SAMSUNG My-mp200 Referenc Information

Samsung Electronics 8-1

8. Referenc Information

MPEG Audio Layer-3
History Quality Details

History

In 1987, the IIS started to work on perceptual audio coding in the framework of the EUREKA project EU147,
Digital Audio Broadcasting (DAB). In a joint cooperation with the University of Erlangen (Prof. Dieter Seitzer), the
IIS finally devised a very powerful algorithm that is standardized as ISO-MPEG Audio Layer-3 (IS 11172-3 and IS
13818-3).

Without data reduction, digital audio signals typically consist of 16 bit samples recorded at a sampling rate more
than twice the actual audio bandwidth (e.g. 44.1 kHz for Compact Disks). So you end up with more than 1.400
Mbit to represent just one second of stereo music in CD quality. By using MPEG audio coding, you may shrink
down the original sound data from a CD by a factor of 12, without losing sound quality. Factors of 24 and even
more still maintain a sound quality that is significantly better than what you get by just reducing the sampling rate
and the resolution of your samples. Basically, this is realized by perceptual coding techniques addressing the
perception of sound waves by the human ear.

Using MPEG audio, one may achieve a typical data reduction of

1:4 by Layer 1 (corresponds with 384 kbps for a stereo signal),
1:6...1:8 by Layer 2 (corresponds with 256..192 kbps for a stereo signal),
1:10...1:12 by Layer 3 (corresponds with 128..112 kbps for a stereo signal),

still maintaining the original CD sound quality.

By exploiting stereo effects and by limiting the audio bandwidth, the coding schemes may
achieve an acceptable sound quality at even lower bitrates. MPEG Layer-3 is the most
powerful member of the MPEG audio coding family. For a given sound quality level, it requires
the lowest bitrate - or for a given bitrate, it achieves the highest sound quality.

Sound Quality

Some typical performance data of MPEG Layer-3 are:

sound quality bandwidth mode bitrate reduction ratio

telephone sound 2.5 kHz mono 8 kbps * 96:1
better than shortwave 4.5 kHz mono 16 kbps 48:1
better than AM radio 7.5 kHz mono 32 kbps 24:1
similar to FM radio 11 kHz stereo 56...64 kbps 26...24:1
near-CD 15 kHz stereo 96 kbps 16:1

CD >15 kHz stereo 112..128kbps 14..12:1
*) Fraunhofer uses a non-ISO extension of MPEG Layer-3 for enhanced performance (“MPEG 2.5”)

In all international listening tests, MPEG Layer-3 impressively proved its superior performance,
maintaining the original sound quality at a data reduction of 1:12 (around 64 kbit/s per audio
channel). If applications may tolerate a limited bandwidth of around 10 kHz, a reasonable
sound quality for stereo signals can be achieved even at a reduction of 1:24.

For the use of low bit-rate audio coding schemes in broadcast applications at bitrates of 60
kbit/s per audio channel, the ITU-R recommends MPEG Layer-3. (ITU-R doc. BS.1115)

8-2 Samsung Electronics

Reference Information

Details

Filter bank

The filter bank used in MPEG Layer-3 is a hybrid filter bank which consists of a polyphase filter bank and a
Modified Discrete Cosine Transform (MDCT). This hybrid form was chosen for reasons of compatibility to its
predecessors, Layer-1 und Layer-2.

Perceptual Model

The perceptual model is mainly determining the quality of a given encoder implementation. It uses either a seper­ate filter bank or combines the calculation of energy values (for the masking calculations) and the main filter bank.
The output of the perceptual model consists of values for the masking threshold or the allowed noise for each
coder partition. If the quantization noise can be kept below the masking threshold, then the compression results
should be indistinguishable from the original signal.

Joint Stereo

Joint stereo coding takes advatage of the fact that both channels of a stereo channel pair contain far the same
information. These stereophonic irrelevancies and redundancies are exploited to reduce the total bitrate. Joint
stereo is used in cases where only low bitrates are available but stereo signals are desired.

Quantization and Coding

A system of two nested iteration loops is the common solution for quantization and coding in a Layer-3 encoder.

Quantization is done via a power-law quantizer. In this way, larger values are automatically coded with less
accuracy and some noise shaping is already built into the quantization process.

The quantized values are coded by Huffman coding. As a specific method for entropy coding, hufman coding is
lossless. Thus is called noiseless coding because no noise is added to the audio signal.

The process to find the optimum gain and scalefactors for a given block, bit-rate and output from the perceptual
model is usually done by two nested iteration loops in an analysis-by-synthesis way: