Interlacing – the hidden story of 1920s video compression technology

By Paul Marshall

Pick-up any worthwhile book on the history of television and turn to the index to look up the word ‘interlace’ or ‘interlacing’. Look at the referenced pages and somewhere you will find an established ‘fact’ that Randall Ballard of RCA invented the ‘clever’ technique of interlacing in 1932. It doesn’t matter whether the book is American, British, German, French or Russian – Randall Ballard invented interlacing. Is this ‘fact’ completely sound though? As with so many issues in the history of the technological development of television the rights to precedence have become distorted over the years by manipulations of corporate image, tweaking for reasons of national pride or just plain acceptance of the status quo.

Interlacing is now viewed by most professionals as a curse – but usually a necessary curse for 1920×1080 HD formats. 1080i (interlaced) is now relatively commonplace but try finding 1080p (progressive) equipment and programming. It exists, but it’s not being received in British living rooms. The well-known problems of interlace inter-line twitter and the difficulties that it causes particularly with moving objects are well known.1 Video technology professionals do not need too much of an introduction about why interlace is still used today. It’s a versatile technique, a kind of video compression technology, squeezing more information down a transmission chain than is possible than with the ‘ideal’ progressive scanning. There is a point of note here though, the technical issues of today are different to when television images were produced by camera tubes and displayed by CRTs. The lag associated with camera tubes tended to mask some of the issues of inter-line twitter at the camera end and at the display end the CRT with its phosphor decay between fields effectively flashed up two discrete but decaying images. This is ideal for motion blur reduction, unlike the so-called zero-order hold displays of today that don’t decay as they are essentially ‘sample and hold’ devices.2 To most ‘traditional’ television engineers interlace is a flicker reduction technology that enables a given number of lines to be displayed with an effective doubling of frame rate with no penalty in line speed and video bandwidth. Two half frames (fields) combine the odd and even lines time sequentially to ensure that the area of phosphor observed by the viewer is refreshed at twice the rate, thus avoiding flicker. This is not the case with zero order hold displays – there is no decay – so today it is simply about bandwidth and data rate reduction. In effect, a first pass video compression system.

Going back further in time, to beyond the ‘traditional’ television engineers of the mid 1930s to 1980s, interlacing was a much more ill-defined principle and Ballard was far from being the first to wonder about non-progressive scanning. In the electro-mechanical era of television, when Nipkow discs and mirror drums spun out a few dozen lines with frame periods of around 15 frames per second or less, the technology was still looking for a viable application. The idea of adding ‘pictures to wireless entertainment’ was taken up in the late 1920s when famous figures such as John Logie Baird in Great Britain and Charles Francis Jenkins in the United States of America, began television broadcasting over the air-waves. There was excitement about the images despite, to our eyes, severe technical limitations and restrictions. For a long time, certainly as early as the 1900s when the idea of television was still firmly theoretical, the need for more lines and more frames was fully appreciated. What was not clear, and this is very hard for us today to understand, is what kind of bandwidth would be needed for television. The idea of bandwidth for speech and music was just about understood but what of the strange television waves? Experimenters around the world began to understand that television needed a lot more space than audio, but how much more? The answer is that nobody really knew, even by the late 1920s. It needed more, a lot more for better pictures than those of Baird at 30 lines and 12.5 frames/second. Baird managed with a sound transmitter – just about – but the pictures flickered terribly and the low resolution was inherent. 

Unknown, or at least unappreciated by Baird, was the fact that he had already stumbled upon a method that could have doubled, or even tripled his picture quality at a stroke either in terms of lines per frame, or effective frames/second or a compromise between the two. All this could be achieved without adding extra bandwidth and with little extra system complication. Did anybody else pick up on this at the time? The answer is yes, and it wasn’t Ballard, at least not at first.

Ulises Armand Sanabria

Who? Or is that a ‘what’?  It’s a ‘who’ and the name is obscure to anybody but the most avid reader of television history. Arguably the most well-respected book on the international history of television is Television – an International History of the formative years by Prof. Russell Burns, but even he fails to mention Sanabria.3 It’s likewise for almost every other television history book. If mentioned at all he only warrants a couple of sentences. The conclusion has to be reached that he wasn’t very important and that he has no legacy that should interest us but the facts of Sanabria’s achievements actually put him alongside Baird and Jenkins as being at the forefront of electro-mechanical television development and deployment. There is only space here for a very brief résumé of his life and work, but his television systems of the late 1920s and 1930s exceeded both technically and commercially, those of his contemporary, Baird. Baird has had dozens, if not hundreds of books written about him but Sanabria’s story has yet to be told anywhere except in snippets from the web and as a chapter in a very rare privately published book.4

Sanabria was born in 1906 in Chicago to a Canadian mother and a Puerto Rican father. Little is known about his early life until the final year of High School where in a brief autobiographical note that he wrote 3 years before his death in 1966 he says:5

Hired by Hearst Newspapers to direct project to create television in six months during last year of high school because television inventions appealed to the publisher’s technical advisers.

