RESOURCE

MP3 Zone


Q1 :	What is MPEG?
A1 :	MPEG is the "Moving Picture Experts Group", working under the joint direction of the International Standards Organization (ISO) and the International Electro-Technical Commission (IEC). This group works on standards for the coding of moving pictures and audio. MPEG has created its own homepage, providing information on the what, where, when and how of the standards. Ą@
Q2 :	Are MPEG-3 and Layer-3 the same thing?
A2 :	No! Layer-3 is a powerful audio coding scheme which certainly is part of the MPEG standard. Layer-3 is defined within the audio part of both existing international standards, MPEG-1 and MPEG-2. Ą@
Q3 :	Talking about MPEG audio, I always hear "Layer1, 2 and 3". What does it mean?
A3 :	MPEG describes the compression of audio signals using high performance perceptual coding schemes. It specifies a family of three audio coding schemes, simply called Layer-1, Layer-2, and Layer-3. From Layer-1 to Layer-3, encoder complexity and performance (sound quality per bitrate) are increasing. The three codecs are compatible in a hierarchical way, i.e. a Layer-N decoder may be able to decode bitstream data encoded in Layer-N and all Layers below N (e.g., a Layer-3 decoder may accept Layer-1,-2,-3, whereas a Layer-2 decoder may accept only Layer-1 and -2.) Ą@
Q4 :	So we have a family of three audio coding schemes. What does the MPEG standard define, exactly?
A4 :	For each Layer, the standard specifies the bitstream format and the decoder. To allow for future improvements, it does not specify the encoder, but an informative chapter gives an example for an encoder for each Layer. Ą@
Q5 :	What have the three audio Layers in common?
A5 :	All Layers use the same basic structure. The coding scheme can be described as "perceptual noise shaping" or "perceptual subband / transform coding". The encoder analyzes the spectral components of the audio signal by calculating a filterbank (transform) and applies a psychoacoustic model to estimate the just noticeable noise-level. In its quantization and coding stage, the encoder tries to allocate the available number of data bits in a way to meet both the bitrate and masking requirements. The decoder is much less complex. Its only task is to synthesize an audio signal out of the coded spectral components. All Layers use the same analysis filterbank (polyphase with 32 subbands). Layer-3 adds a MDCT transform to increase the frequency resolution. All Layers use the same "header information" in their bitstream, to support the hierarchical structure of the standard. All Layers have a similar sensitivity to biterrors. They use a bitstream structure that contains parts that are more sensitive to biterrors ("header", "bit allocation", "scalefactors", "side information") and parts that are less sensitive ("data of spectral components"). All Layers support the insertion of programm-associated information ("ancillary data") into their audio data bitstream. All Layers may use 32, 44.1 or 48 kHz sampling frequency. All Layers are allowed to work with similar bitrates: Layer-1: from 32 kbps to 448 kbps Layer-2: from 32 kbps to 384 kbps Layer-3: from 32 kbps to 320 kbps The last two statements refer to MPEG-1; with MPEG-2, there is an extension for the sampling frequencies and bitrates (see below). Ą@
Q6 :	What are the main differences between MPEG-1 and MPEG-2 in the audio part?
A6 :	MPEG-1 and MPEG-2 use the same family of audio codecs, Layer-1, -2 and -3. The new audio features of MPEG-2 are a "low sample rate extension" to address very low bitrate applications with limited bandwidth requirements (the new sampling frequencies are 16, 22.05 or 24 kHz, the bitrates extend down to 8 kbps), and a "multichannel extension" to address surround sound applications with up to 5 main audio channels (left, center, right, left surround, right surround) and optionally 1 extra "low frequency enhancement (LFE)" channel for subwoofer signals; in addition, a "multilingual extension" allows the inclusion of up to 7 more audio channels. Ą@
Q7 :	Are Layer-3 and MPEG-3 the same thing?
A7 :	No! Layer-3 is a powerful audio coding scheme which certainly is part of the MPEG standard. Layer-3 is defined within the audio part of both existing international standards, MPEG-1 and MPEG-2. But:There is no MPEG 3 defined. Ą@
Q8 :	How do I listen to MP3 file?
A8 :	Get a program to play mp3 file. If you are running Windows, we recommend Winamp. Of course, you need to have a sound card and speakers or headphones, you can find a portable play in this website product category.