Beginning of the last century, the American scientific community has emerged in the heat invention , following Thomas Edison invented the light bulb and the phonograph , Forrest invented the transistor, after Bell invented the telephone , Armstrong joined the ranks of the great inventions. He invented the negative feedback breath , regeneration, super-regenerative , superheterodyne circuit , laid the foundation for modern radio receivers. In 1933 he invented the wideband FM , and since the construction of private testing stations 50 kilowatts . April 1935 , he was simultaneously launched in the Empire State Building in New York FM signal and AM signals , in Springfield , New Jersey Hatton receives his laboratory for comparison . The results showed that the noise amplitude modulated signal has been flooded , and the FM signal is still very clear. The United States FM technology very seriously. 1941 New Year's Day , 25 FM radio stations throughout the United States opened simultaneously , creating the world's first FM radio . In 1958, the American engineer Laina De Kang developed a stereo broadcasting system , in 1960 , the first application of the Montreal radio station Laina De Kang stereo FM radio system . FM stereo mid- 1960s has been rapid development. From the late 1970s , some countries have begun to study the four -channel surround sound broadcasting panorama , but due to the demanding reception conditions , short-lived failed.
FM radio is our New Year's Day 1959 in Beijing began pilot , the band is 64.5 ~ 73MHz, our FM stereo broadcasting began in 1979 in Harbin , the mid-1980s the popularity of FM radio in the country . Since then, the Chinese people ushered in the day against electrical noise interference, and has a new era of radio fidelity .
In the amplitude long wave, medium wave , short wave , shortwave SSB , FM broadcast system in these simulations , FM is the only way to provide high-fidelity broadcast media , music-based broadcast content , known as Joy FM . FM will not only bring joy to our lives , in a broadcast culture, technology exploration, quality evaluation , equipment collection also full of charm.
CD sound quality worthy of comparable
September 23, 2007 , WECWRA held at Tongji University was established at a news conference , Desheng General Electric Company 's total beam brings a TECSUN-1994 Commemorative Edition desktop radio , Shanghai 94.7MHz when playing classical music station program , Shanghai Shen Ying television reporter exclaimed : "This is how radio sounds like the sound ! " This is the FM radio charm. In fact, the only sound to reach the 1994 entry level , it can emit a sense of comfort to listen to the sound Puff , soft and sweet tenor , but the lack of gorgeous treble. Although there is little vocal spectrum components above 8000Hz , but above 20KHz instrumental component spectrum is still rich. Especially the strings, such as the violin and erhu, which has high harmonics extend to ultrasound frequencies. Although these treble level is low , it reflects the music exactly the color of the icing on the cake plays a role in timbre . But in ordinary FM radio design due to both selective and designed to be narrower IF bandwidth , combined to reduce costs , the circuit is programmable , so the sound quality is greatly reduced .
GE-Zenith pilot system is the world's unified standard FM radio , compared with amplitude modulation , 75KHz frequency offset can improve the signal to noise ratio 17 db ; 50 microsecond pre-emphasis and can improve 10.18 db , a total improvement of 27.18 db . If the AM signal to noise ratio of 50 decibels , SNR FM radio can reach 77.18 db . FM radio and analog TV broadcasting equipment has not stopped the digital transformation , especially in stereo exciter experienced a matrix-encoded , time division switching encoding , multi-level soft switching coded into the current software DSP coding, quality of equipment transmitter has been greatly increased. Analog radio program source from the opening tapes, LPs , cassette deck to simulate today's DAT, CD and hard drives. Audio distortion is reduced from 3% to 0.001% , the dynamic range from 50 db to 90 db. Transmitter hardware and software to achieve progress in clearing the way for the high-fidelity .
At the receiver , since the distribution parameters in the VHF FM frequency band , high-frequency characteristics of the device and the circuit will affect the overall performance , design and manufacturing costs are relatively high, with only conventional techniques and inexpensive devices can not make high-fidelity radio. Fortunately, this standard FM has huge potential to be tapped , if at the cost of using adaptive transversal filter , variable parameter processing, digital frequency discriminator , peak sample lock- decoding technology to design a high index receiver. Twenty years ago , when I opened the ST5555 tuner, could not believe it was a radio circuit board. Even in today's era of highly developed microelectronics technology , to design a tuner with CD quality sound , still a few manifold can be achieved . FM technology is really unfathomable , if human habitation house to match , the tuner is a senior five-star hotel , a portable FM semiconductor is a log cabin .
