| Inside DSP on Audio: Inside a Modern Digital Audio Product |
By Amit Shoham, 1/5/2004
The last decade has seen consumer audio
products from home theater systems to car stereos and portable players
go digital. These complicated devices play back compressed audio
formats, compensate for room acoustics, and add effects such as
reverberation, equalization, and dynamic bass, thanks to the power of
digital signal processing.
How do manufacturers pack the DSP punch these applications require into
small, affordable, and power-efficient systems? A portable audio
player, for example, contains a processor, volatile and non-volatile
memory, analog and power management components, and more. These
components need to talk to each other and to the outside world. And
there is more to these devices than hardware: complex signal processing
software, user interface software, and device drivers are also needed.
This article charts the anatomy of a typical
consumer digital audio product, focusing on portable players and
network-enabled devices. Closely related to this topic, the "Under the Hood" column in the January 5 issue of EE Times provides a tear-down of one handheld audio product, the Apple iPod.
Processors
At the heart of today's digital audio product is a programmable
processor or processor core. Roughly speaking, processing requirements
vary from roughly 20 MIPS for MP3 decoding to well over 100 MIPS for
products that feature sophisticated audio encoders or combine high-end
multi-channel decoders with numerous other audio processing functions.
These computational loads are well within reach of many DSPs and
general-purpose processors, giving designers a wide range of options to
choose from. The article, "Digital Audio Technology Guide," discusses considerations and tradeoffs in selecting a processor for a digital audio product.
Some products use a DSP processor, or incorporate multiple processors;
for example, a microcontroller for connectivity, storage management,
and user interface functions, and a DSP for audio processing. The
latter approach is particularly attractive for convergence products
such as audio-enabled digital cameras where a microcontroller or
general-purpose processor is already present, but does not provide
sufficient performance for implementing the necessary audio functions.
Network-enabled A/V receivers, PVRs, home media servers and set-top
boxes are likely to utilize a general-purpose processor for all
networking functions, storage management, user interface, and audio
processing functions.
Often at least one processor core is hidden inside an ASSP (Application-Specific Standard Product chip) such as the PortalPlayer chip used in the Apple iPod (See the "Under the Hood" column in EE Times
for details). ASSPs combine processor cores with appropriate
peripherals and sufficient on-chip memory for implementing a particular
consumer audio product. The cost, size, and power consumption
advantages of integrating many functions on one chip make ASSPs
particularly attractive in portable products, although ASSPs also make
their way into many other applications. Figure 1 compares key
components of Texas Instrument's TMS320DA250 DSP chip and SigmaTel's
STMP3560 ASSP, both targeting portable audio applications.
Click to Enlarge
While ASSPs targeting decoding of compressed audio are common, some
system developers prefer to design their own custom ASICs—if they can
tolerate the long and costly development cycles involved. (See "Digital Audio Technology Guide" for an in-depth discussion of processor selection.)
Memory
ASSPs targeting digital audio products typically include sufficient
on-chip memory for audio decoding and additional processing such as
equalization and dynamic bass processing. Many packaged DSP processors
also offer sufficient on-chip memory for these functions. Avoiding the
use of off-chip memory lowers the system component count, reducing cost
and simplifying hardware design. In general, avoiding the use of
off-chip memory also lowers power consumption by reducing the activity
on off-chip buses. However, in hard-disk based portable players, a
large off-chip memory buffer can actually reduce power consumption.
Compressed audio data is read into memory from the hard drive in quick
bursts, allowing the hard drive to be powered down for long periods
while the contents of the buffer are decoded and played. This power
saving technique is illustrated in the design of the Apple iPod.
Off-chip SDRAM is typically used for this purpose, invariably leading
the designer to select a processor or ASSP with an integrated SDRAM
controller.
Internal connections
Serial buses play an important role in consumer audio product designs:
synchronous serial I2S buses are typically used to connect
digital-to-analog (DAC) and analog-to-digital (ADC) converters to the
chip performing the audio processing. Serial I2C buses are often used
to send volume control, equalization control, and other messages from a
microcontroller handling user interface functions to the chip handling
audio processing. A microcontroller may also use a dedicated serial
connection to send compressed audio read from a hard disk drive,
compact flash card, or a network port to a DSP or ASSP for decoding.
Serial ports typically have ample bandwidth for these purposes, and
because they require fewer signals to connect chips together, serial
ports can simplify hardware design and circuit board layout compared to
parallel connections.
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