A Δ-Σ Digital Stereo Multiplexing–Demultiplexing System

Abstract

Frequency modulation (FM) stereo broadcasting technique is well understood in literature and practice and is regulated by federal law (FCC). This stereo broadcast is not limited to FM; the FCC has authorized some form of stereo AM broadcast as well. There are a number of books and references describing the FM stereo technique. In Fig. 7.1 a generic block diagram of a stereo FM system is shown [1]. Similar block diagrams of a stereo FM system can be found in many other references and on the Internet. Thus, following general notation in this figure, we will briefly describe the operation of the system and point out its disadvantages. The idea of stereo is to provide a sound wave-front which replicates the depth and realism that an individual experiences when listening. As can be seen in Fig. 7.1, the signal broadcast consists of two channels: left (L) and right (R). This signal can be derived from a microphone, tape, or CD player, or another source. In the case of a broadcast of speech or music, the frequency bandwidth is from 30 Hz to 15 kHz. With FM stereophonic broadcasting, voice or music channels are frequency division multiplexed onto a single FM carrier. The L and R audio channels are combined in analog adder networks to produce the (L − R) and (L + R) audio channels. As one can see, the (L + R) signal is used to modulate the carrier just as a non-stereo signal does. The (L − R) signal is shifted by a balance modulator (subcarrier frequency is 38 kHz) to produce a double sideband (DSB) suppressed carrier (SC) signal spanning 38 − 15 = 23 kHz and 38 + 15 = 53 kHz. This process of stereo multiplexing is known as MPX. For demodulation purposes (synchronization), a 19 kHz pilot signal is also transmitted. All three signals are combined and delivered to an FM radio frequency transmitter. Most FM transmitters now use an integrated circuit (IC). Motorola MC1376 IC is a complete FM modulator. Unfortunately, it requires several external analog components to make it operate (including two inductor (coil) components). Similarly, the XR-1310 stereo demodulator has a significant number of analog components. Thus, FM IC chips have to be buffered by a number of analog components, which it is not possible to integrate. There are several US Patents proposing different digital stereo methods, but they are not related to the digital stereo multiplexing–demultiplexing method we are proposing.