All of these advances , the FM radio to reach today's high- fidelity medium fidelity from thirty years ago. FM radio to say comparable with CD, many people will disagree . Indeed , the sound through the transmitter coding and modulation , air transport , the receiver frequency selection, amplification and demodulation , error handling , and there will inevitably be entering into noise , it should be said FM receiver excellent sound design is very close to the CD. If you have the opportunity to personally listen to what MD-108 or L02-T tuner such , you can truly appreciate the original in today's FM stereo broadcast quality .
Broadcast audio from tape to disc
Since the world 's first phonograph since August 15, 1877 Edison , the recording technology has been changing. Broadcast quality FM radio recording technology has been keeping up with the pace of progress. Born in the forties of the last century monaural FM radio sound source is from coarse wire recorder and began LPs , difficult to trace when the quality of FM broadcasts. By the late fifties, LPs and basically opening a tape recorder to record and faithfully restore the original sound , FM radio sound quality is recognized and valued by the people , FM receiver designed specifically for low-noise high-frequency electronic transistor, sharp cutoff pentode power tubes and high transconductance start mass production for the manufacture of high -quality receivers foundation.
Sixties stereo FM radio technology is introduced , due to the weak signal to noise ratio 21.7 db lower than mono , stereo, mono coverage only one-third of the edge of the coverage area will produce hiss , hiss noise must switching to mono in order to eliminate , so there is no cause for praise. The noise floor of the main radio station broadcast the tape drive device more so stereo broadcast quality worse, stereo decoder has as an option in high-end radio components . In 1965, Dolby invented the Dolby A noise reduction technology , all of a sudden the tape noise is reduced by 20 decibels, high fidelity FM stereo see the dawn .
Seventies, the FM station is equipped with a high resolution stereo exciter , combined with noise reduction and LPs deck applications , while live opera and symphony , make people produce immersive feeling before radio, sound localization wide sound field advantage has been widely recognized , is called stereo stereo .
Eighties DAT digital recorders and CD discs , so recording quality undergone revolutionary change , FM radio truly entered the era of high definition . Means of radio broadcast this period gradually move towards automation, production and broadcast of the program is carried out separately , the way to record programs broadcast based. DAT FM station was once the main device , after a good editing program recorded on DAT, as are the PCM signal DAT and CD recording , the sound quality can be guaranteed. The program also has a voice recording on DAT, musical selections directly by the program , with the robot hand changer from the CD. This way ensures the best sound quality, in particular, can achieve the best sound quality when using 24bit/192KHz standard for SACD discs.
The late nineties, the player automatically appears , all the programs first produced and edited later on a computer hard drive with a high-speed Ethernet transmission to broadcast ministry , in order to save space for audio data stored using compression , this situation under broadcast sound quality will play a little discount. Because the current compression ratio of various audio bit rate compression is lossy , including DAB and FM HD audio broadcasts using MUSICAM and AAC. A mention in the audio sector fever compressed audio you will scoff, I do not know these hardcore enthusiasts listened digital compression after what feels like FM radio ?
Stereo exciter from hardware to software
On the transmit side effects of FM broadcast-quality hardware stereo exciter , it is mainly to complete stereo coding tasks. The pilot system and the polarization has made two FM stereo broadcast system in the world , the country is used in the pilot system . FCC and FM broadcasting norms in Western developed countries , the stereo separation index greater than 30 db , corresponding to the main and sub -channel level difference of 0.3 dB and a phase difference of 3 degrees. Under the conditions of the past , to achieve these targets in the analog exciter is quite difficult , as long as the phase characteristics of a filter in the vicinity of 15 kHz, slightly offset , once produced 1.5 db electrical phase difference of 20 degrees or poor, stereo sense of hearing will disappear left to do . Thus , the actuator determines the quality of broadcast -quality radio .
