Could someone, please, explain how old CRT TVs reacted to input signal, specifically to VSYNC and HSYNC pulses?

Do these pulses control the electron beam? For instance, if signal would consist only of VSYNC pulses, would the beam stay at its initial position?

Or does the beam operates on its own and these are needed only for synchronisation of beam and signal, as their name suggests? If so, then, what is the need for HSYNC? Since, once the first frame is synchronised with signal after tune-in, the beam could operate with a constant frequency, matching the signal and therefore always stay in sync.


3 Answers 3


The vertical and horizontal oscillator circuits were analog and had only a small range of variation in frequency (maybe 10%), primarily to allow for drift and component variations. They were not crystal-controlled, and typically had pots to get the frequencies close (called vertical and horizontal hold, because if they didn’t lock in then the image would roll visually). That is what would happen if the sync signals were absent or too far off.

The purpose of the sync signals is not only to lock the frequencies but to lock the phase of the vertical and horizontal oscillators so that the the image would be displayed properly. So they are both required. You will find that even modern LCD panels that receive a video signal use HSYNC and VSYNC signals to route the video data to the correct pixels. In the case of a CRT television, the oscillators drove coils in the magnetic “deflection yoke” assembly that sat on the neck of the CRT. The slow vertical deflection circuitry was a bit like an audio amplifier, but the horizontal circuit had to operate around 15kHz (NTSC) so it used a “flyback” circuit that did double duty to generate the HV for the CRT in all but the very oldest TV sets. Once the beam is waggling around in synchronization with the transmitter, it’s only necessary to modulate the beam intensity with the video luminance signal to display an image properly.

Below, from here is what you would see if the horizontal sync signal was missing or the oscillator failed to lock. There is both a phase and a frequency mis-match.

enter image description here

Loss of vertical sync would result in the image rolling vertically up or down depending on the difference in frequencies.

  • 4
    \$\begingroup\$ I can remember when TVs had knobs labelled Vertical Hold and Horizontal Hold. Their purpose was to prevent rolling of the picture and I seem to remember using them fairly regularly - especially the vertical hold. \$\endgroup\$ Jun 29, 2023 at 22:06
  • 3
    \$\begingroup\$ Also, interlacing is dependent on the fact that HSYNC and VSYNC rates have a non-integer ratio (262.5 lines per field for NTSC, 312.5 lines per field for PAL). The phase relationship between HSYNC and VSYNC is what makes one field per frame be the top field and one field be the bottom. \$\endgroup\$
    – hobbs
    Jun 29, 2023 at 22:11
  • \$\begingroup\$ In the UK the V sync was locked to the 50 Hz national grid for years, to make set design easier, and I remember during power issues in the 1960s when the grid frequency dipped, the picture would kind of writhe so that the sides of the image looked S shaped. \$\endgroup\$ Jul 1, 2023 at 13:13

Your third paragraph is correct. In a typical CRT TV set, the horizontal and vertical circuits operate on their own, and depend on the incoming H and V sync pulses to stabilize the image.

The electron beam is moved by electric potentials between opposing plates at the output of the electron gun. The voltages come from two independent oscillator circuits that produce sawtooth waveforms. These are called the sweep circuits. For each osc, as the voltage slowly increases, the beam is moved (swept) across (left-right or top-bottom) the screen. At any instant in time, the beam location on the screen is determined by the instantaneous value of the outputs of both oscillators. The vertical oscillator controls its y position, and the horizontal oscillator controls the x position.

For a properly synchronized image, each oscillator is running slightly slower than the perfect frequency. The H and V sync pulses reset the outputs of the oscillators to their voltage values for the far left and top positions, respectively.

It is possible to produce accidentally a composite video signal that has the VSYNC component but no HSYNC component. Note that a TV camera also had H and V sweep oscillators. If the H sweep osc in a camera died, the resulting composite video signal could have the V sync pulses but not the H sync pulses. Very strange, and very rare. But once a camera made a correct video signal with all sync pulses, there was no point in the signal chain all the way to the transmitter where a single component failure could strip off the HSYNC pulses but leave the VSYNC pulses. I can think of one completely bizarre set of circumstances that could cause this, but (figurative) alarm bells would go off all over the station.

However, it was common for a TV receiver to have a dead vertical oscillator. The usual symptom was a single horizontal scan line across the center of the screen. That line was actually all 525 (US) scan lines piled on top of each other, and you could see brightness variations in the line as the video image changed.

