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チューナー Creek Audio T40のPDF はじめにをオンラインで閲覧またはダウンロードできます。Creek Audio T40 8 ページ。 Fm radio
ページを印刷するThe control signals are monitored continuously, and are derived and perform control as follows:
Signal-to-Noise information is obtained by extracting a DC signal proportional to the non-audio content of the
received and demodulated signal. The signals present above 75kHz are a function of the noise content of the
demodulator output, while those below 10Hz are a function of inter-modulation and cross modulation products
between adjacent channels. The subsonic content is also used to trigger the blend circuits in the presence of
aircraft flutter, momentarily forcing the tuner to mono to reduce the irritation of this interference.
Signal Strength is extracted from the IF limiter, and is linearised to give control signal proportional to the RF at
the aerial socket over a range greater than 80dB. This control signal is also used to operate the Automatic Gain
Control (AGC) in the RF tuner head to prevent overloading in the presence of very strong input signals, and a
further control signal is derived to ' gate' (or switch) the AGC operation, described below.
The brightness of the frequency display is controlled by a derivative of the signal Strength monitor to give a clear
indication of relative signal strength to the user.
Detuning is a measure of the amount that the tuner is off-station. It is derived from the Automatic Frequency
Control (AFC) signals, with further processing to obtain a linear control voltage proportional to the magnitude of
the difference between the required station and the actual tuning point of the tuner. The Detuning monitor is also
used to disable the AFC action between stations, described in more detail below.
These various control signals are then combined to provide progressive control of the muting circuits and the
stereo decoder. The operation of the system as a whole is best understood by examining in detail what happens
as the tuner is tuned from an empty part of the band onto a strong station...
When the tuner is completely off-tune, and receiving no signals at all, then the Signal Strength monitor is at zero,
the detuning monitor is at zero (cannot be de-tuned from nothing) and the signal/noise monitor shows zero (no
signal – all noise). Combining thse signals on a ' voting' basis gives two out of the three voting for no signal worthy
of any attention, and the audio output is held firmly muted. The stereo decoder is also forced to mono, but as
there is no pilot tone signal for it to lock onto it holds itself in mono, anyway.
As we tune towards a station the Signal Strength monitor starts to rise, as does the Signal/Noise monitor (since
the supersonic content of the recovered audio falls rapidly as a station is approached). However the subsonic
content of the audio is now very high (due to the amplitude modulation products of the FM signal operating in the
non-linear ' skirt' region of the IF filters) and the Detuning monitor shows that we are strongly off-tune, and so the
audio is still held firmly muted by a two-against-one vote as before.
As we now tune more into the range of the IF filters, within 150kHz of the centre of the desired station (75kHz in
Narrow mode) the Signal Strength monitor reaches its maximum, and (if the signal is strong enough) operates
the AGC in the front-end. At this point the Stereo Decoder is released from mono, to allow it to lock onto a pilot
tone if one is present, thereby lighting the Stereo Indicator. The proportional Stereo/Mono Blend is set by the
(now stable) Signal Strength monitor to give the ' amount' of stereo available once the mute is finally released.
This is described in ore detail below.
If AFC is switched On it is also now released to take control of tuning, accelerating the tuner circuits rapidly into
the centre of the station frequency. At the exact centre of the desired station the Detuning monitor returns to zero,
and this un-mutes the audio output fully, and lights the Tuning indicator.
If the AFC is Off, then the muting is progressively released as the station centre is approached, lighting the
Tuning indicator at the exact centre frequency when the mute is fully open.
The monitor circuits continue to operate, so that if a burst of noise or interference is detected the mute will close
briefly to prevent these unwanted signals reaching the amplifier.
The effect of the progressive muting function is to make the tuning knob seem to operate as a ' clickless' rotary
switch. As the knob is turned through each station the tuner un-mutes at the point where the station is perfectly
tuned, lighting the Tuning Indicator. This ' switch' effect is much more pronounced with the AFC switched Off. With
AFC operating the T40 tunes rather like an AM radio – the station is apparently loudest at the point of perfect
tuning, and gets quieter as you tune away to either side of this point. This makes it easy for those users unfamiliar
with FM tuners to achieve the perfect tuning essential to obtain full FM performance. It is quite possible to tune a
station entirely by ear, although the Tuning Indicator lights exactly at the centre point so it may be a little silly not
to use it!
More about Mono/Stereo and Blend
As mentioned earlier, the effect of going from mono to stereo is to require 20dB more RF signal to achieve the
same audio signal-to-noise ratio. The T40 incorporates a progressive changeover stereo decoder, which is
controlled by the Signal Strength and Signal/Noise monitor signals. The decoder operates by mixing or blending