Video Resolutions And The Kell Factor Myth

The Myth Surrounding Video Resolutions

Many websites erroneously give misleading information on video resolution. For instance, if you go to Wikipedia, it will give you a “list” of Horizontal resolutions for various formats when comparing Betamax to VHS or Digital8; something called the Kell Factor gives them their reasoning.

What is the Kell Factor?

Three engineers working on a new display device for RCA developed the Kell Factor in 1933. According to PC Magazine, when it comes to video, the Kell Factor is a “subjective number of lines of resolution that can be visually perceived in a video display system.¹” Now then that is very interesting, since many “professional” websites will just go to Wikipedia and just copy these figures without telling you that that is just a subjective resolution that is not based on any scientific data. It is very similar to when the weather forecaster says that the Humidex will make it feel like 45 degrees Celsius, even though the actual scientific temperature will only be 27 degrees Celsius; it means absolutely nothing, just as the Kell factor, scientifically, means absolutely nothing in terms of video. Even most textbooks avoid using these “subjective” resolution factors when talking about video.

Even when you hear that NTSC and PAL/SECAM video can send and record, respectively, 525 and 625 lines of resolution, this is mere speculation. As I live in Canada we used to receive our TV signals over the air in the NTSC format, and for cable companies that still offer analog service, NTSC was and is still the way that analog was transmitted and delivered. Many cable installers and broadcast engineers would tell me that by the time the signal got to your TV from the station/cable company, you had lost about half your signal and were really only getting between 290 and 325 lines of resolution. Digital satellite, cable and over-the-air-broadcasts use, at least here in Canada, use the ATSC format which digitally encodes the video and gives you a better picture; other countries use other digital transmission formats that also allow for better quality video than NTSC/PAL/SECAM ever did.

So what is the resolution of Betamax, VHS, Mini-DV, etc.?

When you want to know the resolution of any given video format you need to look at the bandwidth for both the luminance (black and white) and the chrominance (color) channels! For NTSC broadcast the luminance bandwidth is 4.2 MHz, while the chrominance bandwidth is transmitted at 3.58 MHz on a subcarrier frequency so that it fits within the 6MHz allotted for NTSC transmission (this does not take into account the audio channels that are also modulated on another subcarrier); this is known as Heterodyning or “color-under”. For resolution you don’t need to worry all that much, since, especially NTSC, chrominance was hastily added to the signal without thinking about how it would affect the signal, and thus it tends to ‘muddy’ up the picture; for resolution the luminance channel is the most important.

Videotape recording required further Heterodyning, so for most video tapes the chroma information was down-converted to the KHz bandwidth. As a result, plus most machines were set to playback at the lowest possible level, most people saw an image that looked worst than broadcast. The majority of analog consumer formats (VHS, Betamax, S-VHS) recorded the signal onto the tape in a composite format where the chroma was phase shifted to a different frequency allowing both channels to be stored in the one channel. For Betamax, if you were recording on a regular Betamax deck, your luminance bandwidth would range between 3.5 and 4.8 MHz, with chroma being stored at 688 kHz; depending on your recording speed (BI, BII, BIII) the slowest speed (BIII) used the smallest amount of storage (3.5 MHz). For Super Betamax the luminance was increased to a range between 4.4 and 5.6 MHz, with Extended Definition Betamax that range was from 6.8 to 9.3MHz, while maintaining the 688 kHz chroma channel. When compared to VHS (3.4 – 4.4 MHz luminance; 629 kHz chroma) or even its “improved” version, S-VHS (5.4 – 7.0 MHz; 629 kHz Chroma), Betamax was clearly the winner. Nothing was able to top ED-Betamax’ performance until DVD came along with a luminance of 13.5 MHz and a chroma of 6.75 MHz.

1982 saw the release of Betacam, a derivative of the Betamax format, for the professional world. So while the Betacam format used a

large tape for editing and a small tape that was the same size as a Betamax tape for use in camera's, Betacam players could not

playback Betamax tapes. Betamax recorded the signal in a composite signal, while Betacam recorded the signal in a component

format where the luminance (Y) was given it's own channel, while the chroma channels (UV) were recorded in their own channel using a

Compressed Time Division Multiplex (CTDM). CTDM allows for the U and V chroma signals to be frequency modulated to provide

better resolution and it cuts down the amount of "crosstalk" between the channels drastically. Although, even with this, when compared

to ED Betamax, the luminance for Betacam is lower for the non-SP Betacam (4.4 - 6.4MHZ) while Betacam SP, which came out in

1985, was only lower to ED by 0.2 MHz (6.8 - 8.8MHz) in terms of its highest peak. But, unlike ED Betamax that relied on the

Betamax's 688kHz chroma storage, the non-SP Betacam could store it's color between 4.0 and 5.7 MHz, while Betacam SP could

store it at a range of 5.6 - 7.3 MHz² (that's better than S-VHS's luminance resolution by a range of 0.2 - 0.3MHz).

References

1. http://www.pcmag.com/encyclopedia/term/57542/kell-factor "Kell Factor". Accessed August 19, 2013

2. http://betacam.palsite.com/format.html "Betacam Format". Accessed August 19, 2013