Because the horizontal oscillator also drove the high voltage circuit necessary to produce the electron beam, the other situation (vertical sweep, but no horizontal sweep) was far less common.

Later vacuum tube receivers used what GE called "compactron" tubes. These were basically a tube version of an early IC - multiple independent tube functions in one glass envelope. If one of these had both the horizontal and vertical video drivers in it and it died, then you got a single bright dot in the center of the screen. The H oscillator was still running and producing high voltage, but neither sweep ramp was reaching the CRT.

  • \$\begingroup\$ "each oscillator is running slightly slower than the perfect frequency" - and with a proportionately higher peak amplitude, so that the portion of the cycle between blanking timings corresponds with the visible portion of the display surface. This is also why you get that leftward bending effect in the picture when HBLANK is lost, and a rolling picture when VBLANK is lost. The oscillators are free-running instead of being properly reset, leading to progressively greater offsets in the image across the frame(s). \$\endgroup\$
    – Polynomial
    Jun 29, 2023 at 14:58
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    \$\begingroup\$ Nice explanation. One nit. You said "The electron beam is moved by electric potentials between opposing plates at the output of the electron gun>" Consumer TV's used a magnetic yoke around the neck of the CRT to deflect the beam, so magnetic deflection. Oscilloscopes used the plates that you described to move the beam electrostatically. \$\endgroup\$
    – SteveSh
    Jun 29, 2023 at 14:59
  • \$\begingroup\$ How does the beam actually correct itself to HSYNC? Does it reset to the beginning of the line? Is there some positional threshold after which it’ll just ignore it? What if HSYNC comes while the beam is still at the end of the previous line? \$\endgroup\$
    – Scylurus
    Jun 29, 2023 at 16:17
  • \$\begingroup\$ Steve - correct. I phased out for a second and described how oscilloscopes and Tektronix precision X-Y displays work with electrostatic deflection. oops. \$\endgroup\$
    – AnalogKid
    Jun 29, 2023 at 16:56
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    \$\begingroup\$ "video signal could have the V sync pulses but not the H sync pulses. Very strange, and very rare" - actually, this was quite often used in (analog) satellite TV encryption (I vaguely remember perhaps Dutch channels being encrypted this way on Astra 19.2°E, though I am not quite sure about the V sync). And of course, there were a lot of DIY "descrambler" circuit designs floating around, adding the sync pulses. \$\endgroup\$ Jun 30, 2023 at 7:19

For a more brief answer -- effectively the pulse triggers a sweep. So it has to trigger each and every time. Which also keeps the timing consistent from line to line, and field to field.

I can't speak for literally all TVs of course, but typically (meaning, probably 99%+ of TV sets sold say from 1950 to 1980?), they used a free-running relaxation oscillator (typically a blocking oscillator) to set H and V sweeps.

The sync signal is added to the oscillator's feedback signal. When a sync pulse arrives, and if the oscillator was close to threshold already, it gets pushed over threshold, triggering a cycle. When this happens on the next cycle, and so on, the oscillator is said to be locked to the sync pulse.

If the sync pulse isn't timed correctly (arrives too soon or too late to trigger a cycle), or is completely absent, the oscillator doesn't lock, and continues to oscillate at its free-running frequency (or at some weird ratio) -- thus a picture is always displayed, though it might be skewed, or "rolling", or nothing but static.

As a general design principle, if there's no reason not to, it is better to leave a system running, than to try and shut it off when not otherwise needed. In this case, shutting off sweep when no signal is available, would shut off the CRT entirely -- no V sweep means no height to the picture, and no H sweep means no width to the picture, or high voltage to light the tube at all (high voltage was derived from the horizontal deflection system). A blank tube would give the user no indication that the set is operational. It would also cause a huge shift in performance when a signal is received again: the picture might take some seconds to stabilize in size, intensity and sync, as components return to normal operating conditions. So it's more beneficial overall, anyway, to leave it free-running like this. (Besides, turning it off would've required additional logic -- more components, more cost!)

Note that the oscillator's frequency can rise, not fall: sync pushes it closer to threshold, not away from it. (It's a bit of both really, but the low duty cycle of the sync pulse means it's mainly in the speed-up direction.) For those of us with keen hearing, the change in pitch was perceptible -- the 15kHz whine would drift down (and become somewhat erratic) when tuned to static, but stabilize to a pure tone when tuned in. The operation of a muted TV was evident from one or two rooms away (sometimes to the confusion or consternation of adults in the room who had lost that range of hearing!).

