So I'm curious to know - is there an easy way to tell whether what I'm seeing on my set could be chroma dots or just a different type of noise which appears similar?

I haven't really been able to find good examples of what they look like in a moving picture, just still images.

Can it be determined by examining the schematic/circuitry whether they can and/or will be visible?

Alternately, I have a VCR attached to the set with composite inputs. If I were to feed it the luminance only of an S-video signal and observe the output for the dot patterns, would that work or does the internal VCR circuitry (e.g. OSD generator) re-insert colour information into the signal?

I realise it's an obscure set of questions but I'm curious to learn and start a discussion on it :)

I thought Chroma Dots were something that the PAL system had but not NTSC?

I don't remember the details but I read that there were some B&W video tapes of what were originally color Dr Who episodes using the dot pattern to determine the correct colors.

http://www.techmind.org/colrec/

I thought Chroma Dots were something that the PAL system had but not NTSC?

I don't remember the details but I read that there were some B&W video tapes of what were originally color Dr Who episodes using the dot pattern to determine the correct colors.

http://www.techmind.org/colrec/

NTSC definitely has it, it's visible on sets where you have more than 3.5MHz bandwidth, since the colour subcarrier is at that frequency. If I had a bandwidth (multiburst) generator I could answer my own question easily as to whether it is possible to see them on my set - alas, I don't, so I'm curious if there's a cheap way to determine this when there is the possibility of other noise sources (e.g. dot crawl)


Bob Andersen talks about it when running some of his sets after alignment and restoration as well.

NTSC definitely has it, it's visible on sets where you have more than 3.5MHz bandwidth, since the colour subcarrier is at that frequency.

The chroma subcarrier has 227 1/2 cycles per horizontal line. Most B&W TV sets made after 1954 limit the luma bandwidth to suppress the chroma subcarrier. Usually done by the way the picture IF strip is adjusted. Older sets can show the entire 4.2MHz of luma bandwidth as there was no chroma subcarrier added in back then.

The chroma subcarrier looks like a fine checkerboard moving vertically in areas of the picture with saturated color. The size of this checkerboard pattern is such that you could have about 330 checkers in a horizontal line (two checkers per cycle of subcarrier).

Here's a close up of chroma dots on a 1949 Admiral TV with a wide video IF bandwidth.

https://farm6.staticflickr.com/5270/5608269695_e1e8cd4001_z.jpg (https://flic.kr/p/9xzRc6)

I've seen that effect on some older B&W TVs, looks like 4.5Mhz beat, but adjusting the 4.5 trap never seemed to help. Always thought it might be some beat interference from the color burst or chroma signals. I've noticed it more now since I have to use a digital converter to pick up broadcast on the older TVs. Interesting!!

Interesting. What I'm seeing does look like the image posted by Bob, yet my set is early 60s and judging by the expected IF response (http://vintagepc.site50.net/site/fleetwoodtv/service/TV_009.png) it looks like it should have 3 Mhz of bandwidth at 50%... which indicates I probably am not seeing chroma dots. On the other hand, it's quite possible it's not aligned to factory specifications given I'm getting bleed from the video signal into the audio (buzzing when bright white text on the screen) but that's an audio trap/alignment issue. One of these days I'll probably splurge and get a VA62 and mod it for 21Mhz IF so I can do something about that.

I'll try to get a good picture of it on my set next time it's playing; I'll see if I can find an old tape or something to feed in a still image so it's easier to snap a pic of what I'm attempting to describe. :)

This thread is confusing. Is your set an early 60s black and white set, or was 60s a typo?
1) An early 60s set would not have a 21 MHz IF. What do you have?
2) A black and white set might roll off the high frequencies, but would not have a specific trap for the color subcarrier, so it could have chroma dot crawl to some degree even if it's not full 4 Mhz bandwidth.
3) you seem to differentiate chroma dots and dot crawl. They are the same thing. The only difference is that in color sets with a subcarrier trap in the luma channel, the dots in large areas of the same color were eliminated, but dots still appeared on the left and right edges of colored objects. In later sets with a comb filter, depending on the design, color dots could appear inside the top edge and just below the bottom edge of colored objects, and were called "hanging dots." Sophisticated comb filter circuits included logic to switch in a chroma trap at these transitions. Fast motion could also fool the comb filter into letting dots through in some cases.

To answer your questions:

1. It has a 21Mhz IF. I refer to it as an early 60s set because I recall finding that date code either somewhere on the chassis or in the accompanying documentation. I suppose it could be late 50s; I haven't found _any_ info on this set online barring a similar-looking but not entirely identical picture from a 50s sale brochure (the 21-41): http://www.tvhistory.tv/1950s-CAN-Fleetwood-Brochure3.JPG. Someone with an RCC Master Index might be able to pin down an actual year.

I'll have another look and see to confirm or deny. I wouldn't be surprised if there were still 21Mhz IF sets in Canada later than most of the US; we were pretty behind with FM adoption in tube radios too (not all that relevant, I guess) and the manufacturers might well have opted to save some re-design costs by extending the 21Mhz design a bit longer...

