| 摘要 |
For the transmission of a number of channels of high quality audio signals via an inductive loop (11), a transmitter includes a digital sinewave pilot tone generator (7, 8, 9), a number of multipliers (4), phaselocked to the pilot tone, to generate sub-carrier signals which are mutually different integer multiples of the pilot tone frequency. The sub-carriers are modulated by the audio signals in respective balanced modulators (3), and the upper sidebands are suppressed (5). The lower sidebands are mixed (6) and are fed to the loop with the pilot tone. A receiver coupled to the loop regenerates the sub-carriers by receiving the pilot tone and multiplying the pilot tone frequency (18) by the same integers as in the transmitter. The resultant sub-carriers, which are also phase-locked to the pilot tone, are used to demodulate (19) the received lower sidebands. The resultant audio signals are fed to a mixer (21) with channel faders (108-112) and thence to a stereo amplifier (127, 128, 129) feeding a common output so that all audio channels can be received simultaneously at a transducer (23) such as stereo headphones, with any desired balance between the channels. Pre-emphasis is applied to the lower sidebands (5) at the transmitter, and the signals are de-emphasised (22) at the receiver. |
