NTSC scan rates during the switch to color

[nasadowsk]

We all know the line rate shifted slightly with color broadcasts.

Did stations switch to that rate before broadcasting in color (i.e. while still in black and white) only change when broadcasting in color.

[Electronic M]

It wasn't only the line rate; the field rate changed from 30 to 29.9something Hz.

As I understand it monochrome shows always used the monochrome rate and color only used the color scan rates and stations would switch between the two on a show by show basis even on locally produced content. Stations would also switch the color burst (color sync pulse) on and off depending on if the show was color. Color killer circuits in TVs would turn off color demodulation if no burst was present to prevent confetti (color snow) and rainbow effects from high frequency monochrome video from appearing on monochrome shows. Sometime between 1970 and 1990 stations transitioned to running the color scan rates and burst regardless of the program.

I think the transition to all color rates was partly due to some systems that used the color burst of OTA TV as a highly accurate calibration frequency for scientific purposes and for simplicity as new monochrome program content dried up.

[nasadowsk]

Oh yeah, the field rate too. I was taking a break on the chest press machine while I typed that up on my phone

Would switching the rates and all happen if a B&W station got a color network feed? Did this cause issues at the station if it was B&W only, or did the equipment not really care?

Did this cause issues with home viewers, or, as I suspect, the differences were so close the TV wouldn't care?

(I wasn't born until 1978, so by then, the only thing I knew about color was our old RCA 'lost' the color when I was about 4...)

[old_tv_nut]

The switch between monochrome and color frequencies would occur depending on the equipment that was originating the program. There were no digital frame synchronizers for a long time, and different sources would have different delays getting to a given station unless they were all fed through a central control point and gen-locked at that point.

There was gen-lock that would synchronize all a station's equipment and prevent vertical jumping from hot-switching between unsynchronized sources, even long-loop systems that would synchronize a station and remote pickup.

If a station was running its old black and white gear and getting occasional color feeds, I'm not sure if the mono gear (based on internal crystal control) could be pulled enough to match the 0.1% slower color feed - so probably a hot switch was used.

Maybe someone knows if any monochrome gear was ever re-crystalled to color frequency, or if they all just waited until new gear was installed.

Later, when essentially all stations were color, the three major networks installed atomic clocks and all stations on the network were constantly locked to the network.
One time at Zenith I had for some reason rigged up a TV that received sync from one network and picture from another. The atomic clocks were so close that the picture was stationary, just with an offset in vertical, horizontal, and color phase. During this time, the national bureau of standards published a paper on using the color burst for an accurate clock reference.

Later, when digital frame synchronizers were introduced, individual stations went back to their own master crystal clocks, so using the broadcast burst as an atomic-related standard was no longer possible; but at least there was nevermore a vertical jump from hot-switching. While crystal reference was less accurate than the atomic clocks, the frame store synchronizers removed problems with subcarrier delays in the station cable runs.

Today, all digital transmissions are synchronized to GPS frequency, which is even more stable than the older atomic clock references.
---
Besides the timing/frequency considerations, some stations had to replace their transmitter before broadcasting color, due to excessive distortion of the color subcarrier.

[kfbkfb]

https://www.freelists.org/post/opend...-to-NTSC-Color

https://www.freelists.org/post/opend...-to-NTSC-Color

https://www.freelists.org/post/opend...ate-BW-Network

In this case, it might be an NBC/ABC affiliate in 1961 (before the 1st ABC color broadcast), the NBC feed was 29.97fps NTSC Color, the ABC feed was 30.00fps B&W.
The local station would do a "glitch take" switching between the NBC and ABC content.


Kirk Bayne

[old_tv_nut]

Those references state that the FCC specified subcarrier frequency as
3579545.00 Hz, which is true, but they also specified a tolerance of +/- 10 Hz.
For drop frame timing to match the wall clock precisely, the ratio of color to monochrome is exactly 1000/1001. This makes the monochrome vertical rate exactly 60 and the subcarrier frequency equal 3579545 + 5/11 Hz (3579545.454545...).

