TX Techniques TX FM1, Техническое руководство

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TX-FM1/S/R/A Technical Manual www.txtechniques.co.uk
4.4 Audio Processor
Audio Processing parameters are set at board level during manufacture, using
carefully selected component values.
When listening off-air, slow variations in loudness may not be apparent due to the
action of the internal ALC, which amplifies or attenuates a wide range of audio levels,
to a single, optimum level.
A +4dB (maximum audio level) test tone will result in roughly 50% modulation level,
due to the nature of the internal Audio Processor, when monitoring off-air using a
Modulation Meter or Analyser. Normal program (music and/or speech) will show full
±75kHz deviation.
PPMs – including the TX-FM1/S/R/A's graphical one – exhibit a fast response
and slow decay, and may not appear to represent music or speech in a predictable
manner. Also remember that meters that indicate the output of a radio station may be
connected to an off-air monitor receiver and will not respond in the same way as the
one on the transmitter (which is ahead of any processing). The graphical PPM is the
sum of both Left and Right channels and is for indication only. It is not intended to be
an accurate measurement of the audio level entering the unit.
By nature, multi-band audio processors greatly amplify any distortion present on
certain elements of source material (a particular instrument, for example), even if it is
not evident when previewed without audio processing. Also, CD recordings mastered
at very high levels, often drive many CD players reproduction circuitry into 'clipping'
(distortion). If, after ensuring that audio levels are correctly set, objectionable
distortion is heard, try alternative programme material and/or source equipment,
before suspecting that the transmitter or its audio processor is at fault. Audio
processing may also accentuate the distortion effect of microphone Voice
Processors.
The addition of an audio limiter in the audio feed, before the TX-FM1/S/R/A,
with the onset of limiting set at slightly above +4dB, is recommended.
Additional (external) audio processing normally has little effect on the sound,
because the internal processing counteracts any external processing. However,
experimentation is encouraged, and ‘mix enhancement’ products with ‘stereo
widening’ often yield excellent results in conjunction with the TX-FM1/S/R/A’s internal
processor.
It is strongly advised that the audio applied to the TX-FM1/S/R/A has little or no 'EQ'
applied. The integral 5-band Audio Processor, acts like a 'dynamic graphic equaliser',
optimising the tonal quality of the programme source. It is likely that this will
counteract any external tone equalisation. The point at which any additional EQ
would become noticeable on the received transmission, would be under extreme
levels of applied EQ. This would cause unpleasant sound, degradation of the audio
and possibly even overmodulation if DC or square waves (ie. distorted or clipped)
audio is applied.