At the beginning of the last century, there was a craze for invention in the American scientific community. After Edison invented the electric light and the phonograph, Forrest invented the triode, and Bell invented the telephone, Armstrong also joined the ranks of great inventions. He invented negative feedback, regenerative, super regenerative, and super heterodyne circuits in one go, laying the foundation for modern radio receivers. In 1933, he invented broadband frequency modulation and built a 50-kilowatt private experimental radio station. In April 1935, he simultaneously transmitted FM and AM signals at the Empire State Building in New York, and compared the reception in his own laboratory in Hattonfield, New Jersey. The result shows that the AM signal has been submerged by noise, but the FM signal is still very clear. The United States attaches great importance to FM technology. On New Year's Day 1941, 25 FM radio stations opened simultaneously across the United States, creating the world's first FM radio. In 1958, American engineer Reinard Kahn developed a stereo broadcasting system. In 1960, the Montreal Radio Station used Reinard Kahn's system for stereo FM broadcasting for the first time. FM stereo was developed rapidly in the mid-1960s. Beginning in the late 1970s, some countries began to study four-channel panoramic surround sound broadcasting, but due to demanding reception conditions, it was a short-lived failure.
my country's FM broadcasting was piloted in Beijing on New Year's Day in 1959. The frequency band is 64.5-73MHz. my country's FM stereo broadcasting started in Harbin in 1979, and FM broadcasting was popularized throughout the country in the mid-1980s. Since then, the Chinese ushered in a new era of radio broadcasting with high fidelity without interference from Tiandian noise.
Among the analog broadcasting formats such as AM long wave, medium wave, short wave, short wave single sideband, and FM, FM is the only medium that can provide high-fidelity broadcasting. The broadcasting content is mainly music, which is known as happy FM. FM not only brings happiness to our lives, it is also full of charm in broadcasting culture, technical exploration, sound quality evaluation, and equipment collection.
Sound quality comparable to CD
On September 23, 2007, at the WECWRA establishment press conference held in Tongji University, Mr. Liang from Desheng General Electric Company brought a TECSUN-1994 commemorative desktop radio to show the program of Shanghai 94.7MHz Classic Music Station. , Shanghai TV reporter Shen Ying exclaimed: "How does this radio sound like stereos!" This is the charm of FM broadcasting. In fact, 1994's sound quality has only reached the entry-level level, it can emit a comfortable, puffy sound, soft and sweet midrange, but lacks gorgeous high pitch. Although there are few components above 8000Hz in the vocal music spectrum, the components above 20KHz in the instrumental music spectrum are still very rich. Especially string music, such as violin and erhu, its high-order harmonics will always extend to the ultrasonic frequency band. Although the level of these trebles is very low, they just reflect the color of the music and play an icing on the cake. However, in the design of ordinary FM radios, due to the need to take into account the selectivity, the IF bandwidth is designed to be narrow, and in order to reduce the cost, the circuit is programmed, so the sound quality is greatly compromised.
The GE-Zenith pilot system is the world's unified FM broadcast system. Compared with the AM system, the frequency offset of 75KHz can improve the signal-to-noise ratio by 17 decibels; the pre-emphasis of 50 microseconds can improve by 10.18 decibels, which can improve a total of 27.18 decibels. . If the AM signal-to-noise ratio is calculated at 50 decibels, the signal-to-noise ratio of FM broadcasting can reach 77.18 decibels. FM broadcasting and analog TV broadcasting equipment has never stopped digital transformation, especially the stereo exciter has undergone matrix coding, time division switch coding, multi-level soft switching coding to the current software DSP coding, and the quality of the equipment at the transmitting end has been greatly improved. To improve. The radio's program sources range from open analog tapes, compact discs, analog cassette decks to today's DAT, CD, and hard disk. The source distortion is reduced from 3% to 0.001%, and the dynamic range is increased from 50 decibels to 90 decibels. The advancement of hardware and software at the transmitter has cleared the way for the realization of high fidelity.
On the receiver side, because the FM frequency is in the VHF band, the high frequency characteristics of the device and the distribution parameters of the circuit will affect the performance of the whole machine. The design and manufacturing cost is relatively high. It is impossible to make high-fidelity with only conventional technology and cheap components. Radio. Fortunately, the FM system has huge potential to be tapped. If you do not hesitate to cost, the use of adaptive transverse filter, variable parameter processing, digital frequency discrimination, peak sampling phase lock decoding and other technologies can design a very high index receiver. Twenty years ago, when I turned on the ST5555 tuner, I couldn't believe it was a radio circuit board. Even in today's era of highly developed microelectronics technology, it is still not possible to design a tuner with CD sound quality. FM technology is really unpredictable. If you compare it to a house where humans live, the advanced tuner is a five-star hotel, and the portable FM semiconductor is a small wooden house.