The other thing you need to know about relaxation oscillators: they aren't very stable. While there was one stable timing element used in color TVs (colorburst crystal), it was a matter of necessity; TVs are all about cost savings (indeed, from where we get the term "Muntzing"), and a relaxation oscillator is cheap as beans -- just one tube/transistor, a coil, and a few resistors and capacitors. The RC (or L/R) time constant has rather modest tolerances -- both in initial accuracy (10% tolerance components might be used!), and in overall stability (frequency variance probably > 0.1% during a vertical sweep?). There's no way such an oscillator will stay in sync at a perfect 262/263* lines/field. Maybe you could get away with sync every other line, or perhaps even every 10th line -- but certainly not once every field.

*Scan rate was actually a constant 262.5 lines. This has to do with how interlace was implemented. It doesn't need any special hardware in the TV set: the lines are simply drawn at whatever phase with respect to vertical sync. With an alternate ± half-line delay written into VSYNC, a full interlaced picture is drawn, no overlap, no gap!

There also may've been enough variation in program material, that a more inflexible (read: crystal controlled) sweep would've been impractical. For example, portable cameras would have to be at least as accurate; VCRs would need much greater precision in their drivetrain; early computer displays and game consoles would've needed much more precise frame rates; etc. It's a burden to many sources, without saving any bandwidth (nothing can (well, should) be drawn to the screen during retrace, why not use it for timing/framing signals instead?).

Incidentally, modern LCDs with analog input do have this issue, and tackle it well. That is, the issue of timing at a precision of one in hundreds, or thousands even. Consider a VGA signal at 1024x768: a thousand pixels must be drawn per line, plus the retrace period, for a total of about 1300 pixel clock cycles per HSYNC pulse. The LCD is a fundamentally digital system, and the RGB signals must be sampled by an ADC at exactly the pixel clock, to recover the data sent by the video card. Typically these monitors are programmed with tables of standard scan rates, and also provide a user adjustment to tweak pixel phase and frequency. When tweaking the phase, you can clearly see when pixels become fuzzy and indeterminate -- the ADC is sampling on an edge between colors.

The precision timing is done through a combination of crystal control (crystals are much cheaper now than they used to be!), PLL and NCO/DDS techniques, and carefully designed, high performance, RC (or perhaps even LC) oscillators embedded on-chip, which are controlled by the PLL, and onboard CPU, to lock onto the HSYNC signal with less than a pixel worth of timing error across the screen.

  • \$\begingroup\$ "...it is better to leave a system running, than to try and shut it off when not otherwise needed". I wish the newer systems did the same thing. Unfortunately, many modern TVs will blank the entire screen if the PLL loses lock for even one HSYNC period. NES and SNES consoles introduce a strange "short line" (only by ~200ns) every frame to hack-in better color decoding/encoding. But some brands of TV will remain in a blank screen indefinitely with these signals. Others will do as you described the older sets did, where there will be a smear/skew at the top of the screen as the PLL recovers. \$\endgroup\$
    – Ste Kulov
    Jun 29, 2023 at 19:45
  • \$\begingroup\$ Nice. I had forgotten that you could hear the horizontal frequency changing. \$\endgroup\$ Jun 29, 2023 at 21:16
  • \$\begingroup\$ I think you meant 262-263 (NTSC) or 312-313 (PAL), though some video games output slightly more or fewer lines than nominal. Also, every field in proper NTSC interlaced video contains 262 full scanlines and one half scanline. Fields that end with a half scanline alternate with fields that begin with a half-scanline, so the two half-scanlines in each complete frame occur consecutively. \$\endgroup\$
    – supercat
    Jun 30, 2023 at 19:47
  • \$\begingroup\$ @supercat Thanks! \$\endgroup\$ Jun 30, 2023 at 19:49
  • \$\begingroup\$ Without a horizontal oscillator running to generate HT at the anode, the CRT cathode overheats, shortening its life (sorry, I can't remember why). The IBM 5151 monochrome PC monitor horizontal oscillator did not free run and this was a problem if it wasn't plugged into the switched mains outlet on the PC and the user left the monitor powered on without the PC for extended periods. \$\endgroup\$
    – grahamj42
    Jun 30, 2023 at 22:02

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