Conversion from Megahertz to Kilohertz

Information about the Megahertz (MHz)

The megahertz is a unit of frequency equal to one million hertz. Its symbol is MHz. It covers the part of the radio spectrum that carries most broadcasting and short-range communication, and it was the unit in which computer speed was measured for two decades.

FM radio occupies 87.5 to 108 megahertz almost everywhere, a band chosen after the Second World War because it was high enough to allow the wide channels frequency modulation needs and low enough for transmitters and receivers to be built cheaply. Terrestrial television took the neighbouring bands, and the switch from analogue to digital broadcasting freed large parts of them for mobile telephony.

Aviation communication sits just above FM radio, from 118 to 137 megahertz, using amplitude modulation rather than the clearer frequency modulation. The choice is deliberate: when two aircraft transmit at once, amplitude modulation produces an audible heterodyne squeal that alerts everyone to the collision, whereas frequency modulation would simply capture the stronger signal and silently lose the weaker one.

Computer processors were rated in megahertz from the late 1970s until around 2000. The original IBM PC ran at 4.77 megahertz, and the megahertz race that followed ended when clock speeds stalled near four gigahertz because of heat, after which manufacturers competed on core count instead. Memory bus speeds and display timings are still commonly quoted in megahertz.

Medical ultrasound works between about 2 and 18 megahertz. Higher frequencies give finer resolution but penetrate less deeply, so an abdominal scan uses a lower frequency than an examination of a superficial structure, and the operator chooses the probe accordingly.

Radio-frequency identification, garage door remotes, wireless microphones, marine VHF and amateur radio all live in this range. So do the harmonics that make poorly shielded electronics interfere with each other, which is why regulatory testing concentrates on emissions in the tens and hundreds of megahertz.

Mobile telephony has made the unit a matter of money. Spectrum is auctioned in blocks measured in megahertz, and governments have raised sums in the tens of billions from bands at 700, 800, 900, 1800 and 2600 megahertz. Physics sets the value of each: lower frequencies travel further and penetrate buildings better, so the 700 and 800 bands cover rural areas cheaply, while the higher ones carry more data over shorter distances and suit cities. An operator's coverage map and its data speeds are therefore both consequences of which megahertz it managed to buy, which is why the auctions are followed as closely as any other national asset sale.

One megahertz equals one million hertz, 1000 kilohertz, or 0.001 gigahertz.

Information about the Kilohertz (kHz)

The kilohertz is a unit of frequency equal to 1000 hertz. Its symbol is kHz. It covers the upper part of human hearing and the lowest part of the radio spectrum, which makes it the meeting point of two very different technologies.

Audio lives here. The upper limit of human hearing is about 20 kilohertz in a young person, and the frequencies that carry speech intelligibility cluster between 1 and 4 kilohertz. Digital audio is sampled at 44.1 kilohertz for compact discs and 48 kilohertz for video work, rates chosen because the sampling theorem requires more than twice the highest frequency to be reproduced, with a margin for filtering.

Radio broadcasting began in this range. Long wave runs from 148 to 283 kilohertz and medium wave, the AM band, from 526 to 1606 kilohertz. These low frequencies propagate along the ground and reflect off the ionosphere at night, giving them ranges of hundreds or thousands of kilometres from a single transmitter, which is why they carried the first national broadcasting services and still serve maritime and aviation navigation beacons.

Submarine communication uses even lower frequencies because seawater absorbs radio energy, and only signals in the range of a few kilohertz and below penetrate more than a few metres. Naval very low frequency transmitters occupy enormous antenna arrays for this reason, and their data rates are correspondingly tiny.

Ultrasound for medical imaging sits far above the audible range, in megahertz, but industrial ultrasonic cleaning and animal deterrents work between 20 and 80 kilohertz, just above what people can hear. Bats echolocate in the same territory, from roughly 20 to well over 100 kilohertz.

Older computers had clock speeds in kilohertz. The processor in the original Apple II ran at just over one megahertz, but the earliest microprocessors and many microcontrollers still operate at a few hundred kilohertz where low power matters more than speed.

Digital audio put a specific number from this range into everyday use. The compact disc samples at 44.1 kilohertz, chosen because the sampling theorem requires more than twice the highest frequency to be reproduced, and twice 20 kilohertz leaves a margin for the filter that removes everything above it. Professional recording commonly uses 48 kilohertz to match video, and 96 or 192 for mastering. Telephony sits far lower, sampling at 8 kilohertz and so cutting off above about 3.4, which is why a voice on an ordinary phone line loses the sibilance that makes the letters s and f easy to tell apart.

One kilohertz equals 1000 hertz, 0.001 megahertz, or one millionth of a gigahertz.