All the above-mentioned advancements have enabled FM broadcasting to reach the high-fidelity level of today from the medium facsimile thirty years ago. To say that FM radio is comparable to CD, many people would disagree. Indeed, after the sound is encoded and modulated at the transmitter, transmitted over the air, selected at the receiver, amplified and demodulated, processing errors and mixed noise will inevitably occur. It should be said that the sound quality of a well-designed FM receiver is very close to that of a CD. If you have the opportunity to listen to a tuner like MD-108 or L02-T, you can truly appreciate the original sound quality of today's FM stereo broadcast.
Radio source from tape to CD
Since Edison invented the world's first phonograph on August 15, 1877, recording technology has been changing. The broadcast quality of FM radio has been following in the footsteps of recording technology. The sound source of mono FM radio, which was born in the 1940s, started with wire recorders and rough-grain records. It is impossible to verify the quality of FM radio at that time. By the end of the 1950s, compact discs and reel tape recorders were basically able to faithfully record and restore the original sound. The sound quality of FM broadcasting was recognized and valued by people. The low-noise high-frequency electronic triode specially designed for FM receivers, Sharp cut-off pentodes and high transconductance power tubes began mass production, laying the foundation for the manufacture of high-quality receivers.
Stereo technology was introduced into FM broadcasting in the 1960s. Because the signal-to-noise ratio of weak signals was 21.7 decibels lower than that of mono, the stereo coverage was only one-third of that of mono. Hiss and hiss noises would be generated at the edge of signal coverage. Switching to mono can be eliminated, so it has not aroused people's praise. The noise floor of the main broadcasting equipment of the radio station, the tape player, makes the stereo broadcast quality worse. The stereo decoder was used as an optional component in high-end radios. In 1965, Dolby invented the Dolby A noise reduction technology, which suddenly reduced the tape noise by 20 decibels. FM stereo saw the dawn of high-fidelity.
In the 1970s, the FM station was equipped with a high-resolution stereo exciter, coupled with the application of noise reduction decks and compact discs, when live opera and symphony, it can make people feel immersive in front of the radio and locate the sound source. The advantages of a wide harmony field are widely recognized, and stereo is called stereophonic sound.
The advent of DAT digital recorders and CD discs in the 1980s revolutionized the recording quality, and FM broadcasting truly entered the era of high-fidelity. During this period, the broadcasting methods of radio stations gradually moved towards automation. The production and broadcasting of programs were carried out separately, and the broadcasting method was mainly based on recorded programs. DAT was once the main equipment of the FM station. After the program was edited, it was recorded on DAT. Since both DAT and CD record PCM signals, the sound quality can be guaranteed. There are also voice programs recorded on DAT, music programs are directly selected from the CD according to the program, and the automatic manipulator is used to change the disc. This method can guarantee the best sound quality, especially when using 24bit/192KHz SACD discs.
At the end of the 1990s, the automatic player appeared. After all the programs were made and edited on the computer, they were transmitted to the hard disk of the broadcasting department by high-speed Ethernet. In order to save space, the sound data was stored in a compression method. The sound quality of the next broadcast will be a little discounted. Because various audio code rate compression methods with large compression ratios are lossy, including MUSICAM and AAC used in DAB and FM HD audio broadcasting. When it comes to compressed audio in the audio enthusiast industry, everyone will sneer at it. I wonder how these hardcore enthusiasts will feel after listening to the digital compressed FM broadcast?
Stereo exciter from hardware to software
The hardware device that affects the quality of FM broadcasting at the transmitting end is a stereo exciter, which mainly completes the task of stereo encoding. There are two FM stereo broadcasting systems in the world: pilot system and polarization system. Our country adopts the pilot system. In the FM broadcasting specifications formulated by the FCC and Western developed countries, the stereo separation index should be greater than 30 decibels, the level difference corresponding to the main and sub channels is 0.3 decibels, and the phase difference is 3 degrees. Under the past conditions, it is quite difficult to achieve these indicators in an analog exciter. As long as the phase characteristics of a certain filter near 15 kHz are slightly shifted, once a level difference of 1.5 decibels or a phase of 20 degrees is generated If it is bad, the stereo sound will disappear. Therefore, the quality of the exciter determines the sound quality of the radio.
