Design Considerations for High-End Consumer and Pro-Audio Equipment 2012/07/20 Silicon Laboratories
Analog
High-end consumer and professional audio (pro-audio) equipment represents a specialized and performance-driven market that benefits from the talents of passionate engineers and designers who concentrate on delivering the highest fidelity sound quality possible.
To achieve the highest possible performance for high-end audio applications, designers often use solutions that rely on discrete components and apply their know-how to fine-tune the radio to the desired level of performance. The downside to discrete radio implementations is that discrete components can drive up the cost of an audio system, and the complexity of the radio introduces design risk. Discrete-based audio architectures have inherent weaknesses arising from component variability, high component count and system cost and often unreliable performance.
Digital low-intermediate frequency (low-IF) CMOS AM/FM receivers are now able to achieve very high performance levels while integrating the majority of their external components on a single monolithic IC. In addition to being used in the handset and portable media player market for a number of years, digital low-IF CMOS AM/FM receivers are also seeing widespread adoption in the OEM automotive market where performance requirements are as demanding as in the high-end consumer audio market. This growing market acceptance of digital low-IF technology speaks to the level of technological advancement achieved by the latest generations of single-chip AM/FM receivers.
Designed by radio enthusiasts specifically for the pro-audio market, the Silicon Labs RF team recently introduced the Si477x digital low-IF AM/FM receiver family to address the requirements of high-end consumer audio/video receiver (AVR) applications. The Si477x family reduces system cost and bill of materials (BOM) count for high-end consumer and pro-audio applications while ensuring outstanding RF performance.
Let’s examine several key requirements and performance considerations for an AM/FM radio receiver solution targeting high-end consumer and pro-audio applications.
Sensitivity
Sensitivity refers to an AM/FM tuner’s ability to tune in weak stations. Sensitivity becomes extremely important when the receiver is in a rural area some distance away from the transmitters of most radio stations. High-end audio applications in today’s market typically target a microvolt level of sensitivity, which allows them to tune to radio stations dozens of miles away. To achieve sensitivity on the order of a microvolt, discrete designs use external low-noise amplifiers (LNAs). This approach sets a high bar of performance for integrated CMOS receivers in which the RF front end including the LNA is integrated on the same monolithic IC as the high-speed digital processing engines.
The Si477x receiver, for example, offers -3.5 dBμV of FM sensitivity, enabling it to tune to signals as low as 0.7 μV and pick up stations from as far away as 100 miles. The Si477x receiver achieves this level of FM sensitivity with an on-chip LNA and without the need for any external active components.
Third-Order Inter-Modulation (IP3)
Third-order intercept point (IP3) is a figure-of-merit measurement for the linearity of a radio tuner. Higher IP3 means better linearity and less distortion. The most severe distortion is third-order intermodulation distortion (IMD3) resulting from two close-in blocker stations that show up at the desired station frequency. The FM demodulator will end up demodulating both the desired and the IMD3 product formed by the blockers. The result is unpleasant, sub-par audio performance.
To eliminate the IMD3 distortion products, current audio designs use an external tracking filter on the front end of the receiver to attenuate the blockers. However, this technique increases system cost and design complexity. In crowded FM spectrums, the blockers are too close to the desired weak station to be easily filtered out. To minimize IMD3 distortion products, the IP3 of the receiver front end must be adequate. Tuners with low IP3 require high Q tracking filters to avoid IMD3 break-in, which can result in an extremely unpleasant experience for the listener.
The Si477x receiver features IP3 of 117 dBμV coupled with sensitivity at -3.5 dBμV at full RF gain. This best-in-class linearity protects against IMD3 break-in without using expensive external filtering blocks.
Selectivity and Dynamic Bandwidth Control
Selectivity refers to a tuner's ability to tune to a weak station in the presence of stronger stations at ±100 kHz and ±200 kHz frequency offset from the desired station. This feature is especially important for receivers in crowded FM spectrums in urban environments. Being able to tune to the desired station in the presence of strong blocker stations is very important for listener satisfaction.
Most current tuner designs suffer from poor selectivity at ±100 kHz and ±200 kHz blocker offsets because of the inability to dynamically change the bandwidth based on blocker strength.