Early actuator tube is level and phase equilibrium matrix format , and the difference signal between a great impact on the overall performance , technical indicators barely broadcast requirements. After the transistor switch actuators are improvements became standard . Switches and standard deviation of the signal path is implied , the factors affecting the degree of separation is transferred to the sub-carrier and pilot phase , if we make a sampling signal subcarrier frequency , stereo separation can reach 35 decibels, has exceeded the FCC Rules . Human nature with perfection , then three times subcarrier frequency harmonics for sub-sampling , may be superimposed to the MPX composite signal cancellation subcarriers in three harmonics up to 40 db of separation . 1980 soft-switching technology makes Mike Martin invention greatly improve the performance of the actuator , the encoder 14 in the soft switching , the harmonic component in the composite signal 13 is zero or less , up to 60 db of separation . Later, various manufacturers competing first raise soft switching frequency, 38 soft switches , actuators separation 3.444MHz sampling frequency reached 70 decibels. The beginning of this century, the processing speed of the DSP has been able to cope with complex coding algorithm, a 2048 soft switch , 77.824MHz sampling frequency once the DSP encoder resolution up to 90 db or more. Modern DSP stereo exciter are all standard , including the simulation of the pilot system and the polarization system, DAB digital and HD Audio formats , direct digital frequency synthesis (DDS), easy to use, cost is declining.
In 1986, CBS (CBS) and the American Association of Broadcasters (NAB) two engineers invented FMX, it adds a differential quadrature signal coding method using pressure to expand the coverage of the stereo FM station expanded three times to reach the same single -channel transmission distance , and so greatly improved stereo separation . Another advantage of this system is standard and is compatible with common FM , FM receiver can receive the common FMX signal , but not being cross demodulated signal . FMX receiver in the marginal areas of radio coverage can be stably receiving a stereo broadcast , without switching to mono . FMX on radio stations and listeners are attractive , the U.S. FCC did not impose FMX system , implemented by the local radio station of their choice . AM stereo as our country does not like our country did not FMX station.
Tuner- FM receiver Emperor
Tuner exact meaning is broadcast tuner , the country known as radio head . It is a high-end broadcast receiver products generally contain only FM / AM two bands , the most attractive of course, is the FM band , manufacturers have resorted to body stops the FM band to do our best . The 1960s and '80s FM radio 's most prosperous period , this time there is no Internet, no digital audio configuration typical home music center is tuner and Dolby noise reduction iron phono cartridge . Early sixties, FM mono to stereo in the transition phase , the tuner is born in this period , circuit tube or transistor discrete components , the sound quality is worse than vinyl and tape . In the 1970s , the full realization of a stereo FM radio , stereo excitation enables excellent strides to improve the quality of broadcast tuner design technology has also made considerable progress , outstanding integrated circuits and solid filter ( SAW crystal and ceramics ) for large-scale use , the FM stereo cassette sound quality than Dolby B , close to vinyl records . Eighties was the golden age of FM radio is commonly used as DAT and CD sound source . The microprocessor introduced tuner circuit design pursuit of innovation. Excellent tuners have available, mono reached 0.003% distortion , stereo 0.01%. Sound quality than iron gramophones, smoother than Dolby C deck . Dolby C due to wheezing sounds no FM beautiful effect . During this period , Japan and Europe for advanced amateur radio tuner respected reached frenzy , we eagerly await the announcement of new aircraft models , listed rushed to buy the new machine , and then compare the performance and comment are not busy also music almost . There are many published introduce the same new technologies tuner magazine article , " Radio Science ", " wireless and experiment " and other times , the temptation for lovers like a magnet attracts iron , like , who can bear to read two or three days later , without breaking the fasting person .
Nineties the popularity of digital audio , CD replaced almost all of the audio , vinyl and decks quickly exit the stage of history , FM tuner began falling out of favor . The launch of digital broadcasting , the original FM / AM tuner evolved into the current DAB / FM tuner and HD Audio / FM tuner , FM reduced to a subordinate position in the tuner .
Glorious history of the last century for FM radio tuner collectors left a rich legacy , when the world has sixty-five electronics companies produced a total of two thousand tuner models , most notably the 18 kinds of tuning is shown in Table 1 . China 's radio enthusiasts in the past no access to these elite equipment , today was able to procure the occasional traces of them in Beijing , Shanghai and Guangzhou, the e-waste market .
