Phonovision

The Discoveries

What we knew before

Prior to the work described here, all we knew about the discs was what had been published. The previous attempts to replay the discs only showed that Phonovision was poor and couldn't produce recognisable pictures. Indeed the late Ben Clapp , Baird's first engineer, told me that he thought what we now know as the 'Stookie Bill' recording looked to him like a cabbage! Even the 'Miss Pounsford' recording describes on the label that she was smoking a cigarette. Maybe she did smoke but not on this recording!

What we know now

It is surprising what we can find out from studying and correcting these recordings. Here is a summary of the findings:

The video format is 30 lines per picture with each line scanned vertically, bottom to top, and each picture scanned horizontally, left to right. (NB the Baird 'standard' had right to left scanning)
There were no sync pulses in the video (unlike today). Providing a stable playback for these recordings was to be done by mechanically linking the display to the disc. This proved too difficult, with the attempts to provide this synchronisation being counter-productive for the following reasons:
- Baird had to drop his frame rate from 12.5 per second right down to 4 per second in order to record three frames per Phonovision disc revolution with a 78rpm disc recorder. (This removed the low frequencies thus distorting the grey levels)
- The 'Stookie Bill' disc is distorted by a speed variation indicating that there was a linkage between 'camera' and disc and that there was a problem with it.
- The 'Miss Pounsford' disc has a different type of distortion caused by mechanical resonance in the linkage. The nature of the distortion looks like the effect caused by using a universal joint or off-centre drive shaft.
The lighting in Baird's studio was unusual - almost like a single light shining down on the subjects. As 'Wally' (from 10th Jan 1928) moves forwards into the light he appears 'nose-first'.
The pictures show the distortion caused by the lines being scanned as arcs and not as straight lines. From measuring the amount of distortion, we can tell that:
- The Phonovision discs all used a single 30-lens-spiral Nipkow disc as a 'camera'. (Later, Baird moved over to mirror-drum scanning which had straight-line scanning)
- The aspect ratio was measured to be 7:3 (The Baird standard)
The Nipkow 'camera' disc had errors in positioning the lenses. We know which ones were in error and by how much and that the same 'camera' disc was used for all the Phonovision recordings.
Baird's video amplifiers were prone to burst into oscillation.

Remarks on this 'Dig'

To find out all this information just by analysing the signal from the discs was a major surprise. But this needs to be seen in context:

To have a recorded television signal from the 1920's is a wonder in itself.
To see these restored pictures today as they would have appeared then is of immense importance for television history. (As one Granada researcher pointed out to me, these pictures rank with those of Armstrong's first steps on the moon.)
To be able to confirm certain aspects of Baird's endeavours is a added bonus.
To have an insight into how Baird tried to develop this invention goes beyond what is written in the history books.
To have an understanding of the difficulties he encountered in trying to bring this invention into practicality almost puts you 'there' in the laboratory with him.

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Last updated by Don McLean on 14th April 1998