This was in 1924, at the age of 17 or 18. Some details about his staff and photoelectric cell construction follow in various sources but then this dramatic claim for 1926 appears:6

First to produce television using interlaced scanning on January 26, 1926 – financed by Illinois Publishing & Printing Company. Demonstrated successful television to 200,000 people attending Chicago Radio Show from October 10th through 17th, 1926 at Chicago Radio World’s Fair, Chicago Coliseum.

Sanabria-triple-Nipkow-disc
Figure 1: Sanabria’s triple interlace Nipkow disc

This is a significant claim, not just because of the claimed (and documented) demonstration of television, but the mention of ‘interlaced scanning’. It’s also in the same date range as Baird’s significant early demonstrations. Sanabria’s interlace system is not like anything that we normally think of as interlace but the principle is identical. His system was a 3:1 scheme (rather than the virtually universal 2:1 of today). It was accomplished using three offset spirals on the sending and receiving Nipkow discs. It worked, and it worked well, with 45 line 15 frames per second images being transmitted. With 15 frames per second and a 3:1 interlace the effective frame rate was 45 fields per second yielding (for the time) a very steady image. Strangely, a patent for his interlaced scanning was not applied for by him until May 1931.

His other major innovation utilised the synchronised mains electricity grid system used in the mid west, then already locked to 60 c/s. By using synchronous motors both at the transmitter and receiver rock steady synchronisation was achieved requiring only an initial phasing by applying a brake to the disc until phase synchronisation was achieved. Jumping forward and glossing over much important further work, Sanabria formed Western Television Corporation and by 1932 had as many as 7 broadcasting stations all using his 45 line, 15 frame 3:1 interlaced system.

Under the brand name ‘Echophone’ receiving sets were made for the general public called ‘Visionettes’ and, like Baird’s ‘Televisors’, were relatively expensive for what they offered. The picture quality would quite likely have been somewhat better on one of Sanabria’s sets compared to those of Baird and potentially owners would have had a choice of ‘station’, unlike the BBC 30 line transmissions with just the one. Figure 2 shows a ‘Visionette’ – note the ship’s wheel in the middle used for the manual phasing operation.

Western-Televison-display
Figure 2: Western Televsion Corp Visionette domestic television display

Sanabria and his associates went on to develop all-electronic television systems and his companies became a significant pre and post World War II CRT manufacturer. After the war mass production of television sets became the main focus of Sanabria’s operations but alongside this was a television engineering school. Ultimately his businesses collapsed in the mid 1950s because of tax irregularities. Sanabria always maintained that technically and commercially the businesses remained sound but the truth of that claim is something yet to be investigated.

The Sanabria claim to interlace

Undoubtedly Sanabria had invented a viable kind of interlacing by 1926, using it regularly by the late 1920s and patented in 1931. Our RCA man, Ballard, did not patent his system (based on a Nipkow disc based film scanner device) until July 1932. It wasn’t actually granted until 1939 which suggests that there were legal issues. Sanabria’s patent would likely have been a major obstacle but there was other prior art too that could have caused problems. I have already mentioned that Baird had had ideas about interlacing. He called his method ‘intercalated scanning’ patented in January 1925. His system was more about increasing resolution using a mechanical offset control applied to the Nipkow disc and it is highly unlikely to have worked. Other patents stretch back even further to that of Samuel Lavington Hart in 1914, William Samuel Stephenson & George William Walton in 1923 and Marius Latour in 1926. Examination of these patents reveals only tenuous links to interlacing as a technique as understood today, but another inventor, Russian Lev Sergeyevich Termen, much better known for his musical instrument, the Theremin, claims an interlaced scanning demonstration in 1926. This has yet to be substantiated.

On current information Sanabria would appear to be the true inventor of interlacing but then he spoils it a little himself with a very strange description of his invention in his patent:

I propose to eleminate (sic) this objectionable interference caused by the side-bands by interposing dissimilar wave forms so that no particular wave form will be repeated a sufficient number of times to overcome the inertia of the circuit having selective frequency characteristics, therefore, a signal is not permitted to build up to sufficient strength to be a factor within the range of practical perceptibility.

The meaning is far from clear, but it seems that he was worried about sidebands from his transmissions interfering with other users. It must be remembered that they were using the medium wave broadcast band initially before moving up into the 2 mc/s range under Federal Radio Commission orders. This suggests that bandwidth reduction was his aim, not flicker reduction. Whatever his intended use for the invention was it appears that Sanabria was there first – unless Terman’s claims can be substantiated.

Interlacing is still with us and likely to be so for a long time to come yet. Sanabria’s idea is one of the few surviving television technologies from the earliest of days and Ballard probably needs to at least share his crown with Sanabria, if not surrender it.

© Paul Marshall

  1. Wikipedia http://en.wikipedia.org/wiki/Interlace Accessed 30-05-09 This account of interlacing describes the technical method and problems well but misses or misinterprets most of the historical issues.
  2. Newer LED backlight LCDs and certain Plasmas do not exhibit this behaviour utilising black field insertion and some even add active predictive inter-fields.
  3. Burns R W  (1998)  Television – an International History of the formative years,  IEE
  4. Martinez, M. A. (1989) Chicago: Historia de Nuestra Comunidad Puertorriqueña  The History of Puerto Ricans in Chicago, Private.
  5. Ibid
  6. Ibid