Xilinx XAPP169 Solution Overview

2www.xilinx.com XAPP169 (v1.0) November 24, 1999

1-800-255-7778

MP3 NG: A Next Generation Consumer Platform R

MP3 refers to the MPEG Layer 3 audio compression scheme that was defined as par t of the

International Standards Organization (ISO) Moving Picture Exper ts Group (MPEG) audio/video

coding standard. MPEG-I defined three encoding schemes, referred to as Layer 1, Layer 2, and

Layer 3. Each of these schemes uses increasing sophisticated encod ing techniques and gives

correspondingly better audio quality at a given bit rate. The three layers are hierarchical, in that

a Layer 3 decoder can decode Layer 1, 2, and 3 bitstreams; a Layer 2 de coder can decode

Layer 2, and 1 bitstreams; and a Layer 1 decoder can only decode Layer 1 bitstreams. Ea ch of

the layers support decoding audio sampled at 48, 44.1, or 32 kHz. MPE G 2 uses the same

family of codecs but extends it by adding support for 24, 22.05, o r 16 kHz sampling rates as well

as more audio channels for surround sound and multilingual appl ications.

All Layers use the same basic structure. The coding scheme can be described as "perceptual

noise shaping" or "perceptual subband / transform coding". The enc oder analyzes the spectral

components of the audio signal by calculating a filterbank (transform) a nd applies a psycho-

acoustic model to estimate the just noticeable noise-level. In i ts quantization and coding stage,

the encoder tries to allocate the available number of data bit s in a way to meet both the bitrate

and masking requirements. In plain English, the algorithm exploits the fact that loud sounds

mask out the listener’s ability to perceive quieter sounds in the same frequency range. The

encoder uses this property to remove information from the signal that w ould not be heard

anyway.

Like all of the MPEG compression technologies, the algorithms are desi gned so that the

decoder is much less complex. Its only task is to synthesize an aud io signal out of the coded

spectral components. All Layers use the same analysis filter bank (polyphase with 32 sub-

bands). Layer 3 adds a MDCT transform to increase the frequency resolutio n.

All layers use the same header information in their bitstream to support the hierarchical

structure of the standard.

Solution

Overview A key design objective for this application was the creation of a solution with the lowest possible

cost, while at the same time providing support for value added f eatures. These features include

the ability to store contact information and record memos and other functions commonly fou nd

in Personal Digital Assistants (PDAs).

Figure 1 gives an overview of the design. The key features of which are:

•128 x 128 pixel graphical touch screen.

•USB interface for download music and network connectivity.

•IRDA compliant infrared interface for exchanging data with other units.

•32 MB of on board FLASH storage.

•CompactFlash interface for storage expansion using CompactFla sh cards or MicroDrive

hard drives.

All of this is driven by a high-performance IDT RC32364 32-bit RISC processor and interfaced

using a next generation Spartan-II FPGA. Before the functions implemen ted in the Spartan

device and the software function running on the RC32364 are examined, the following gives an

overview of the Application Specific Standard Products (ASSPs) tha t are included in the

design.