[Mi40793]

B&W television in the early days at local tv stations were commonly locked locked to 60Hz power line to synchronize the 16mm film projector shutter to the blanking time in video, due to wide use of iconoscopes for film. Into 1960’s. With vidicons for film this issue went away and the projector could be non synchronous with the film shutter.

You could also lock the mono sync gen to its internal 94.5 khz crystal (3 x 31.5) if you had unstable 60hz, or no synchronous film projectors.

Lock to 60 Hz also helped with hum bars floating through due to various sources of 60 hz ac in video, so that any ‘ hum bars’ were stationary on home receivers.

During any major network show coming in on ATT microwave, the station just switched to the net source and all home receivers tuned in would do a vertical roll since the network and local station vertical sync had random relationship. Coming from network back to the local station, the tv station operator would take ‘black’ momentarily first so all home receivers would roll in black then punch the local station source. Going from local to network that doesn’t work, so the switch always caused a roll at home. Crude, but that’s the way it was.

The big glitch and roll was due to the random vertical sync relationship between local station sync and network sync, NOT due to the minor change in sync rates. This happened with mono as well as color.

BTW, Frame sync’s weren’t until mid 70’s and cost a fortune due to the 3MB digital memory.

Genlock on a sync gen could be added (an option) but local station didn’t stations didn’t stay genlocked to network feed because any disturbance on the network would cause your whole station sync to ‘glitch’ sync. If you were recording locally on videotape this disturbance caused a huge glitch in the recording.

No one at a local station or at home even noticed if the network were sending color unless you had a color monitor, or noticed the color burst on a scope. The color H &V rate changes had no effect since by intent they were so close to mono standards.

[Mi40793]

All of the earliest mono broadcast sync generators could be modified to color including the RCA TG-1, circa 1946. RCA sold a option chassis with a 3.58 oven-controlled crystal that counted down and output a substitute 94.5 kHz (31.5 x3 offset from 3.58 rate. 3.58 divided by 455/2 gives 15734 khz) input for the internal TG-1 crystal. A second chassis, connected to the TG-1 generated a burst flag pulse required for the early color camera encoders (along with 3.58 Mhz SC). The 3.58 frequency was held within a few Hz, close enough. Atomic standards for 3.58 are not really necessary except for synchronizing network facilities in the days before frame syncs.

[Mi40793]

If a station went into genlock this typically also caused a roll on all viewing home receivers because the genlock resets the sync gen counters so it will coincide with incoming video sync. So genlock was done sparingly to super a staion ID for example, then drop out of genlock.

[Mi40793]

once a color film chain, for example, was added at a monochrome tv station, and the sync gen was changed to ‘color’ then the station forever just ran on color sync standards, 15734H and 29.97V. Only the color equipment output burst and chroma.
At home tv sets or to other tv station equipment there was zero noticeable difference except: if you had any hum in video equipment (common was weak mercury rectifiers in the transmitter) any hum bars now float slowly through the pix. and, heater to cathode leakage was a common problem with tubes.

[dtvmcdonald]

Originally Posted by Mi40793

No one at a local station or at home even noticed if the network were sending color unless you had a color monitor, or noticed the color burst on a scope. The color H &V rate changes had no effect since by intent they were so close to mono standards.

That's not true. On a good B&W set the difference is very obvious due to dot crawl on a color picture.

The difference is vertical rate is also obvious if the set has any hum in either
vertical sync or video.

[Mi40793]

Yes, I agree if the b&w set has resolution at 3.5 mhz, and if you know what to look for. But I’d say most folks didn’t notice. Initially chroma was tried at 3.89 MHz and other frequencies pre-1953, during the CPA trials, to minimize ability to see the chroma on the b&w sets of the day with really good video response.

I mentioned 60 Hz hum, both in the home receiver and at the tv station equipment. That mostly went away as transistors replaced tube equipment. All you could do is minimize it like in audio. I heard of ONE station that ran all video equipment filaments on DC! And yet we managed for many decades.

[Electronic M]

I notice dot crawl on my monochrome sets.

[nasadowsk]

I notice it too

[kfbkfb]

drifting OT, however...
https://www.youtube.com/watch?v=CjK-b4x9ZmQ
^^^
colour restoration technique using chroma dots


Has this technique been used for NTSC color recorded on B&W film?


Kirk Bayne