Does 32 kHz or 3.58 MHz mean anything to you? part 2

These two frequencies played major roles in electronics; one is still viable, while the other is entirely obsolete. Find Part 1 here. 

Color TV was developed in the 1950s and 1960s, largely by Radio Corporation of America (RCA) (see Related EE Word content), as a compatible upgrade to black and white (monochrome) TV. Adding color while maintaining monochrome performance and doing so within the same signal envelope and bandwidth was a major challenge for this all-analog design. Key to the enhancement was the use of a 3.58 MHz subcarrier.

What about 3.58 MHz?

There’s a major contrast between 32 kHz and 3.58 MHz. The rationale for using the former is clear, but the reason for using the latter is not. In the analog world, 3.58 MHz is a complicated story of signal processing, piggybacking, upward and downward compatibility, and much more.

RCA set out to create a color TV system compatible with the monochrome TVs that were already becoming commonplace in homes. EIA standard RS-170 defined monochrome TV signal levels, timing, and more.

The challenge was to develop an analog signal-encoding scheme that could meet some challenging objectives (“digital” circuits and functions as we know it was not available then): any color-encoded signal had to appear correctly as grayscale on existing and new monochrome TVs as it did under RS-170; inherently monochrome video content had to still seem that way on color TVs and without color “artifacts.” Upward and downward compatibility across both image modes and systems — color versus monochrome images and color versus monochrome TVs — had to be transparent to the user.

RCA’s massive effort was very successful. They developed a system adopted by the FCC and industry as the National Television Standards Committee (NTSC) video standard, despite competition from a rival electromechanical system based on synchronized spinning color filters. To achieve this, RCA spent hundreds of millions of dollars of their own money in dollars of those days to develop the system.

However, they did not just do this as an academic exercise. They did all the required scientific and engineering analysis, of course. They also created vital, leading-edge components such as the color image-capture vidicon and the complementary color-display cathode ray tube (CRT) with an internal shadow mask, built their factories, and fabricated almost every active and passive component needed in the TVs. To complete the total broadcast system, they also designed, built, and sold the studio equipment and transmitters required to support the color-TV signal chain from end to end.

It’s a sad yet revealing note of history that this company, which was a top-tier source of technology, components, and systems in consumer, military, space, and other markets in the middle and mid-late 20^th century, deteriorated and faded away to become just a marketing nameplate in the 21^st century.

So how did the RCA scientists and engineers implement the compatibility “trick”? They relied on a deep understanding of how the eye and brain perceive color, including the relationships among the primary colors, color space, luminance (brightness), which colors needed more detail (bandwidth) for acceptable images, and which could tolerate less and more. They looked at the frequency spectrum of the TV signal and found “gaps” that they could exploit to insert color-encoded information as a “color burst.”

The Wikipedia entry under that phrase is fairly straightforward and informative, yet only the tip of the technical iceberg: “The original black and white NTSC television standard specified a frame rate of 30 Hz and 525 lines per frame, or 15,750 lines per second. The audio was frequency modulated 4.5 MHz above the video signal. Because this was black and white, the video only contained luminance (brightness) information. Although all the space in between was occupied, the line-based nature of the video information meant that the luminance data was not spread uniformly across the frequency domain; it was concentrated at multiples of the line rate.”

They realized that if the needed chrominance (color) information was modulated on a carrier that was a half-integer multiple of the line rate, its signal peaks would fit neatly between the peaks of the luminance data. When they went through all the analog-signal spectrum calculations, the unique frequency for the color-burst subcarrier, which would be used for encoding color information, was derived from 315/88 = 3.579545… MHz (an irrational number).

A crystal and oscillator established this frequency in TV receivers and then used it to synchronize and decode the analog color-information signal, which was dropped into the gaps in the 6-MHz bandwidth TV signal timing.

The transmitter sent a burst of just nine cycles at this 3.58 MHz frequency within each video signal line. The TV receiver synchronized the phase of its own free-running 3.58 MHz oscillator to the phase/frequency of this burst signal to enable analog decoding of the color information. The 3.58 MHz signal was simply ignored for monochrome receivers, and it did not affect the grey-scale signal presentation (Figure 1 and Figure 2).

The frequency-domain representation was also complicated and showed the placement of the nominal 3.58 MHz chroma carrier for the color burst in the 6-MHz bandwidth (Figure 3). For reasons we won’t go into here, the video signal was amplitude-modulated onto the carrier while the audio signal was frequency-modulated.

Where does this leave us?

Both the 32 kHz and 3.58 MHz frequencies and their crystals have a life beyond the original application. The 32-kHz crystals are still used in various unrelated applications; they are widely available from dozens of crystal and oscillator vendors as standard, low-cost components with different packages, tolerance, and other specifications.

The follow-on effects of 3.58 MHz were also significant., Asl with 32 kHz, the high volume of use led to manufacturing advancements, which made these crystals into low-cost, widely available components. Non-TV applications soon leveraged them. Radio amateurs and others adopted the frequency and its subharmonics and harmonics for unrelated uses.

Some early “personal” computers from Radio Shack, Commodore, and others used fractional ratios of this frequency as their clock since the crystal was so cheap and available. The 10.7 MHz intermediate frequency (IF) of standard superheterodyne FM radio, which was also commercialized during the 1950s and 1960s, was derived from a tripling of this frequency.

While analog TV defined by NTSC is now officially opposite, having been superseded by digital TV in the early part of the 21^st century, there are still many nooks and crannies of present-day systems that were defined by the NTSC color standard and 3.58 MHz subcarrier.  Adding color to monochrome TVs while maintaining compatibility was a brilliant exercise spanning theory, component innovations, circuitry implementation, and large-scale production.

In the funeral oration in Shakespeare’s “Julius Caesar,” Marc Antony proclaims, “The evil that men do lives after them; the good is often interred with their bones.” The opposite is true for 32-kHz RTC crystal and especially for the 3.58-mHz color-burst scheme: the good they did lives long after them and is not buried, even if we don’t realize it.

Related EE Word content

RCA & Color TV: A dominant company and standard, both now gone – Part 1
RCA & Color TV: A dominant company and standard, both now gone – Part 2
Radio receiver architectures, Part 1—TRF and Superhet
Quartz crystals and oscillators, Part 1: Crystal basics
Quartz crystals and oscillators, Part 2: Advanced crystals
Real-time clock modules feature a wide temperature range, low power consumption
What’s the difference between a clock and a real-time clock?

External references

David E. Fisher & Marshall Jon Fisher, “Tube: The Invention of Television” (Sloan Technology Series)
Science Direct, “Color Subcarrier”
Wikipedia, “NTSC”
Wikipedia, “Color burst”
IEEE, “3.579545 MHz Can be More Than the Color Burst”
Hackaday, “Know Your Video Waveform”
Britannica, “Basic principles of compatible color: The NTSC system.”
Renesas, “IDT1337AG Real Time Clock Data Sheet”
ST Microelectronics, AN3060, “Application note: Applications guide for serial real-time clocks (RTCs).”
SIWARD Crystal Technology Co., “Why Are 32.768 kHz Crystals So Important?”