|Home Analog Devices Feedback Subscribe Archives 简体中文 日本語|
The Successful Implementation of High-Performance Digital Radio
Evolution of Digital Radio
The Satellite Digital Audio Radio Services (SDARS) enabled mobile car audio listeners to tune into the same radio station anywhere within the satellite’s coverage map, limited only by intermittent blockage of satellite signal due to buildings, foliage, and tunnels. XM satellite radio took the lead in circumventing the blockage problem by installing terrestrial repeaters, which transmit the same satellite audio in dense urban areas and create a hybrid architecture of satellite and terrestrial broadcasts.
Around the same time the traditional terrestrial broadcasters also charted a digital course—for two reasons. First, they perceived that their life span on the analog concourse had to be quite short, as the world migrates to the higher quality digital runway. Second, the frequency spectrum is getting scarce, so additional content within the same bandwidth could be delivered only by digitizing and compressing the old and new content, packaging it, and then broadcasting it. Thus, the world started migrating from analog to digital radio. These techniques for radio broadcast had the advantages of clearer reception, larger coverage area, and ability to pack more content and information within the existing bandwidth of an available analog radio channel—as well as offering users increased control flexibility in accessing and listening to program material (Figure 1).
Figure 1. Digital radio on a convergence processor.
Digital Radio Development Example: India
Useful propagation in the VHF bands is essentially limited to line-of-sight in small geographic regions. Propagation in short wave, on the other hand, can go almost anywhere in the world due to multiple reflections in the ionosphere. For countries that are densely populated and have small geographic regions, DMB transmitting in VHF Band III and L-Band functions very efficiently. For countries that have large geographic areas, transmissions in medium and short wave provide effective coverage. For this reason, after a few years of trials of DAB and DRM, India decided to adopt DRM.
During 2007, All India Radio (AIR), Asia-Pacific Broadcasting Union (ABU), and the DRM Consortium conducted the first field trial for DRM in New Delhi. The experimental trial was conducted over three days with three transmitters, with measurements of various parameters. Besides these tests in New Delhi, AIR also did these measurements at long distances. It became clear that DRM had the advantage of serving a larger population with a limited number of transmitters. In addition, the increasing need for energy conservation raises power saving considerations to paramount importance. DRM’s 50% greater power efficiency plays a vital role in supporting the ecology and a “greener” Earth.
Digital Radio Receivers and DSP
Development of powerful and efficient DSPs—along with advancements in information and communication theory—enabled the convergence of media technology and communications. Digital radio owes its existence to these technological advances.
Digital radio receivers were initially designed as lab prototypes and then moved to pilot production. Like most technologies, the first generation products are generally assembled using discrete components. As the market size and competition increase, manufacturers find that markets can be further expanded by bringing down the price of the finished product. The prospect of higher volume attracts semiconductor manufacturers to invest in integrating more of these discrete components to bring down the cost. With time, the shrinking silicon geometries lead to further cost reductions and improvements in the product’s capability. Such has been the continuing evolution in many products, including FM radios and mobile phones.
Signal Processing in Digital Radio
Figure 2. Software architecture of digital radio.
Signal processing algorithms in a digital radio receiver can be classified into the following categories:
In digital radio, the source coding and channel coding can respectively be mapped to an efficient audio codec (coder-decoder) and error control system components. Practically, error control can be performed better if the codec is designed for error resilience.
An ideal channel coder should be resilient to transmission errors. An ideal source coder should compress the message to the highest information content (Shannon entropy), but highly compressed messages would lead to very high audio distortion if the input stream contains errors. Thus, effective source coding should also ensure that the decoder can detect the errors in the stream and conceal their impact so that overall audio quality is not degraded.
DRM applies relevant technological innovations in source coding and channel coding to deliver a better audio experience. The DRM audio source coding algorithm that is selected ensures:
Efficient Audio Source Coding
Let us consider AAC from the MPEG community to understand some of the important technologies involved in source coding. Psycho acoustic model (Figure 3) and time-domain alias cancellation (TDAC) can be considered as two initial breakthrough innovations in wideband audio source coding.
Figure 3. Understanding psycho-acoustic tonal masking.
Spectral band replication (SBR, Figure 4) and spatial audio coding or binaural cue coding techniques from industry and academia can be considered as the next two game-changing innovations. These two key breakthrough innovations further enhanced AAC technologies to give scalable coding performance, which resulted in standardization of HE-AAC v2 and MPEG surround—which received overwhelming responses from the industry. Industry-driven standards, like Dolby®, AC3, and WMA®, also took similar steps to leverage similar technological innovations for their latest media coding.
The spectral band replacement (SBR) tool doubles the decoded sample rate relative to the AAC-LC sample rate. The parametric stereo (PS) tool decodes stereo from a monophonic LC stream.
Figure 4. AAC-LR, SBR, and PS in audio decoding.
Like any other improvement initiatives, measurement technologies also played their role in audio quality improvement initiatives. Audio quality evaluation tools and standards, like perceptual evaluation of audio quality (PEAQ) and multi-stimulus with hidden reference and anchor (MUSHRA), aided faster evaluation of technological experiments.
Graceful Degradation/Error Resilience
Error Resilience (ER) AAC coding guarantees graceful degradation from bit-stream errors with the help of these additional tools:
The ER-AAC features, together with UEP, will provide adequate error resilience characteristics for DRM.
Table 1. DRM bit-rate-bandwidth.
This requirement demanded the use of highly efficient audio coding: Meltzer-Moser MPEG-4 HE-AAC v2 (International Standardization Organization/International Electrotechnical Commission—ISO/IEC) was a good choice, but the robustness against channel fading made an error-resilient version of HE-AAC v2 (Martin Wolters, 2003) the best choice.
Table 2. Different codecs supported by DRM.
Besides AAC, the DRM standard defines the harmonic vector excitation coding (HVXC) and code-excited linear prediction (CELP) codecs to be used for transmitting speech. Streaming raw data for image slideshows, HTML pages, and the like is also allowed by the DRM standard.
Figure 5. Multiplexing and channel coding in DRM.
The MSC encodes the frame generated by the multiplexer. One can choose between standard mapping, symmetrical hierarchical, or mixed hierarchical mapping. The MSC uses unequal error protection (UEP, Figure 6), in which the multiplex frame is split into two parts with different levels of protection: higher- and lower-protected data parts.
Figure 6. Unequal error protection in DRM.
Digital Radio with Blackfin
Figure 7. Blackfin processor based digital radio.
Blackfin processor based digital radios, Internet radios, and multi-featured products can be created using the existing ecosystem that ADI created for these products.
In addition to creating the required ecosystem and sourcing the various software modules, ADI also created its own decoder libraries for digital radio. One such key component is an HE-AAC v2 decoder, which optimizes the performance available from the large number of required MIPS.
Architecture of HE-AAC V2 Decoder
Figure 8. MPEG-4 HE-AAC v2 decoder.
Key features include:
Table 3. MPEG-4 HE-AAC v2 decoder performance.
The decoder implements all required audio coding tools specified by the standard, including:
Digital Radio Test Results
Table 4. Digital radio test results.
Now, additional companies are using this design for making digital radios in India and other countries. The ADI Blackfin processor has the right combination of DSP and microcontroller features to form the core of a very cost effective DRM radio receiver. Availability of software tools, support by experienced applications teams, and the required software modules and reference designs from third parties make this implementation a good choice for manufacturers in India and elsewhere to adopt the design and mass produce DRM radios that use it.
Copyright 1995- Analog Devices, Inc. All rights reserved.