Table 1: 18 kinds of history 's most famous tuner
Manufacturer Model Date
SAE SAE Mark Six 1971
Yamaha VT7000 1974
Sony ST5555 1974
Sherwood Micro/CPU100 1977
Revox B761 1978
Toshiba ST910 1978
McIntosh MR80 1980
Revox B261 1982
Kenwood L02-T 1982
Tandberg 3001A 1983
Accuphase T-106 1984
Naim NAT 01 1985
Revox B260S 1989
Meridian Model 504 1993
Fanfare FT-1 1993
Rotel RHT10 1993
Linn Kudos 1995
Marantz Dynalab MD-108 1996
Note : FTA-100 and MD-109 is a product of this century , it is not included .
Solve intermodulation spurious response starting from LNB
FM tuner has a metal shield with a high-frequency circuit , said tuner, it contains high-level radioactive , mixed , and tuning the oscillation circuit . LNB is in the forefront of signal processing , its quality directly determines the sensitivity of the receiver , intermodulation spurious response and other indicators. Since the sixties FM radio a few areas , the design is very simple tuner , dual tuner will be able to get good reception . Seven years of intensive FM stations in big cities , in order to improve the selectivity , the tuner is designed to more than even the tuning up of up to 13 even . Even after using the multi- structure really improves selectivity , but also increases the tracking error , the group delay characteristics deteriorate the sound quality deteriorates . At that time, there is no high-quality sound source , it did not significantly change in perceived sound quality . Eighties tuner into the ranks of high-fidelity equipment , became the first important indicator of quality , it is recognized that to improve the sound quality must first eliminate false response intermodulation caused LNB obligatory take on this responsibility. The number of spurious response generated on the number of stations , the radio station number is set n, the number of spurious response (n-1) n. At present China's coastal cities and eastern generally received more than 30 FM stations , then leave it up to 870 the number of responses , we can see how serious the problem is . Therefore, a city in the FM radio frequency setting time, will be carefully calculated , so that the receiver band fall to minimize the number of false response . Spurious response caused an increase in the radio can be received on the surface , but when tuned to the frequency response will be accompanied false cries , cries , cries and chirp , chirp , chirp sounds.
Since the mixer is to rely on non-linear characteristics of the device to achieve, while the non-linearity is the root cause intermodulation , spurious response and therefore in principle impossible to completely eliminate the superheterodyne receiver , so excellent linearity , wide dynamic range devices become a tool to improve the performance of the tuner . From intermodulation and cross modulation index, the worst bipolar transistors , JFET slightly better , MOS FET is better, GaAs FET best . Since GaAs single crystal very easily broken , manufacturing difficulties , the price is expensive. Depletion -type double-gate silicon MOS transistor equivalent common source - common-gate amplifier series , its large dynamic range , low Miller capacitance , good stability , linearity is better than six balanced analog multiplier tube , a high-level radioactive and mixed the ideal device .
How to use the tuner in the end even better ? Just from selective consideration , even the number , the better ; But linearized group delay characteristics , improve the sound quality way , even the number the better. In order to take into account the quality and selectivity , select 4 to 5 even better. The next question is a tuning variable capacitor device with air or varactor ? The mid-seventies previous tuners all use air variable capacitor , the first since the 1974 frequency synthesizer tuner ST-910 available, various manufacturers have generic . Japan is the largest country in the production of the tuner world , was discontinued in 1983, Alpine air than even the last one , from the tuner to the variable capacitance a stop . The insertion loss and capacitance - frequency characteristic view, significantly better than the air variable capacitor varactor diode. In order to improve the Q value of the varactor diode can be made of the two back to back varactor twin tube form , close to the performance of variable air capacitors . 5 pairs of varactor tuning system performance even with the air four fairly , using varactor biggest advantage is to achieve digital tuner and multi-point adjustment system , get rid of the trouble of manual tuning .
Multipath signal interference is the culprit pop sound
When we receive FM radio , in addition to the transmitting antenna can receive direct access to foreign direct wave receiver also will also receive the reflected wave from the mountains , buildings and ground reflection formation. When using an indoor antenna from ghosting on the TV can directly feel the harm of the reflected wave . However, ghosting when receiving FM broadcasts are pop , pop sound and hiss , hiss form appears . When you move the position and orientation of the antenna of the radio , the radio sound mixed with intermittent pop , pop sound and hiss , hiss , this is the effect of multipath interference . FM multipath interference received the most harmful , and difficult to eliminate . Because this standard itself does not have anti-multipath capability. Because of serious harm , FM radio is not suitable for mobile reception .
