How to modulate a marker with a square wave

[Jon1967us]

In the book Guide to Color TV Servicing, published in 1970, it states that one way to assist diagnosing poor video response is with a 15.75khz square wave, and that there are 2 ways of doing so. The first is to feed the signal into the video amp, after the detector diode, adjusting the amplitude and DC offset to match the video signal that would normally be there. The second method is to use a square wave to modulate a marker generator, which is fed into the antenna terminals. By analysis of the effect on the square wave form, one is supposed to determine if there is excessive or insufficient response of low/high video response. Im not understanding the second method here. How would it be modulated? By frequency or amplitude? How would this be helpful?

[Jon1967us]

https://ibb.co/Z1cvrC5
https://ibb.co/L0D9x6z

[Notimetolooz]

I have never heard of this method being used before.
Seems kind of kludgy to me.
The author is suggesting to use a marker generator as a TV channel RF source.
Video is always an AM modulation of the RF in the NTSC system.
However this suggested method does not provide the proper sync signals to the TV. Also any shortcomings of the modulation of the RF generator will also show up in the video amp output. Maybe he happened to find a generator that did a good job at modulation.
I would think that a good video pattern generator, which is intended to be used for this sort of things, would be a much better way to make this test.
Also in 1970 affordable digital electronics was just getting good enough to generate high quality video test signals, so perhaps the author's method made more sense.

[Electronic M]

The proper way to assess the video response with a test pattern generator is to get a pattern generator that has a multiburst test pattern. The other way is using a standard sweep marker generator.

[old_tv_nut]

This was simply using a "marker generator" that had modulation capability. The TV would hopefully sync horizontally on the square wave. Today, a MUCH better way is to use a pattern generator and modulator, or a DVD player with a test pattern DVD.

Multiburst is fine for detecting high video frequency problems, but a square wave (like a white square in the middle of a black background or vice-versa) is better for detecting the low-frequency smears. These smears can also be observed with the long horizontal black bars in the Indianhead test pattern.

Tektronix produced the most comprehensive book on video distortions, although it contains much that mainly applies to transmission gear rather than TVs. The section on linear distortions is useful to get an understanding of what distortions are possible and how to measure them. In your case, precise measurement is not needed, but recognition of the various types could help.
https://download.tek.com/document/25W-7049-4.pdf

[old_tv_nut]

https://en.wikipedia.org/wiki/Indian...ith_Labels.png

[Jon1967us]

The idea is that looking at the effect on a simple square waveform fed through the video amps can tell you a lot about the types of frequency response you're getting. A marker generator seems more useful for helping to pinpoint problems in the IF or Chroma sections, whereas this method can specifically be employed for the video section. The video section is where my smearing issue is suspected. I've fed composite signal into the 1st vid amp and there is smearing. Signal fed into the 2nd amp shows no smearing on screen. The tuner AGC line may be playing a part, but I thought I'd start with testing signals around the 1st and 2nd vid amps. Note, I am unable to get snow on the screen without an RF input signal. Again I haven't figured out if the tuner has anything to do with my positive and negative smearing issue.

It may be worth trying, or perhaps I could just stick with something that can do a bar sweep, like a Sencore VA-48 and analyze the responses that way.

An Indian pattern was certainly available at the time of the article I referenced, but I don't think the second generation pattern generators were out yet, or at least not common.