Creek Audio T40 Getting Started - Page 3
Browse online or download pdf Getting Started for Tuner Creek Audio T40. Creek Audio T40 8 pages. Fm radio
Print Pagean FM signal carrying our 15kHz of audio is dramatically greater than for AM, because the maximum deviation of
the carrier to either side of its normal frequency is proportional to the maximum amplitude of the audio being
transmitted, so the limitation on radio bandwidth is imposed not by the audio frequency range but by its dynamic
range. For good fidelity of reproduction, dynamic range Is an equally important factor to bandwidth, and our FM
system will need something like 300kHz of radio bandwidth to transmit our 15kHz audio signals, or equivalent to
nearly one third of the Medium Waveband. To use FM for good quality broadcasting it is therefore necessary to
transmit at frequencies high enough to give enough bandwidth for each channel, hence VHF.
Why, then, is FM used? Well, it has many inherent advantages for transmission of high quality music signals.
Most interference and static tends to be AM by nature, and so an FM receiver can be made to reject this
unwanted AM, and be relatively interference-free. The FM transmitter runs at constant power, and can be
designed to work more efficiently. It is possible to design an FM receiver which detects if it is off-tune, and applies
correction automatically to keep locked to the station being received. This is called Automatic Frequency Control
(AFC). Also F receivers exhibit a phenomenon called ' capture effect' . If two stations transmit on closely adjacent
frequencies, instead of the receiver detecting both together it will be ' captured' by the stronger signal, allowing
clean reception. The ration between the strengths of the two signals need not be very great for this effect to work,
and with the added benefit that VHS signals travel only short distances (the line of sight, more or less) this allows
many stations to share the same channels without interfering with one another. The most important benefit of FM
for Hi-Fi users is that of greatly improved Signal-To-Noise ratio which can be obtained. Again, without getting too
deeply into the technicalities, to transmit information (and speech or music are a form of information) there is a
possible trade-off between three parameters, being rate of information transfer, signal-to-noise, and transmission
bandwidth. In the case of broadcast radio, the rate of information relates to the received audio bandwidth – the
higher the audio frequency we wish to receive, the faster must the modulation take place at the transmitter. To
achieve this fast rate of information transfer with FM we have to use a great chunk of bandwidth, anyway which
means that we can achieve a great benefit in improvement of signal-to-noise performance.
With AM, the weaker the received signal, the weaker also is the recovered audio – eventually the signals just fade
away into nothingness. With FM, however, the recovered audio is independent (to a point) of the received carrier
strength. What happens is that the audio gets noisier with weak signals, eventually disappearing into the noise.
An AM receiver receiving no signals is silent. An FM set under the same conditions produces nothing but (lots of)
noise. Therefore if we make sure that the signal reaching our FM tuner is as big as possible, then we will ensure
that we are receiving as noise-free (and interference-free) a signal as possible, and therefore obtaining the Hi-est
possible Fi. That is why a good aerial system is vital to good Hi-Fi FM reception.
The importance is greatly increased by Stereo. Returning to our simple information theory for a moment, you can
probably see that a stereo signal must contain more information than a mono signal of the same strength. In fact
probably twice as much, since there are now two different signals where before there was only one. Since they
are occupying the same bit of transmission bandwidth as before, and we are now passing information through it at
a faster rate, something in our trade-off must get worse, and it does. In return for our increased information we
lose signal-to-noise performance – the recovered audio gets noisier.
In fact our surmise that two channels in the space of one will mean a factor of two worsening in noise
performance is wildly wrong. Because of the need to make stereo signals compatible with (receivable n) mono
receivers, the extra information to transmit stereo is sent in a clever way, modulated onto a sub-carrier above the
15kHz of mono audio. The combination of this, plus the fact that the mathematics of stereo versus bandwidth are
somewhat more complicated than our surmising would indicate, actually makes for a dramatic increase in noise,
of about 20dB or a factor of 10. In other words, to receive a stereo version of a mono signal requires ten times
more signal at the aerial socket of the tuner to get the same audible noise level. For stereo reception a good
aerial is not a luxury, it is an essential.
Since we have designed the T40 to appeal to those who do not have unlimited resources to spend on their Hi-Fi,
but nevertheless enjoy listening to the highest quality sound possible, we have incorporated several unusual
features to minimise the effects of weak signals. Even so, you will greatly improve your listening pleasure if you
have a good aerial to pull in the signals. Your dealer can advise you on what is necessary for your location, since
VHF reception is variable from place to place. There are also specialist companies who are experts in aerial
installation, and will advise and estimate fairly on your particular needs.
In general if you live within a few Kms of the transmitters then a simple indoor dipole will be fine. Out to about
25Km a 3 or 4-element loft aerial will also be perfectly adequate. Beyond this range it is impossible to generalise.
If you live on high ground, with nothing much between you and the transmitter, then an indoor aerial can give
good results from 80Kms away. Conversely a gasometer or similar large steel structure can completely mask a
transmitter only 3Kms away.
If you live near an airport, or under the path of a busy air-lane then you may experience ' aircraft flutter' which is a
rapid fluctuation in signal strength due to reflections of the radio signals from aircraft, and sounds like a fluttering
noise. In severe cases you may see the stereo indicator flashing as the signal varies. It may be possible to point
your aerial at a different transmitter in another direction to overcome this problem, but if there is only one source