Vertical interval signals on 405

[ppppenguin]

T= 1/(2*BW) so with bandwidth taken as 5MHz for a 625 system T=100ns. Hence half amplitude width of a 2T pulse is 200ns. Eaxctly what Terry said.

For 405 the vision BW is usually taken as 2.5 to 3MHz. The nominal 405 BW from an Aurora would be 5 * (405/625) = 3.24MHz but this is a bit high really seeing as the sound carrier spacing is 3.5MHz. Ignoring the infamous Kell factor, 625 with 5MHz BW is about 10% deficient in H resolution compared to V. At 3MHz BW 405 would be about 17% deficient in H compared to V.

These figures are reasonable, as the effective V resolution is smaller than the nominal. This is often called the Kell factor though I don't think that usage is strictly accurate.

[Terry]

It never occurred to me that 405 had VITS. I had thought of VITS as being more modern than that.

I can't put a precise date on it but the BBC started using it not long after I started work in 1960, so 61 - 62 at the latest.

I remember that the first time I saw them in all their glory was on a Bush TV53 where they appeared very prominently about half an inch from the top of the screen.

Replacing the field output valve restored things to normal.

I can't remember any precise details but there were a few sets around where the designers had continued to take full advantage of the entire blanking period a long time after everybody else was using much faster designs and these sets needed to be modified. I have a feeling that one was a particular PYE model.

[ppppenguin]

Since we are using 405 converters to feed a variety of sets from 1936 to the late 1960s that's a very good reason not to have VITS on their output. What practical use is it anyway? Unless you're monitoring a trasnsitter network or a substantial CATV installation they don't have any use at all. The Dulwich Museum system may well be the largest 405 distribution system now in existence. I can't see any use for VITS in that. If we had any awkward problems we could use full screen test signals, a lot easier than strobing out the VITS line(s) on a high performance waveform monitor.

[`FRANK.C]

I was curious as to how well Hedghog could produce a 2T pulse.
The results pictured below are not from Hedghog but a unit using a similar clocks, DAC and video amplifier.
The pulse is 343 nS as opposed to 333 nS just because that was convenient as it was a quick lash up. With some more work it could be adjusted to 333 nS.
A clock used for oversampling running at 34.992 MHz was used to generate the pulse.

Frank

[ppppenguin]

Looks nice and accurate. It's a quick way to prove that the DAC and output parts of a converter are working well.

Other useful test patterns are multiburst and ramps. Multiburst gives less information than a 2T pulse but can sometimes show anomalies that are hard to spot on 2T. Ramp tests are a sensitive check on the diferential linearity of the DAC. The ramp should be smooth. Integral linearity is less important in a video DAC. A slight overall curve is a minor problem but differential errors at major bit transitions can potentially be visible on real pictures. When you change (say) from 511 to 512 all 9 bits are switching at the same time, many DACs will show a visible glitch.

An especially evil test pattern is a single pixel of white against a black background. This will reveal the impulse response of the oversampling filter and any analogue filter after the DAC.

[Terry]

Some other components of the ITS waveform are a composite pulse and a staircase.

The composite pulse is a 10 or 20T raised cosine luminance pulse combined with a burst of subcarrier shaped using an identical filter. When these are combined, the positive peaks follow the raised cosine outline and the negative peaks produce a flat line across the base. It is used to check for and measure group delay - the time difference between delays to the luminance and chrominance components of the signal.

Viewed on a scope, movement of the chrominance component will cause the flat baseline to become a horizontal 'S', the height of which increases as the delay increases. A multiburst signal can be used to measure group delay across the entire spectrum by using pulses containing, say, 1, 2, 3, 4 & 4.43MHz signals (for 625 lines).

The staircase performs a similar function to the ramp and may or may not have subcarrier superimposed on it.

At first sight of a 5-step staircase, the obvious conclusion would be that you would just measure the relative heights of each step using a scope. Modern scopes are capable of very accurate waveform measurements but that wasn't the case in the heyday of ITS - such methods would not not give the accuracy demanded in a broadcast environment!

Instead, the waveform was differentiated - basically passed through a low value capacitor to remove the low frequency content and retain only the vertical transitions. These could then be amplified and the peaks viewed on the scope. In an ideal world they would all be the same height but it was now much easier to measure any differences.

[Terry]

Here are the ITS waveforms used by the IBA:



Note the error in the caption at the very bottom - Interval instead of Insertion!

This taken from the first article in the first issue of the IBA Technical Review which can be downloaded from here.

[ppppenguin]

The staircase with chroma was used to measure differential phase and gain.

[valvekits]

I have no idea how differential phase and gain errors occur, or even manifest themselves.

In mitigation, it’s many years since I worked on colour tv but I thought that swinging burst and an automatic colour control took care of anything we repair engineers would have needed to worry about?

[ppppenguin]

Indeed from the POV of a TV repairer diff gain and phase are really only of academic interest. Absolute (as well as differential) phase errors can cause big colour problems in NTSC but PAL converts these into saturation errors. The swinging aspect of the burst is merely to ensure that the PAL sequence is correct. ACC looks after colour saturation, on the assumption the burst and chroma are equally affected by amplitude errors. This is true for NTSC as well as PAL.

Diff gain and phase were very much problems for the broadcasters. VTRs, transmitters, and to some extent the national distribution system were the big offenders. As digits and YUV components (analogue or digital) found their way into broadcasting so diff phase and gain problems went away for all but the actual transmitters.