In the fixed receiving conditions , multipath delay and amplitude of the signal is fixed , the position and the rotation direction of the antenna, the multipath interference can always find a small dot . But to eliminate multipath interference cars and air planes caused only by the strong point of the antenna to work. Experience abroad FM enthusiasts are erected 4 5 element Yagi antenna array , can get 18 degrees around the main lobe , set multipath signal absorption mosaic wall in the direction of the ferrite antenna sidelobes , weaken the intensity of the reflected wave , can significantly reduce the nonlinear distortion caused by multipath , to obtain excellent quality. However, this antenna is expensive , the cost is too high listening to the radio , only a handful of Shaoyou will do it.
Adaptive transversal filter is an effective weapon to eliminate multipath interference from the circuit , the last lab test results are encouraging . In the tall buildings of the city, only a rod antenna can get excellent sound quality, even under the conditions as long as the moving speed of not more than 60 kilometers, can get a good reception. Due to the complexity of such a filter structure , the tuning process requires high-speed amplitude and time delay judgment processor real multipath signal , automatically switches to the best offset node cost is relatively expensive , has not been realized in the consumer electronics , cast regret generations broadcast lovers. Today, software-defined radio to address this chronic problem provides a simple and inexpensive means of C code describes an improved anti- multipath transversal filter clock frequency of the DSP in real-time detection of the reflected signal 700MHz microsecond , and automatically selects the appropriate delay sessions and attenuation coefficient , fully offset multipath signals . Unfortunately, this technique missed opportunities late in FM tuner applications , if you Amoy to a fine tuner, frame an outdoor directional antenna is resistant to multipath interference and improve the quality of the most simple and effective way.
IF amplifier is the source of distortion
The place is the core of the FM receiver sensitivity, signal to noise ratio , capture ratio, distortion , and selective other indicators are directly related to the place of performance related , FM put the application in new devices and new technologies of the most concentrated areas , tuning the technology has been used as follows :
1 ) Ultra- linear solid filter : the place had been used in the LC in the week, quartz crystal, ceramic and SAW four kinds multimode filter . LC in the week is the oldest and classic devices, from 4 to 6 circuits can be designed to combine Butterworth or Gaussian amplitude-frequency characteristics . In order to improve the selectivity of early multi Butterworth type , due to the group delay characteristics of poor sound quality and later popular attention in the group delay characteristics of a good machine Gaussian . Crystal filter has the best shape factor , but the group delay characteristics are poor . Ceramic filters small size, low price, early product group delay characteristics is poor , then the products have been greatly improved , became the mainstream of the IF filter . The disadvantage is that the center frequency varies large , you need to select the pairing . SAW filter amplitude-frequency characteristic and phase -frequency characteristics can be separately designed , group delay characteristics can be done well, but there are side-lobe corresponding . In order to take into account the selectivity and distortion, tuners generally use a variety of filters used in combination. For example, a state with a narrow-band crystal and ceramic filters priority to ensure selectivity, the general state of ceramic filters and SAW filters both sound quality and selectivity, LC filters guarantee the state of broadband quality and capture ratio.
2 ) the frequency of negative feedback and variable parameters in place : the idea of the frequency of negative feedback is to reduce the deviation to narrow the width of the distribution FM sideband waves , you can take advantage of a narrow band group delay characteristics of the ceramic filter center frequency of the most flat straight section of the curve , the minimum distortion . And make absolutely sideband filter through , do the whole spectrum transmission . After the high-frequency signal to noise ratio will decline to reduce offset , so after positive feedback filter frequency offset and then returned to 75KHz. The technology first appeared on Onkyo 's T-727 tuner, only a 6- dB frequency of negative feedback , distortion reached 0.1%. After , Kenwood On this basis, the company invented the non- spectrum technology to offset compressed to almost zero, the application of this technology in the history of the famous L-02T tuner, distortion of the machine down to 0.003%. Negative feedback is used to change the frequency offset parameters to improve linearity , but also can improve the signal to noise ratio by changing the offset method . Because SNR is directly proportional to the frequency deviation of FM waves , with a simple multiplier can exponentially increase the frequency offset . Offset each doubling SNR will increase by 6 dB. If 5 octave frequency offset can be increased to 375 KHz, signal to noise ratio can be increased by 30 dB. Let 75 KHz offset when the SNR is 65 db , after five octave is 95 db , and a CD of the same indicators . After the increase of the frequency discriminator offset the linear range of requirements also increased , thus the multiplier factor for not more than five -fold. Another parameter that can be changed is the relative frequency deviation , it can be changed to improve the sensitivity of the frequency discriminator . It is achieved by double-conversion , reducing the IF frequency increases the relative deviation . Wide linear frequency discriminator tend to have lower sensitivity , this method can improve the frequency discriminator output amplitude.