3. You're absolutely right; I don't think dot crawl is the term I was looking for. I was reading about other types of noise and how one of them could be mistaken for chroma dots, but the name escapes me. Dot crawl must have been the first thing that latched into my mind.

I run all my black and white sets through the Luma line of s-video or from component when set at 480i.

I run all my black and white sets through the Luma line of s-video or from component when set at 480i.

I gather you have an RF modulator of some sort, or did you modify your set to allow for a non RF video signal?

To answer your questions:
...I was reading about other types of noise and how one of them could be mistaken for chroma dots, but the name escapes me...

Thanks for the clarifications. 4.5 MHz sound carrier can look a bit like chroma, but it dances around and changes rapidly from vertical to diagonal to dots (possibly too fast to see) as the sound modulates the frequency. It is much more stationary during silence. This is suppressed by sound traps providing the fine tuning is correct. Even when alignment is perfect, if you tune toward the high end of the channel, you may be able to see the sound carrier as it comes out of he traps, provided the IF and video aren't too narrow to pass it.

I gather you have an RF modulator of some sort, or did you modify your set to allow for a non RF video signal?

Rf modulator.

IIRC Canada was slow to adopt color too so the IF bandwidth may not have been designed to eliminate a chroma dot issue the set likely would not have had to deal with when new.

Thanks for the clarifications. 4.5 MHz sound carrier can look a bit like chroma, but it dances around and changes rapidly from vertical to diagonal to dots (possibly too fast to see) as the sound modulates the frequency. It is much more stationary during silence. This is suppressed by sound traps providing the fine tuning is correct. Even when alignment is perfect, if you tune toward the high end of the channel, you may be able to see the sound carrier as it comes out of he traps, provided the IF and video aren't too narrow to pass it.

You may also be right on the date. I was able to peg the turntable (Collaro RC456) to around 1956 or 57.

I re-read my notes from the restoration and I now recall the CRT seems to have been a rebuild/replacement; I suspect very much the date I found was the date code on the CRT label, which would account for the discrepancy, unless those RC456s were in use for a long period of time.

OK, here's the effect I'm seeing (that background is a saturated blue in colour).
Also... looking at the text in the first image... is my picture tube under-focused (i.e. focal point behind the phosphor face)? The scanlines seem to expand to a larger "blip" when the brightness is turned up.

(clickable for full-size)

http://i.imgur.com/HbSvtAOl.jpg (http://imgur.com/HbSvtAO)

And another with a "real" image - visible on the hand/glove

http://i.imgur.com/wTKbLQ4l.jpg (http://imgur.com/wTKbLQ4)

It's a little fuzzy since my camera doesn't like the TV screen and wants to use the flash but I won't let it.

I'm pretty sure this isn't audio carrier as I experimented with tuning off-channel and I can see what old_tv_nut is talking about - especially on channels with just speech since there are naturally periods of silence and the effect will come and go with the sound.

Looks a decent match for what Bob posted earlier but not as pronounced, so if anything I suspect either IF alignment is a bit off, or the bandwidth is just barely wide enough to let some of that effect bleed through.

I'll be honest, looks fine to me.

https://scontent-a-mia.xx.fbcdn.net/hphotos-frc3/t1/1656341_10201625370460148_1270668802_n.jpg

Ignore my horizontal issue in the pic. But yea, that's my chroma dots.

OK, here's the effect I'm seeing (that background is a saturated blue in colour).
Also... looking at the text in the first image... is my picture tube under-focused (i.e. focal point behind the phosphor face)? The scanlines seem to expand to a larger "blip" when the brightness is turned up.

(clickable for full-size)

http://i.imgur.com/HbSvtAOl.jpg (http://imgur.com/HbSvtAO)

And another with a "real" image - visible on the hand/glove

http://i.imgur.com/wTKbLQ4l.jpg (http://imgur.com/wTKbLQ4)

It's a little fuzzy since my camera doesn't like the TV screen and wants to use the flash but I won't let it.

I'm pretty sure this isn't audio carrier as I experimented with tuning off-channel and I can see what old_tv_nut is talking about - especially on channels with just speech since there are naturally periods of silence and the effect will come and go with the sound.

Looks a decent match for what Bob posted earlier but not as pronounced, so if anything I suspect either IF alignment is a bit off, or the bandwidth is just barely wide enough to let some of that effect bleed through.

Yes, these are chroma dots - quite normal. As discussed, level depends on IF and video response. What you have may very well be in spec - no realignment needed. In a B&W set with a "sharpness" control, it affects them too.

Regarding focus: All electron guns make a larger spot at higher current ("bloom") to some extent. The effect is more or less non linear and somewhat different in different tube types depending on the gun design. Color tubes for a long time were run with a cyanish white point in order not to have the red current be way more than the blue and green(maybe 2:1 max - still unbalanced!), because the blooming in the red gun would produce red edges around small bright white highlights. Blooming gets worse as the cathodes age and the settings are cranked up to pull current from a larger area of the virtual cathode (the electron cloud in front of the cathode).


