Composite video via RF modulator, negative image?

On Dec 4, 3:42=A0pm, Berend wrote: > All, >

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).

G=B2

Composite video has negative sync.

Are you sure the channel tuning is correct?

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

Isaac

On Dec 5, 5:25=A0am, snipped-for-privacy@seas.upenn.edu (Samuel M. Goldwasser) wrote: > snipped-for-privacy@yahoo.com writes: >

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.

G=B2

On Dec 5, 9:20=A0pm, isw wrote: > In article , > = snipped-for-privacy@seas.upenn.edu (Samuel M. Goldwasser) wrote: > > Composite video has negative sync. >

Also not necessarily true.

G=B2

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)

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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 > All,

from

connects an old DVD player to an

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

Isaac

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

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

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On Fri, 5 Dec 2008 21:28:22 -0800 (PST), snipped-for-privacy@yahoo.com put finger to keyboard and composed:

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.

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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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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

Quite the reverse, actually.

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    Dave Plowman        dave@davenoise.co.uk           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

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.

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Michael A. Terrell
Central Florida

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.

has a condensed descripton of various TV transmission systems.

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Central Florida

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.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

"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.