3 ) signal : after changing the frequency deviation of FM -IF The down- Limiting later became sparse ranging pulses , with a simple digital circuit can be converted into a pulse width modulation (PWM) signal , and which CD the same as in the one-bit digital amplifier quantized signal , but the signal is not modulated audio, but MPX signal. If using digital frequency discriminator , the IF signal will be carried out this transformation. In software radio , 10.7MHz IF sampled directly into the ADC with DSP processing , past the place , frequency and decode the new technology can be used software algorithm.
Table 2: tuner design points and distortion distribution
LNB frequency discriminator in place Decoder
Design features intermodulation spurious response selectivity and distortion bandwidth and linear separation
Distortion Distribution ( % ) 580 105
The key is frequency discriminator linearity and bandwidth
FM receiver frequency discriminator is the second largest source of distortion , seen from Table 2, the frequency discriminator impact on the sound quality is higher than the tuner and decoder. In a tuner , amplifier and discriminator in common determines its performance, so extra attention by the manufacturers . In order to compete in the market, the level in the history of ever used 11 kinds of frequency discriminator , which is the ratio of the frequency discriminator , phase discriminator , the product phase frequency detector , PLL frequency discriminator , phase tracking frequency discriminator , pulse counting frequency discriminator , delay line frequency discriminator , differential discriminator , PWM frequency discriminator , digital parametric frequency discriminator and DSP frequency discriminator . Manufacturers and designers of their own hype frequency discriminator advantages , some circuits are touted hype . NHK order to assess the performance of these frequency discriminator , once in 5 ~ 10Hz frequency deviation FM , for passband frequency discriminator scan to check their linear with 12KHz audio , and found not to mention what frequency discriminator better, because no matter what form of frequency discriminator circuit , as long as linear and bandwidth requirements , differential gain is a horizontal line , you can get good sound quality .
In the end multi- band and multi- wide good linearity to meet the high fidelity demands? In order to prevent the temperature and frequency error caused by the partial transfer detuning , ordinary radio , the linear frequency discriminator bandwidth of the IF bandwidth should be higher than 100KHz, the tuner should be higher than 200KHz.. If the tuner has a bandwidth selection , broadband is generally 400 KHz, narrowband typically 200 KHz, so the linear bandwidth of frequency discriminator need to reach 600KHz. In the center frequency of 10.7MHz frequency discriminator analog circuits , the ratio and phase frequency detector must meet the requirements in order to double -tuned circuit . Also useful for generating a linear tracking bandwidth , e.g., the tracking phase of the FM discriminator is the wave frequency and phase modulation wave becomes , the MPX signal is demodulated at the phase detector , the phase locked loop reference signal is reproduced. Because the circuit is more complex, it made a Hitachi integrated circuit HA11211. JVC 's most favored such a circuit , in their upscale tuner can often see it as T7070, JT-V77 and so on .
