Early BBC NTSC Colour Tests

[Mr Hoover]

This is a very interesting historical BBC document,describing the results of the BBC colour tests in the mid-late 1950's.

Interestingly there was a channel 5 transmitter used at Kingswood Warren
so that some tests could be carried out in normal TV hours

I haven't read much of it yet but it's fascinating reading

http://www.bbc.co.uk/rd/pubs/archive...nograph_18.pdf

[Mr Hoover]

There's a lot more to read at

http://www.bbc.co.uk/rd/pubs/archive...hs_listing.pdf

[Andy Green]

Its interesting to note the effect of positive video modulation as opposed to negative video mod on the 'look' of the picture - according to their research , the BBC note that +ve AM gave less colour saturation and increased panchromacity (Panchromacity is, if I recall correctly from photgraphy is the rendering of different colours by different shades of grey in a black and white picture) while -ve gives the reverse effect.

Also interesting to note that the sound to video carrier frequency spacing should ideally be an exact multiple of the colour subcarrier frequency - and both oscillators should be derived from the same frequency source. I expect that with FM sound, this would have less effect - maybe Darryl (Tubesrule) could comment?

Both the above observations are due to deficiencies in real world receiver detectors and IF strips, I would imagine.

[Mr Hoover]

I remember years ago when we had a dual standard BW Radio Rentals set
that the 405 line pos modulation pictures always looked more "dynamic" than
the 625 line negative modulation ones which always seemed rather "flat"
looking,especially peak whites which would figure(peak white being maximum on pos modulation pictures).
French Uhf TV using 625 line positive mod vision also looked more dynamic than the UK 625 line signals.Then along came colour & I really forgot about this.

Here multisystem dual sideband Uhf modulators are avilable which include the French system so a test could be done-The only two "identical" sets to hand
are a Sony 990UB for 625 line negative & a Sony 990UM for 625 line
positive which might not give a fair test though.

Must put this on the "To Do" list !!

[Ray Cooper]

...sound to video carrier frequency spacing should ideally be an exact multiple of the colour subcarrier frequency - and both oscillators should be derived from the same frequency source. I expect that with FM sound, this would have less effect...

With A.M. sound, intermodulation between the sound carrier and the colour subcarrier will produce a pattern that is static, since the sound carrier is of constant frequency. So precision offsetting will give a noticeable improvement. With an F.M. sound carrier, the intermod pattern will also be frequency-modulated, so produces a less static pattern: offsetting will have less effect.

...Its interesting to note the effect of positive video modulation as opposed to negative video mod on the 'look' of the picture - according to their research , the BBC note that +ve AM gave less colour saturation and increased panchromacity (Panchromacity is, if I recall correctly from photgraphy is the rendering of different colours by different shades of grey in a black and white picture) while -ve gives the reverse effect...

Yes, it is interesting, that.

Use of an envelope detector for vestigial-sideband vision demodulation introduces an effect called, variously, 'quadrature distortion' or 'chrominance/luminance cross-modulation'. The effect of this is, on a -ve modulation system, that the luminance ('brightness') signal is depressed in amplitude in areas of the picture that contain high chrominance subcarrier - i.e., brightly-coloured areas. Presumably, the reverse effect would occur on a +ve modulation system. So a -ve mod signal would look slightly duller than a +ve mod one, if the two could be viewed side-by-side on otherwise identical monitors.

This quadrature effect disappears if you use a synchronous vision demodulator instead of an envelope one. Early colour TVs invariably used envelope detectors - synchronous, or even quasi-synchronous demodulators had to wait until the consumer integrated-circuit revolution had arrived to make them economically viable.

[murphyv310]

Originally Posted by Ray Cooper

Use of an envelope detector for vestigial-sideband vision demodulation introduces an effect called, variously, 'quadrature distortion' or 'chrominance/luminance cross-modulation'. The effect of this is, on a -ve modulation system, that the luminance ('brightness') signal is depressed in amplitude in areas of the picture that contain high chrominance subcarrier - i.e., brightly-coloured areas. Presumably, the reverse effect would occur on a +ve modulation system. So a -ve mod signal would look slightly duller than a +ve mod one, if the two could be viewed side-by-side on otherwise identical monitors.

