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
- 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.
Other Phonovision Pages
What we have Learned, | Phonovision in Print, | The Phonovision Discs, | The Recovered
Images, | The First TV Recording Studio, | Further Reading
Main Index, | The World's FIRST TV
Recordings, | Early Television History, | The Earliest Recording of Broadcast TV: Silvatone 1933, | The First Recording to be Sold - Major Radiovision 1934 | The "Marcus Games" discs
Back to Main Page
All material in this page is copyright ©DFMcLean 1998 except where
Last updated by Don McLean on 14th April 1998