The Si477x receiver provides 65 dB selectivity at ±100 kHz and 72 dB at ±200 kHz frequency offset from the desired station, which is an exceptional figure of merit for single-chip tuners. This outstanding selectivity is realized with powerful, state-of-the-art radio DSP algorithms, such as channel bandwidth control, that dynamically optimize the desired channel bandwidth by gauging several signal metrics including adjacent and alternate channel conditions, as well as narrowing the channel bandwidth in the presence of strong nearby blocker stations.
Multipath Handling
While multipath handling is often thought of as only a “mobile” issue, it is also a static phenomenon that is very important to pro-audio enthusiasts who rate AM/FM receivers based on their ability to manage multipath fading in static environments. Multipath distortion is caused when two or more radio signals broadcasting from the same source arrive at a receiver at different times with different phases and attenuation levels due to reflections of the signal from various objects.
The current approach to mitigating the effects of multipath distortion is to apply techniques, such as blending from stereo audio to mono audio, deploying low-pass audio filters (hi-cut and hi-blend), attenuating the audio level when there are unavoidable harsh pops, and, as a last resort, applying full soft muting. All of these mitigation engines run on the Si477x receiver autonomously and are activated by signal quality metrics that are continuously monitored by the Si477x. These signal quality metrics and the effects they apply to the signal are all configurable by the designer.
The Si477x receiver also introduces an on-chip, pro-audio class FM channel equalizer designed to eliminate multipath fading, thus yielding audio with minimal distortion. The adaptation algorithm of the equalizer continuously seeks to restore spectral components attenuated due to multi-path fading effects, thus restoring the signal even if the radio experiences multiple fading conditions. The result is reduced audio distortion and a lower level of sound-compromising mitigation techniques, such as stereo-mono blend, hi-cut/hi-blend and soft mute, being applied to the audio.
State-of-the-Art Low-IF Technology for High-End Consumer Audio
Single-chip low-IF receivers should integrate all tuner RF blocks, have an MCU and DSP signal conditioning, and support I2S digital audio and/or left/right high-fidelity stereo audio.
Reduced System BOM Cost: Digital low-IF AM/FM receivers offer very high levels of integration that minimize component counts in audio systems, support the use of lower-cost external components and reduce PCB size through smaller packages. Single-chip low-IF receivers integrate numerous external components on-chip, resulting in an external BOM that contains no manually tuned components, active components or filters. The only external components required for a typical high-end audio system are inexpensive passives. This high level of integration significantly reduces the external BOM count and system cost.
High Performance: Low-IF AM/FM receivers incorporate advanced DSP algorithms designed to enable outstanding RF sensitivity, adjacent and alternate channel selectivity, linearity and audio preservation. The result is superior audio quality under the most demanding environments.
Simplified Design: Because of their high level of integration, digital low-IF receivers require minimal external components to deliver optimal RF performance. Since the majority of the external components are integrated on-chip, the audio system designer can be confident that the RF performance of the front end will operate according to data sheet specifications over temperature and voltage. Single-chip integration resolves a serious design challenge for pro-audio system developers who would otherwise need to compensate for the drift of the front end components in their designs. Reduced BOM count also simplifies testing of the final design, further reducing development costs.
Radio Configurability: High-end consumer and pro-audio customers are similar to automotive customers in that they both require excellent receiver performance. However, each customer base has a different definition for “excellent receiver performance.” Automotive customers focus on audio performance in a mobile environment, while pro-audio customers focus on pure audio reproduction in a stationary environment.
In some respects, high-end audio equipment designers build their brands by making equipment that reproduces the transmitted broadcast sound as true to the original content as possible. For this reason, true audio representation is a primary concern in the high-end audio equipment market. The high level of configurability available from state-of-the-art, digital, low-IF AM/FM receivers enables designers to tweak their designs based on their performance targets. Ultimately, this configurability is fundamental to achieving a particular “sound” that is unique to an audio equipment maker’s brand.
Companies Website:
http://components-asiapac.arrow.com/en/new_product/4743/
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