & FCC about a change in the laws, regarding tower
lighting. We had 72 hours to let them know if we already had a remote
monitoring system in place, or buy one from an approved equipment list.
I designed and built a system that day, and installed it the next
morning. It used a1024 Hz tone on a spare audio channel in the STL from
our old transmitter site, to the new one. A current transformer
monitored current to the tower lights. The tone was on when the lights
were off, and went off when all the lights were on. A tone decoder at
the other end was mounted in the main console, with a small lamp beside
the master monitor. It was just annoying enough for the operator to
notice, and log the start & stop times each day. I designed and built a
lot of interfaces to connect incompatible hardware, too. The most fun
was transmitting a color station ID at a B&W AFRTS station with no color
equipment, at all.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida