Reception performance improvement of AM/FM tuner by digital signal processing technology Akira Hatakeyama Osamu Keishima Kiyotaka Nakagawa Yoshiaki Inoue Takehiro Sakai Hirokazu Matsunaga Abstract With developments in digital technology, CDs, MDs, DVDs, HDDs and digital media have become the mainstream of car AV products. In terms of broadcasting media, various types of digital broadcasting have begun in countries all over the world. Thus, there is a demand for smaller and thinner products, in order to enhance radio performance and to achieve consolidation with the above-mentioned digital media in limited space. Due to these circumstances, we are attaining such performance enhancement through digital signal processing for AM/FM IF and beyond, and both tuner miniaturization and lighter products have been realized. The digital signal processing tuner which we will introduce was developed with Freescale Semiconductor, Inc. for the 2005 line model. In this paper, we explain regarding the function outline, characteristics, and main tech- nology involved. 22 Reception performance improvement of AM/FM tuner by digital signal processing technology Introduction1. Introduction from IF signals, interference and noise prevention perfor- 1 mance have surpassed those of analog systems. In recent years, CDs, MDs, DVDs, and digital media have become the mainstream in the car AV market. 2.2 Goals of digitalization In terms of broadcast media, with terrestrial digital The following items were the goals in the develop- TV and audio broadcasting, and satellite broadcasting ment of this digital processing platform for radio: having begun in Japan, while overseas DAB (digital audio ①Improvements in performance (differentiation with broadcasting) is used mainly in Europe and SDARS (satel- other companies through software algorithms) lite digital audio radio service) and IBOC (in band on ・Reduction in noise (improvements in AM/FM noise channel) are used in the United States, digital broadcast- reduction performance, and FM multi-pass perfor- ing is expected to increase in the future. In addition, a mance) state of technological maturity has been reached in ・Improvements in interference prevention (improve- AM/FM broadcasting, making it difficult to make further ments in interference from adjacent AM/FM signals) cost reductions and improvements in reception for con- ②Reductions in cost through functional integration ventional analog technology. (development of integrated radio processing/audio Through digital signal processing for AM/FM IF and processing DSP) beyond, we are attaining performance not possible with ③Miniaturization (reduction of large parts such as conventional analog signal processing, while also develop- ceramic filters) ing a car digital signal processing tuner that shares pro- cessing with future digital media. 3 Outline3 Outline of ofentire entire system system In December 2004, this tuner was added to our lineup of products for 2005. 3.1 Outline of entire system A system block diagram of the digital signal process- 2. Development goals ing tuner is shown below. 2 Development goals It is made up of 3 primary parts. 2.1 Background ①Tuner module Although certain manufacturers have been develop- This module converts RF signals to IF signals, and ing products for the digital processing of AM/FM radio outputs the IF signals. It is constructed of a tuner LSI for approximately 10 years, little progress has been made and its circuitry, as well as a Memory IC that stores para- over conventional products (those with analog processing meters. IF signals are then input to the analog interface. of signals) in terms of performance and cost reduction. ②Analog interface However, with the recent improvements in DSP perfor- This interface converts the IF analog signals input mance, there have been significant reductions in cost. from the tuner into digital signals. After the digital signal The results of car ratings performed by JD Power, a is processed by the audio processor (described below), it North American consumer satisfaction indicator, showed is re-input to the analog interface, where it is converted that the cars with the highest ratings had radios with to analog audio and output. functions to prevent noise and interference from adjacent ③Audio processor FM signals. This processor applies a variety of radio and audio As mentioned above, it has become difficult to make processing to the digital signal input from the analog improvements and differentiate with analog technology. interface. With digital signal processing, performance can be Radio processing includes the following algorithms: improved for clear differentiation with other companies, AM/FM noise reduction, AM/FM detection, FM stereo but this had not yet been achieved due to the processing demodulation, FM diversity, RDS, and multi-pass noise capabilities and cost-related issues of DSP. reduction. Due to recent improvements in both signal processing Audio processing includes the following