Put the circuit in the case of variable parameters is more complicated to treat specific for the changed parameters. When the offset is changed , frequency bandwidth is necessary to follow the change , if the deviation becomes 150 KHz, 225 KHz, 300 KHz, 375 KHz later, according to the end of the Japanese island of good vice calculation method, the corresponding linear bandwidth 800KHz, 1.2 MHz, 1.6 MHz, 2MHz. Analog frequency discriminator circuit more difficult to achieve linear bandwidth of 600 KHz , then switch to a digital manner. The easiest way is to put the sine wave frequency modulation digital signal into a pulse width modulation , restore the MPX signal low -pass filter , such as a pulse counting discriminator and PWM frequency discriminator . This frequency discriminator in the early 1970s in the U.S. Heathkit 's AJ510 tuner, the 1976 Trio Company Institute , used on all his high-end tuner. Another approach is to put the digital FM IF frequency 2MHz following a pulse , the delay time by two different CMOS gate , with the XOR gate MPX signal is demodulated . Digital frequency discriminator algorithm is very simple and can be completed with a quadrature signal multiplier , and there is no linearity and bandwidth issues in the DSP . Now the DAB / FM tuner , frequency and decoding software is used in DSP algorithm implementation .
In place and the frequency discriminator is broadcast once a paradise for lovers of DIY , there are many ingenious, excellent performance of the classic circuits , it is still discussed today many fans relish .
The most reassuring is the stereo decoder
Today, both factory or your own DIY FM radio , the most reassuring part of the stereo decoder . Even two battery-powered portable computer , a decoder is a TA7343, you can easily get 40 dB stereo separation . In the past, it is almost unthinkable .
In order to improve stereo separation receiver historically spent two decades. Pilot -system , the sum signal and the difference signal amplitude difference and a phase difference between ; subcarrier regeneration subcarrier transmission side will affect the separation phase . If the difference between the signal amplitude and phase 3 db difference or presence of 20 degrees , regenerated subcarrier phase and the presence of former vice -carrier phase difference of 20 degrees , the stereo effect will disappear without a trace . Differential phase and amplitude are simultaneously present in the decoder , these parameters will change with temperature and time . Stereo decoder and matrix switches have two forms , the matrix is simple and easy to implement , but the demanding circuits and devices , which doomed the early design with tube or transistor matrix decoder born there will be high degree of separation . Have tested the history of astronomical advanced stereo radio , separation is generally around 12 decibels, far less than the palm machine stall selling today . Switching the decoder to obtain a high resolution in principle , but requires a regenerated subcarrier with no phase difference between the transmitter switching signal . Phase Locked Loops do not meet the regeneration phase of the switching signal requirements, so the decoder is not the switching of high resolution , the highest at about 20 decibels. Thus, in the twenty years after the launch of FM stereo , FM separation has been the soft underbelly of the receiver .
While the Europeans and the Japanese in the early years of stereo separation headaches , when in 1972 the United States Motorola developed the world's first integrated PLL stereo decoder MC1310, stereo separation from ten decibels jumped up to 40 db , distortion is reduced from 1% to 0.3% . Afterwards. Japanese manufacturers have to learn to imitate, to produce a variety of performance is more excellent decoder. For example cheap machine commonly used TA7343, separation 45 db , distortion of 0.08% , SNR 74 db . Stereo decoder using a special separation has in the tuner 65 dB THD 0.006% , SNR 89dB. Since the emergence of such a device , FM radio in stereo only become truly . And breaking the boundaries of the senior and popularity , people have lamented the advances in technology and changes of the era .
Bass preamp can not be ignored
Although the low-frequency preamplifier is obscure position in the tuner , but some can not ignore its role as a receiver in a good tuner , you should have the following features:
1 ) deemphasis circuit : mono receivers , 50 microseconds in the deemphasis circuit is connected after the frequency discriminator , a stereo receiver, and the difference in order to ensure the pilot signal is not attenuated in the de-emphasis circuit connected to the stereo after decoder .
2 ) and the sub- carrier frequency of the pilot filter : The main purpose is to remove the residual pilot audio subcarrier frequency component and prevent intermodulation distortion caused by the low frequency amplifier . They also at the time of recording and AD conversion and partial magnetic beat frequency and sampling frequency chirp generated .
3 ) squelch circuit : FM receiver gain is high , it will produce a lot of noise when no signal is input. Set squelch the past in order to avoid noise in the main tune , no radio station is still very quiet location . Digital memory tuning tuning noise does not exist , since many retained the flywheel manually tuned receiver squelch circuit is still necessary .
4 ) loudness control : According to the human ear 's auditory response curve compensation narrower frequency response at low volume when hearing defects , this function when the volume of the background music for a small family get to high bass rich effect.
