C-QUAM
C-QUAM , English C ompatible QU adrature A mplitude M odulation , is a common in North America AM stereo method for the analog transmission of stereophonic signals broadcast transmitter in the medium wave range . It was developed in 1979 by Norman Parker and Francis Hilbert at Motorola and is one of the most widely used AM stereo processes in the USA and Canada in the medium wave range.
Procedure
Similar to other analog stereo transmission methods, C-QUAM is compatible with mono broadcast receivers. Conventional medium wave receivers with amplitude demodulation continue to receive a mono signal; a C-QUAM-compatible receiver is required to receive the stereo signal.
A stereo signal generally consists of two separate signals: the left (L) and right (R) audio channels. With C-QUAM, two auxiliary signals are generated in the transmitter by adding and subtracting:
The first signal (1 + L + R) is referred to as the sum signal; it represents the mono signal, which also ensures compatibility with mono radio receivers. The second signal (LR) is called the difference signal and is received by the C-QUAM radio equipment.
These two auxiliary signals are modulated by means of quadrature amplitude modulation (QAM) on the I component and the Q component of the carrier signal , which is phase-shifted by 90 ° . A double sideband signal Q is added to the monophonic AM signal I, the suppressed carrier of which is in quadrature to the AM carrier.
To ensure mono compatibility, the sum signal is provided with a constant offset ; in the sum signal this is expressed by the addend with +1 (this means the carrier), which also represents the highest occurring amplitude. In the adjacent modulation diagram, the addition shifts the permissible signal space into a circle with a center by +1. In mono receivers, the amplitude demodulation is primarily used to receive the sum signal (L + R). This portion causes a significant change in the amplitude; in the diagram opposite, this means that the signal vector shown in red moves vertically. - With QUAM, the difference signal (L − R) has only a slight effect on the amplitude; in the diagram this signal component corresponds to a horizontal curve and primarily leads to a change in the phase position φ of the carrier signal. To detect in the receiving device whether a C-QUAM signal is being received, a pilot tone with 25 Hz is added to the difference signal on the transmitter side . With C-QUAM, the difference signal has no influence on the carrier amplitude. It is fully compatible with it.
In the receiver, the low amplitude modulation is obtained by comparing the amplitude difference between the I and the envelope curve demodulation and regenerated with a voltage-controlled IF amplifier.
C-QUAM receivers are constructed similarly to receivers for modulation with suppressed carrier: The carrier frequency generated in the receiver and synchronized with the received carrier is used for demodulation by mixing the two auxiliary signals (synchronous demodulation). From the sum and difference signal obtained in this way, the original stereo signal can be generated again by adding and subtracting for the output on the receiving device:
The envelope demodulation is used for the sum signal and the synchronous demodulation is used for the difference signal.
Integrated circuits such as the MC13022A are available for the practical construction of C-QUAM receivers .
literature
- Motorola (Ed.): Introduction to the Motorola C-QUAM AM Stereo System . 1992 ( PDF [accessed on February 9, 2013] company publication).