algorithms: performance and cost reduction in DSP, the standardiza- Vol. control, Tone control, CSII (circle surround II), and tion of radio signal digital processing DSP with other FixEQ. functions (specifically, the standardization of audio pro- The audio processor also includes a digital input, so cessing such as sound field controls with DSP) has result- that by connecting a CD player, for example, you can cre- ed in the achievement of costs equal to and lower than ate a CD tuner system. conventional systems (analog processing tuners + audio Next, we will take a closer look at each part. processing DSP). In addition, through digital processing FUJITSU TEN TECH. J. NO.25(2005) 23 FUJITSU TEN TECHNICHAL JOURNAL Previous Product Sub Main Tuner module Tuner LSI Audio Processing AM RF ＬＷ／ＭＷ：MIX+IF+DET ＦＭ：NC+MPX+Diversity Ａ Ｍ ＳＰ ＦＭ：MIX+IF+DET ＬＷ／ＭＷ：NC Ｐ FM RF 2 2 ＤＡＣ ＰＬＬ Ｉ ＣＢＵＳ Electronic VOL Ｉ ＣＢＵＳ Memory(E2PROM) Parameter storage Parameter control μ－ＣＯＭ Developed Product Sub Main Tuner module Analog interface IF-ADC Ａ Ｍ Audio-DAC ＳＰ Tuner LSI Synthesizer Ｐ Reference AM RF ＬＷ／ＭＷ：MIX+IF ＦＭ：MIX+IF Serial Audio processor FM RF ＤＡＣ ＰＬＬ ＢＵＳ ＬＷ／ＭＷ：NC+DET Vol+Tone control Ⅱ Memory(E2PROM) ＦＭ：NC+DET+MPX CS +RSE +DIV+RDS Serial ＢＵＳ Parameter storage Parameter control μ－ＣＯＭ Fig.1 System block diagram (comparison with current product) 3.2 Tuner module functions of the tuner module, allowing the removal of 3.2.1 Tuner LSI several large parts and the use of compact parts. In conventional analog tuners, processing is per- Table 1 Tuner part composition formed by the front-end tuner LSI from the RF area to detection. However, since the tuner module we have Item Previous product Developed product developed performs digital processing after IF, no detec- Height 40mm 30mm tion circuitry is needed. In addition, while a crystal oscilla- Miniatur- ization Area 10 8 tor is required in conventional tuners for the PLL refer- IC Reflow 2 2 ence frequency, in our newly developed system it is sup- Transformer, DIP 8 4 plied from the analog interface. With these changes, the coil Reflow 0 2 construction is simpler than conventional products. Filters DIP 5 2 Large parts Crystal oscillator DIP 1 0 Previous product LW/MW: MIX + IF + DET 1005-size chip parts are not used in the previous prod- FM: MIX + IF + DET uct. However, they make up 40% of part in the developed Shared areas: PLL + DAC + BUS product, and contribute to miniaturization. In addition, Developed product parts were successfully installed on one side, as with the LW/MW: MIX + IF previous product. FM: MIX + IF 3.2.3 Improvements in productivity Shared areas: PLL + DAC + BUS As you can see in Table 1, although ALL reflow is not achieved through cost restrictions, DIP parts are reduced 3.2.2 Sliming of tuner module to 40% from the previous product (from 14 to 6), con- Other companies have also been adding multiple func- tributing to improvements in productivity and quality. tions to car audio devices over the years, resulting in In addition, we are making rapid in-house progress for large installation spaces. Our newly developed tuner lead-free compatibility. needed to be 75% the height of the previous product. This sliming was achieved through a reduction in the FUJITSU TEN TECH. J. NO.25(2005) 24 Reception performance improvement of AM/FM tuner by digital signal processing technology ・Decimation filter for speech input ・Analog volume attenuator ・Audio DA converter: 6 channels ・Interpolation filter for audio output ・Control register After the intermediate frequency signal from the tuner module goes through AD conversion, the analog interface uses filters to convert the sample rate to the optimum value for processing by the base band chip in a later step. The AD converter for IF uses a ΣΔ AD con- verter with band-pass functionality to achieve an input Previous product dynamic range of approximately 80 dB at a transit band- width of ±140 kHz. A sampling frequency of 14.4 MHz is used, and the transit bandwidth is set to 280 kHz to avoid any effects due to aliasing noise. Sine the sampled IF sig- nal becomes a 480 kHz signal when sent to the bass band chip in a later step, oversampling is performed 30 times. Two systems are included so that the AD converter is compatible with twin tuners. For external analog input, there are two systems for differential input stereo ADC, one system for differential monaural audio ADC, and two systems for single-end monaural audio ADC. Each of Developed product these can be used for cassette tape input, microphone input, intercom, external CD changer analog input, and Fig.2 Outer shape comparison of developed and previous product auxiliary input. These analog signals input to the analog interface are processed by the audio processor in a later 3.3 Analog interface step, and then sent back to the audio interface, where 3.3.1 Process they are converted to analog signals by the DA converter A 0.35 (m 3.6 V standard CMOS process was used, and output to the speaker amplifier.