5 ) Tone Control : To compensate for the defect of the speakers and the listening environment
6 ) passband control : FM radio to listen to the edge in a weak signal area , put in 150 ~ 8000Hz can significantly reduce high-frequency noise bandwidth setting low frequency circuits.
7 ) intimacy Control: appropriate to enhance the range of 2000 ~ 3000 Hz amplitude can make people feel warm sound , appropriate restrictions on the rate of low-frequency amplifier can weaken articulate a sense of softness to enhance the sound .
Not suitable for listening to FM radio with Hi-Fi headphones
With Hi-Fi headphones always felt when listening to the FM radio sounds a little rough , you will hear a hissing noise and crackling sound when the discharge interval and low in the program , which is the background noise inherent in FM radio . Sky from the outside , the interference of industrial and domestic appliances , which can be 99.9% by weight of the amplifier and discriminator suppressed by the limiter , the remaining parasitic modulation, thermal noise and flicker noise of a transistor on the slicer powerless , they will become the background noise superimposed on the audio signal.
So why the speaker can not hear the noise floor it? There are two reasons , one is the weight of the headset is very light voice coil and diaphragm . If a pair of headphones and speakers also marked with 90 dB sensitivity , headphones refers to 1 milliwatt of electrical power drive get 90 dB SPL at one centimeter ; speaker is using a watt electric power drive away from the speaker 1 m achieve the same sound pressure sensitivity than the speaker apparently headset thousands of times higher . Another reason is that the sound transmission direction , sound unit area can be inversely proportional to the square of the distance , but up to the sound frequency from 1KHz 10KHz, frequency increased by 10 times , to absorb the loss of air has increased by 100 times. Low energy distribution of interference and noise in the high frequency acoustic attenuation in the air very quickly , almost one meter outside decay to zero . Played a role in the distance and the air filter , make ear completely feel the presence of background interference and noise. Use headphones to listen to the situation is completely different, very close to the eardrum from the headset , this is equivalent to the noise floor of the bypass filter , combined with the high sensitivity of the headphones , the music and noise are collected ears, so we feel the sound hair rough .
In addition , speakers and headphones for music listening time adaptability and psychological sense are not the same , speaker appreciation for boldness , such as the symphony , concerts and opera. Program traumatic Betts , soft sweet tenor , treble gorgeous bright as the heart of India as to induce the listener to feel the overall outline of the music , and attend to minor . Headphones for listening details Ruqirusu violin and erhu, hidden when the triangle and so on. Sweet vocals and delicate nuanced treble will induce the listener to feel the melody and musical skills to capture the rich levels and relatively minor differences . Therefore , the experience of the old burned with headphones to listen to CD, listen to the radio using the loudspeaker .
Meet tomorrow digitizing
FM radio portable music and joy gone through 66 years of history , China's radio enthusiasts to enjoy the good fortune did not last century music FM , FM radio around the reform and opening up have mushroomed as take out. But the majority of fans plagued two regrets : a broadcast content is mixed , ground , county FM stations only poor equipment , program content is also to be commended. Unfortunately, there is no longer a good receiver to enjoy the broadcast . 2002 Central People's Broadcasting Station that " the frequency of specialization, the frequency of administration ," the slogan of reform since the Voice of China , Voice of the economy , The Sound of Music has been introduced. Also follow up the reform of the local radio stations broadcast programs, carefully compare the Shanghai FM radio , 107.7MHz sound of music sound better ; contents 94.7MHz best classical music , but unfortunately the minimum transmission power . Qiujiang electronics market and Xiangyang modern electronic mall you can find cheap used to tuner .
Although the danger of being replaced by FM digital broadcast ending, but now the quality of broadcasting has reached a peak, in the future , whether alternative format or IBOC DAB have been a bit-rate compression . Theoretically compressed signal is redundant , but listening evaluation is a complex matter , theory and laboratory concluded not cover individual psychological and physiological differences in each person . UK DAB radio has been very popular in Europe enthusiasts careful comparison of the DAB and FM sound quality , crystal- on tagline DAB sound questioned. No matter what our future digital broadcasting system , the human inherent nostalgia is sure to make many people miss gave us endless joy FM radio . Where a pastor in the Evangelical Church had enlighten me: " To cherish today , only today is the happiest ."
