Creek Audio T40 Getting Started - Page 6

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the Left and Right stereo signals to give partial mono over a 20dB range of signal strength. The effect of this is to

maintain an almost constant audio signal/noise ratio during the transition from full Mono to full Stereo. Even partial

Mono is still very pleasing, certainly much more listenable than full, but 20dB noisier, Stereo. When the decoder is

in the fully Mono condition the Stereo Indicator is extinguished, although the pilot tone oscillator is left running to

maintain stability. This ' Monoing' can happen very quickly and briefly during flutter conditions, effectively masking

this irritating interference.

The Blend Function is also controlled by the Signal/Noise monitor, so that in the presence of Adjacent Channel

Interference (birdies) a degree of blend is introduced regardless of signal strength, reducing the audibility of this

interference considerably.

The decoder' s oscillator operates at 76kHz, and is divided down to 38kHz to obtain the stereo sub-carrier, phase-

locked to the 19kHz pilot tone. This feature makes the T40 immune to birdie-like interference problems which

result from other transmitted signals such s for the German VWF/ARI traffic information system, which operates

on a 56kHz sub-carrier and is used in many European countries.

More about AGC and AFC

The T40 incorporates two additional features using its other signal monitors to enhance performance – Gated

AGC and Detuning-limited AFC.

In some areas there are one or two very powerful local transmitters which can prevent reception of weaker

stations adjacent to them in the FM band by dominating the front-end AGC. Normally the AGC signals are derived

from the first IF amplifiers, which look at a10.7MHz wide ' window' onto the band. Obviously, one very strong

station within this wide window will cause the AGC to reduce the sensitivity of the front end, preventing it from

detecting any weaker signals in the same window. In the T40 an extra monitor signal is obtained from the second

IF circuits, which look through a window only 300kHz wide (150kHz in Narrow mode).

If this second signal indicates that the AGC is reducing front-end gain due to a strong station outside the second

IF window, and therefore not of interest, the AGC circuits are turned off, or ' gates' . This allows reception of any

weaker transmitters being masked by the presence of one dominant station elsewhere in the band.

Normal AFC control can also sometimes be self-defeating, since the AFC system must be a compromise between

holding on firmly to the tuned station, and allowing the tuner to be deliberately moved to another one when

wished. The two extremes of this trade-off either allow the tuner to drift away from the desired station too easily,

or alternatively hang on to it permanently, preventing selection of a different station without first disabling the

(' grim death' ) AFC.

In the T40 the AFC is itself controlled by the detuning monitor, so that we can achieve very firm AFC once the

desired station is tuned, but while actually tuning along the band there is no AFC action at all. The prevents the

tuner from ' flipping' from one strong station to the next as it is tuned, missing out any weaker signals in between.

This Detuning-limited AFC allows the T40 to eliminate the compromise and use both extremes when

appropriate, ensuring both easy tuning and then very stable locking to the required station.

Frequency Display

The green, digital frequency display indicates the frequency to which the tuner is actually tuned and is calibrated

in MHz to a resolution of 100kHz.

Although the display cannot therefore show the correct frequency for one or two European broadcast stations

which do not transmit on exact multiples of 100kHz (e.g. 95.125MHz), the tuning system in your T40 will ensure

that the tuner locks accurately to the centre of any channel to which it is tuned.

Finally if you do have any queries about the operation of the T40 which are not covered in this Handbook, or you

wish to clarify something that is, then please contact your dealer, Creek Audio Systems or our overseas agents.

B. FRONT PANEL CONTROLS

Wide/Narrow

This switch changes the bandwidth of the IF amplifiers. In simple terms this means that the tuner is

selecting a narrower ' window' of the FM band to extract signals. As discussed in the Aerials,

Signals and Noises section, we need a window of about 300kHz wide to transmit and receive

15kHz of audio with adequately high fidelity, and a 300kHz window is used by the T40 with this

switch in the normal ' WIDE' position.

In normal conditions this bandwidth is perfect, because FM transmitters are cunningly placed within

the band (by international agreement) so that they are at least 300kHz apart in any particular area.

This assumes that the normal range of a VHF transmitter is restricted to somewhat less than

100Km. However in occasional freak weather conditions (called a ' lift' ) the propagation of VHF