The early tube exciters were of matrix format, and the level and phase balance between the sum and difference signals had a great impact on the performance of the whole machine, and the technical indicators barely reached the broadcast-level requirements. All subsequent transistor drivers have been improved to a switching system. The signal channel of the neutral difference in the switching system is implied, and the factors that affect the separation are transferred to the phase of the subcarrier and pilot. If the subcarrier frequency is used as the sampling signal, the stereo separation can reach 35 decibels, which has exceeded the FCC specification. . Humans have the nature of pursuing perfection, and then use the third harmonic of the subcarrier frequency as auxiliary sampling, which can be superimposed into the MPX to cancel the third harmonic of the subcarrier in the composite signal, and the separation can be increased to 40 decibels. In 1980, the soft-switching technology invented by McMartin Company greatly improved the performance of the exciter. In the 14-level soft-switching encoder, the harmonic component below the 13th order in the composite signal was zero, and the separation was increased to 60 decibels. Various manufacturers later competed to increase the soft-switching frequency, 38-level soft-switching, and the separation of the exciter with a sampling frequency of 3.444MHz reached 70 decibels. At the beginning of this century, the processing speed of DSP has been able to cope with complex coding algorithms. A 2048-level soft switch, 77.824MHz sampling frequency DSP encoder suddenly increased the resolution to more than 90 decibels. Modern DSP stereo exciters are full systems, including analog pilot system and polarization system, digital DAB and HD Audio systems, and direct digital frequency synthesis (DDS). They are very convenient to use, and their cost is decreasing year by year.
In 1986, two engineers from CBS and NAB invented FMX, which added an orthogonal difference signal and used companding encoding to cover the stereo range of the FM station. It has been expanded by 3 times to reach the same transmission distance as mono, and the stereo separation is greatly improved. Another advantage of this system is that it is compatible with the ordinary FM system. Ordinary FM receivers can receive FMX signals, but the quadrature difference signal cannot be demodulated. The FMX receiver can stably receive stereo broadcasts even in the marginal areas of the radio wave coverage without switching to mono. FMX is attractive to both radio stations and listeners. The FCC in the United States does not enforce the FMX system, and local radio stations choose to implement it. Just as there is no AM stereo in our country, there is no FMX radio in our country.
Tuner-the emperor in the FM receiver
Tuner accurately means broadcast tuner, which is called radio head in Chinese. It is a high-end product in broadcast receivers, and generally only contains two bands of FM/AM. Of course, the most attractive is the FM band. Manufacturers also use whole-body solutions to make the FM band the best. The 1960s and 1980s were the most prosperous period of FM broadcasting. During this period, there was no Internet and no digital sound source. The typical home music center configuration was a tuner, a moving iron record player and a Dolby noise reduction deck. In the early 1960s, FM broadcasting was in the transition stage from mono to stereo. The tuner was born during this period. The circuit was composed of electronic tubes or transistor discrete components, and the sound quality was worse than vinyl and tape. In the 1970s, FM broadcasting had all realized stereo sound. Excellent stereo exciters greatly improved the broadcasting quality. Tuner design technology also made great progress. Excellent integrated circuits and solid-state filters (crystal sound meters and ceramics) were widely used. , So that the sound quality of FM stereo surpasses that of Dolby B cassettes and is close to vinyl records. The 1980s was the golden age of FM broadcasting. Radio stations generally used DAT and CD as sound sources. The microprocessor introduces the tuner, and the circuit design pursues innovation. Excellent tuners have appeared one after another, with a distortion of 0.003% for mono and 0.01% for stereo. The sound quality is better than that of a moving iron record player and smoother than the Dolby C deck. Dolby C does not sound as beautiful as FM due to its gasping effect. During this period, broadcast enthusiasts in Japan and Europe were enthusiastic about advanced tuners. Everyone was eagerly waiting for the announcement of the new model. After the new model was launched, they rushed to buy first, and then compared the performance and commented. Also happy. In the same period, "Radio Science", "Wireless and Experiment" and other magazines published many articles introducing the new technology of tuners. The temptation to enthusiasts is like a magnet magnet. After reading it, who can bear three or two days, Do not break the fast.
With the popularity of digital audio sources in the 1990s, CDs almost replaced all audio sources, vinyl and decks quickly withdrew from the stage of history, and FM tuners began to fall out of favor. With the start of digital broadcasting, the original FM/AM tuner evolved into the current DAB/FM tuner and HD Audio/FM tuner, and FM became a subsidiary status in the tuner.
The glorious history of FM broadcasting in the last century has left a rich legacy for tuner collectors. At that time, 65 electronic companies around the world produced more than 2,000 models of tuners, of which 18 were the most famous. See Table 1. Chinese broadcasters did not have the opportunity to access these noble equipment in the past, but today they can occasionally find them in the e-waste markets in Beijing, Shanghai and Guangzhou.
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