This is very interesting, I remember very clearly the early BBC2 transmissions that were mono and noting that the whites were poor in contrast to 405.
It wasn't untill later and an apprentice station engineer that I could see the benefit of a syncronous detector. British Relay used two receivers for 625 UHF one a MR745 was conventional with an envelope detector and a MR746 with syncronous detection, the envelope detector was flatter with "something" missing from the whites, but examination of the two units, with the scope showed no visible difference.
Comparison now cannot be done as there are so many variables, the scource material may be compressed etc, but one thing is sure the converters (aurora) we have now will give a faithfull conversion of what is thrown at them. My philips generator will do positive and negative modulation and on my modded sony TV990 shows no difference between either, this is with program or test card.

Trevor

[ppppenguin]

The BBC report and Ray Cooper both talk about effects caused by detector non-linearity. With colour signals this causes intermodulation between luma and chroma. Effectively the luma is modified by rectified chroma and the chroma is distorted by luma, effectively differential gain.

Did some early CTVs have separate detectors for Y and C to reduce these effects?

[Andy Green]

Originally Posted by Mr Hoover

I remember years ago when we had a dual standard BW Radio Rentals set
that the 405 line pos modulation pictures always looked more "dynamic" than
the 625 line negative modulation ones which always seemed rather "flat"
looking,especially peak whites which would figure(peak white being maximum on pos modulation pictures).

I remember thinking the exact same , and surmising that it may, in some way be down to the sense of the modulation. Interesting to read the technical reason for this. With regards to envelop detectors vs synchronous detectors with VSB , the reason for the distortion heard under skywave fading on the AM broadcast bands is down to a similar mechanism - a synchronous detector sounds fine - the only trouble is if the modulation gets too close to 0% (pinch off) the synch demodulators can lose lock and produce 'popping noises. This might cause a problem , I would think, if a synchronous detector was used for +ve AM TV, such as system 'A', since the sync pulses are close to if not at 0% modulation.

[Ray Cooper]

... a synchronous detector sounds fine - the only trouble is if the modulation gets too close to 0% (pinch off) the synch demodulators can lose lock and produce 'popping noises. This might cause a problem , I would think, if a synchronous detector was used for +ve AM TV, such as system 'A', since the sync pulses are close to if not at 0% modulation.

A valid point. I don't think that synchronous demodulators were ever used on 405 - the advantages that they provide are fairly minimal for a monochrome signal. But they were very widely used for 625 colour: all BBC 'rebroadcast' main stations (those that get their programme feed off-air from another main station, rather than from a dedicated link) used a synchronous receiver for the RBL (re-broadcast link) receiver, with a plain envelope receiver for the RBS (re-broadcast standby).

Whether or not a synchronous demod would have difficulties with 0% carrier depends largely on how it was implemented. A 'quasi-synchronous' demod, which derives its demodulating local oscillator feed by heavily slicing the input RF around the zero-crossing points, would certainly have problems. But the BBC receivers were a bit of a cut above this, using a crystal-controlled L.O. phase-locked to a 'constant-amplitude' part of the waveform: either sync tips or blanking level. Such a device could be made to work on blanking-level of a +ve-mod system. The resulting complexity would probably place it out-of-court for consumer electronics, though.

Teetering dangerously close to being OT here, mind...

[FERNSEH]

In my Pye 405 CTV, set does the AGC job properly. The tuner, IF's and AGC circuits resemble those of the Pye VT17 series which were a range of high performance sets made in 1955.
I'm of the opinion that the Pye 405 CTV was the only model the company produced. The set was not offered for general sale so there was no point introducing any cabinet changes although there must have been some circuit changes during it's production run.Throughout the 1950s Pye continued to make hi-spec sets which were intended to be used in fringe areas. Of course they made cheapo models to compete with the low cost models offered by other firms.
As for those 1960s mono TVs. Let's be honest, most of them were junk.

DFWB.

[Mr Hoover]

If someone has a d/s largish screen set + an aurora converter it would
be instructive to feed the same video source into the aurora and
a Uhf modulator and note any difference.Or just compare an off air uhf
transmission with a modern set

Were there any d/s mono sets without mean level agc? Few of them had
black level clamping either which wouldn't help.

The radio rentals Baird set we had was used in weak and strong signal
areas & always looked grey on 625.It also had plenty of gain,remember seeing
a lot of German testcards on Uhf in the late 60's,my first DXTV reception.

I vaguely recall the early single standard 1970's sets giving a better picture.