TV receiver guns are carefully designed to eliminate a worse effect - a large circular halo around the spot, which reduces detail contrast. You can see such halos in some oscilloscope CRTs when the beam current is turned way up. The compromise may be that you may see more of the TV CRT central spot blooming if it is driven hard, whereas the oscilloscope spot stays sharper but gets foggier.

By the way, too sharp a spot just makes the scan lines more objectionable - there is an optimum range. Tektronix once had a project to build a professional NTSC monitor with essentially high definition guns and phosphor dot pattern, and they didn't go to production because the video professionals told them it was impossible to judge video quality with those fully defined scanning lines flickering at them.

I have a case of --something--like this at the moment.

I have an old Bush 405 line set Ive modded to run at the UK's 625 line, and feeding video compo direct into the video-amp stages, via a small gain stage.
--I should point out--the set is a 14" Bush TV62 monochrome set and from 1955-7 vintage, when 405 line and VHF was the only show in town.

Ive potched about with the video amp to remove an overshoot issue--that has also extended its bandwidth probably to over 4 meg according to test-card images
Our chroma sub-carrier is 4.433MHz here....

The effects I get using a Composite Video signal are the herringbone-patterning on parts of the picture of high colour content, but also have an odd vertical strip approx 1/3 picture width that can only be noticed on dark/low contrast scenes, and can be shifted sidewise by use of line-osc setting. It looks exactly like a honeycomb or 'eye pattern' a CD player laser if scoped shows...

Switching the compo video from a PAL 4.43 signal to NTSC 3.38 has the effect of changing the pitch/coarseness of the herringbone and the honeycomb Surprisingly, the set is just as happy to run at 525 line as 625 line, although its not so keen on 60Hz frame-rate, and has somewhat reduced height, and hum on picture becomes an issue. 405 line UK VHF transmissions were frame-frequency locked to 50Hz mains freq so maybe are not as tolerant of hum issues as later 625 line sets ...

Most odd! Really need to try the set out via an S-Video source of just the Luma signal, see if I can banish that darned honeycomb. The herringbone I can live with...
Sure the colour-burst is doing summit odd when it gets in there....

I have a case of --something--like this at the moment.

... but also have an odd vertical strip approx 1/3 picture width that can only be noticed on dark/low contrast scenes, and can be shifted sidewise by use of line-osc setting. It looks exactly like a honeycomb or 'eye pattern' a CD player laser if scoped shows...

Switching the compo video from a PAL 4.43 signal to NTSC 3.38 has the effect of changing the pitch/coarseness of the herringbone and the honeycomb Sure the colour-burst is doing summit odd when it gets in there....

Exactly - you are seeing the peaks of the colour-burst during retrace.

I have a case of --something--like this at the moment.

I have an old Bush 405 line set Ive modded to run at the UK's 625 line, and feeding video compo direct into the video-amp stages, via a small gain stage.
--I should point out--the set is a 14" Bush TV62 monochrome set and from 1955-7 vintage, when 405 line and VHF was the only show in town.

Ive potched about with the video amp to remove an overshoot issue--that has also extended its bandwidth probably to over 4 meg according to test-card images
Our chroma sub-carrier is 4.433MHz here....

The effects I get using a Composite Video signal are the herringbone-patterning on parts of the picture of high colour content, but also have an odd vertical strip approx 1/3 picture width that can only be noticed on dark/low contrast scenes, and can be shifted sidewise by use of line-osc setting. It looks exactly like a honeycomb or 'eye pattern' a CD player laser if scoped shows...

Switching the compo video from a PAL 4.43 signal to NTSC 3.38 has the effect of changing the pitch/coarseness of the herringbone and the honeycomb Surprisingly, the set is just as happy to run at 525 line as 625 line, although its not so keen on 60Hz frame-rate, and has somewhat reduced height, and hum on picture becomes an issue. 405 line UK VHF transmissions were frame-frequency locked to 50Hz mains freq so maybe are not as tolerant of hum issues as later 625 line sets ...

Most odd! Really need to try the set out via an S-Video source of just the Luma signal, see if I can banish that darned honeycomb. The herringbone I can live with...
Sure the colour-burst is doing summit odd when it gets in there....

What is your audio carrier frequency? Someone here pointed out earlier that if you have too much bandwidth (which may be the case if you've modded yours) you can also get that to bleed into the picture - and the result was a herringbone pattern.

The Audio carrier--IF I remember correctly for VHF/405 was 3.5MHz, although I may be wrong....

Ive completely disconnected the input to the video stage from the detector/IF strip, but guess there could be some sound carrier leaking in somewhere....

The Burst visible on flyback/retrace sounds very possible though--I really will have to drag the set back out and try with straight luma signal.
I keep getting diverted to do these damned new-fangled flat-screen lcd things.....

The Audio carrier--IF I remember correctly for VHF/405 was 3.5MHz, although I may be wrong....

Ive completely disconnected the input to the video stage from the detector/IF strip, but guess there could be some sound carrier leaking in somewhere....

The Burst visible on flyback/retrace sounds very possible though--I really will have to drag the set back out and try with straight luma signal.
I keep getting diverted to do these damned new-fangled flat-screen lcd things.....

IIRC if you have a source with component out (R/G/B connectors, G will also provide a chroma-dot free black and white signal.