An army of amateur astronomers is eagerly awaiting the collision of Comet Shoemaker-Levy 9 with Jupiter. But not all of them will be peering through optical telescopes. Some will be listening to the changes in radio static from the giant planet.

Signals at what are known as HF frequencies – between about 500 kilohertz and 40 megahertz – were first picked up from Jupiter in 1955. They are thought to be generated by electrons breaking free of the vast tube of magnetic flux that connects Jupiter and its moon Io. Because the process is intermittent, the HF radiation is sporadic.

However, a good short-wave radio receiver, equipped with a directional loop (see Figure) or dipole antenna, is perfectly capable of receiving these signals. They are spread over a broad frequency band, so no sophisticated filtering is required. Jovian HF signals have even been detected on a 35-year-old domestic valve radio. The ‘sound’ of Jupiter has been described as the rushing of waves on a beach or, more prosaically, a rapid modulation of white noise.

However, the Earth’s ionosphere can create problems. HF emissions from Jupiter are strongest at around 21 megahertz. Under normal conditions this frequency penetrates the atmosphere. But, if solar activity has been high, the upper F layer of the ionosphere (otherwise known as the Appleton layer) will block out higher frequencies than normal, and completely mask any incoming signals at 21 megahertz.

The only alternative would be to monitor at a higher frequency. Jupiter also emits radio signals at the higher VHF/UHF frequencies between about 100 megahertz and 1 gigahertz. These signals come from synchrotron radiation emitted by electrons spiralling within the planet’s magnetic field. It is more consistent than the HF signal, and not usually blocked by the ionosphere. However, these frequencies suffer severe attenuation as they travel, so a high-gain steerable antenna to concentrate the radio waves and a receiver with a low-noise amplifier are needed to pick them up. Such specialised equipment is used by amateur radio operators to bounce signals off the Moon, and they may provide useful data on the impact.

What will actually be heard is a matter of some conjecture. It would be nice to hear a cataclysmic burst of radio noise at the moment the comet fragments strike Jupiter. Ultimately, though, it might be long-term changes in Jupiter’s radio emissions that are more revealing.

Possible repercussions after the collision include large fluctuations in the radio signal, twists in the polarisation of the radio waves, which would be detectable only at VHF and higher, and radio blackouts caused by the cometary dust mopping up the charged particles that generate radio waves.

Bodies coordinating radio observations, such as the US-based Society of Amateur Radio Astronomers, are keen for people to start monitoring as soon as possible. It is important to establish a record of Jupiter’s normal radio emissions before any possible effects of the collision emerge. All the signals that are monitored, from amateur listeners as well and professional radio astronomers, will doubtless be closely analysed worldwide.

In Britain, Geoffrey Grayer will coordinate reception reports via the Radio Society of Great Britain at Lambda House, Cranborne Road, Potters Bar, Herts.

The Tokyo-based company Ikegami and Avid Technology of Iver in Buckinghamshire
intend to have a prototype of a digital broadcast-quality camcorder ready
by the first quarter of 1995 and to be selling commercial versions by the
end of 1995. They announced their plans at the National Association of Broadcasters
exhibition in Las Vegas last month.

Conventional lightweight broadcast cameras consist of a main camera
section, behind which is mounted a miniature videotape recorder that stores
data in analogue form. The recorder works on the helical-scan principle:
magnetic tape passes longitudinally around an angled head that is spinning
at high speed. This gives the tape speeds essential for recording the wide-bandwidth
video signals needed for broadcast-quality pictures.

Ikegami’s design will replace this section with a single detachable
unit containing magnetic disc drives with enough capacity to capture up
to 20 minutes of digitally encoded video signals. This is enough time for
news inserts, or even separate ‘takes’ in outdoor drama productions. Professional
camcorders now use Betacam tapes costing about £20, which can hold
about 20 minutes’ recording. These cannot be reused more than a handful
of times before the picture quality degrades too far for it to be acceptable
for broadcasting.

Conventional methods for coding pictures in digital form would produce
too much data to be stored on even a large disc drive. To overcome this
problem, Ikegami intends to use a data compression system devised by Avid,
known as AVR26. This would provide a compression ratio of roughly 10 to
1 and would allow about six minutes of audio and video to be stored on a
1-gigabyte disc drive. Three such drives would be coupled together to form
a single store.

One of the problems with analogue video is that it restricts the amount
of editing that can be done. Every time titles or special effects are added
to a sequence it has to be re-recorded onto another tape, causing a slight
drop in the picture quality for each edit as tape noise builds up and bandwidth
is lost. Sometimes a maximum of only two or three generations is all that
is possible, especially if the tape has been reused.

Avid already offers digital editing suites for TV production in which
the analogue signals are converted to digital format, which can be edited
over and over again without losing quality. With a digital camcorder and
digital editing suite, there would be no need for the data ever to be decoded
or decompressed except for monitoring and final transmission.

Digital camcorders will only gain the widest possible commercial acceptance
if their storage modules are compatible with other cameras. Avid says it
has been approached by other interested parties that want to use its system
within their own products, but declines to name them.

The two companies do, however, have a number of obstacles to overcome.
One of the first is to find high capacity disc drives that are small and
light enough to be fitted into a camcorder. At present Avid get its discs
from the California-based companies Hewlett-Packard and Seagate, but it
says no decision has been made about a supplier for the new camera. The
disc drives will have to withstand the hard knocks and gyroscopic forces
encountered in everyday use. The power consumption may also be greater than
for a helical tape deck, so the camera would require heavier batteries.

Though digital camcorders may cost more than the conventional equivalent,
the companies argue that this will be offset by lower long-term costs. The
present generation of camcorders relies on tiny and complex moving parts,
with video-head assemblies which wear out and are easily damaged. Disc drives
could be more reliable, and unlike videotape, be used repeatedly.