MPEG Audio FAQ

MPEG-4 Audio: coding of natural and synthetic sound

Overview of MPEG-4

I'm looking for an introduction to MPEG-4 Audio?

Here is an introduction to MPEG-4 Audio:

Coding of Audio Objects

MPEG-4 Audio provides tools for coding of both natural and synthetic audio objects. It permits to represent natural sounds (such as speech and music) and to synthesize sounds based on structured descriptions. The representation for synthesized sound can be derived from text data or so-called instrument descriptions and by coding parameters to provide effects, such as reverberation and spatialization. The representations provide compression and other functionalities, such as scalability or play-back at different speeds.

Test Results:

The MPEG-4 Audio coding tools covering 6kbit/s to 24kbit/s have undergone verification testing for an AM digital audio broadcasting application in collaboration with the NADIB(Narrow Band Digital Broadcasting) consortium. With the intent of identifying a suitable digital audio broadcast format to provide improvements over the existing AM modulation services, several codec configurations involving the MPEG-4 CELP, TwinVQ, and AAC tools have been compared to a reference AM system. It was found that higher quality can be achieved in the same bandwidth with digital techniques and that scalable coder configurations offered performance superior to a simulcast alternative.

Natural Sound

MPEG-4 standardizes natural audio coding at bitrates ranging from 2 kbit/s up to and above 64 kbit/s. The presence of the MPEG-2 AAC standard within the MPEG-4 tool set will provide for general compression of audio in the upper bit rate range. For these, the MPEG-4 standard defines the bitstream syntax and decoding processes in terms of a set of tools. In order to achieve the highest audio quality within the full range of bitrates and at the same time provide the extra functionalities, three types of coding structures have been incorporated into the standard:

Parametric coding techniques cover the lowest bitrate range i.e. 2 - 4 kbit/s for speech with 8 kHz sampling frequency and 4 - 16 kbit/s and for audio with 8 or 16 kHz sampling frequency.

Speech coding at medium bitrates between about 6 - 24 kbit/s uses Code Excited Linear Predictive (CELP) coding techniques. In this region, two sampling rates, 8 and 16 kHz, are used to support both narrowband and wideband speech, respectively.

For bitrates starting below 16 kbit/s, time-to-frequency (T/F) coding techniques, namely the TwinVQ and AAC codecs, are applied. The audio signals in this region typically have sampling frequencies starting at 8 kHz.

To allow optimum coverage of the bitrates and to allow for bitrate and bandwidth scalability, a general framework has been defined. This is illustrated in Figure 3.

Figure 3: General Framework of MPEG-4 Audio.

Starting with a coder operating at a low bitrate, by adding enhancements, such as the addition of BSAC (Bit sliced arithmetic coding) to an AAC coder for fine grain scalability, both the coding quality as well as the audio bandwidth can be improved. These enhancements are realized within a single coder or, alternatively, by combining different coding techniques.

Scalability:

Bit rate scalability allows a bitstream to be parsed into a bitstream of lower bit rate that can still be decoded into a meaningful signal. The bit stream parsing can occur either during transmission or in the decoder. Bandwidth scalability is a particular case of bit rate scalability whereby part of a bitstream representing a part of the frequency spectrum can be discarded during transmission or decoding. Encoder complexity scalability allows encoders of different complexity to generate valid and meaningful bitstreams.

The decoder complexity scalability allows a given bitstream to be decoded by decoders of different levels of complexity. The audio quality, in general, is related to the complexity of the encoder and decoder used. Error robustness provides the ability for a decoder to avoid or conceal audible distortion caused by transmission errors.

Scalability works within some MPEG-4 tools, but can also be applied to a combination of techniques, e.g. with Twin VQ as a base layer and AAC for the enhancement layer(s).

The MPEG-4 systems layer facilitates the use of different tools and signaling this, and thus codecs according to existing standards can be accommodated. Each of the MPEG-4 coders is designed to operate in a stand-alone mode with its own bitstream syntax. Additional functionalities are realized both within individual coders, and by means of additional tools around the coders. An example of a functionality within an individual coder is pitch change within the parametric coder.

Synthesized Sound

Decoders are also available for generating sound based on structured inputs. Text input is converted to speech in the Text-To-Speech (TTS) decoder, while more general sounds including music may be normatively synthesized. Synthetic music may be delivered at extremely low bitrates while still describing an exact sound signal.

Text To Speech:

TTS coders bit rate range from 200 bit/s to 1.2 Kbit/s which allows a text or a text with prosodic parameters (pitch contour, phoneme duration, and so on) as its inputs to generate intelligible synthetic speech. It includes the following functionalities:

Note that MPEG-4 provides a standardized interface for the operation of a Text To Speech coder (TTSI = Text To Speech Interface) rather than a normative TTS synthesizer itself.

Score Driven Synthesis:

The Structured Audio tools decode input data and produce output sounds. This decoding is driven by a special synthesis language called SAOL (Structured Audio Orchestra Language) standardized as a part of MPEG-4. This language is used to define an "orchestra" made up of "instruments" (downloaded in the bitstream, not fixed in the terminal) which create and process control data. An instrument is a small network of signal processing primitives that might emulate some specific sounds such as those of a natural acoustic instrument. The signal-processing network may be implemented in hardware or software and include both generation and processing of sounds and manipulation of prestored sounds.

MPEG-4 does not standardize "a method" of synthesis, but rather a method of describing synthesis. Any current or future sound-synthesis method can be described in SAOL, including wavetable, FM, additive, physical-modeling, and granular synthesis, as well as non-parametric hybrids of these methods.

Control of the synthesis is accomplished by downloading "scores" or "scripts" in the bitstream. A score is a time-sequenced set of commands that invokes various instruments at specific times to contribute their output to an overall music performance or generation of sound effects. The score description, downloaded in a language called SASL (Structured Audio Score Language), can be used to create new sounds, and also include additional control information for modifying existing sound. This allows the composer finer control over the final synthesized sound. For synthesis processes which do not require such fine control, the established MIDI protocol may also be used to control the orchestra.

Careful control in conjunction with customized instrument definition, allows the generation of sounds ranging from simple audio effects, such as footsteps or door closures, to the simulation of natural sounds such as rainfall or music played on conventional instruments to fully synthetic sounds for complex audio effects or futuristic music.

For terminals with less functionality, and for applications which do not require such sophisticated synthesis, a "wavetable bank format" is also standardized. Using this format, sound samples for use in wavetable synthesis may be downloaded, as well as simple processing, such as filters, reverbs, and chorus effects. In this case, the computational complexity of the required decoding process may be exactly determined from inspection of the bitstream, which is not possible when using SAOL.

I'm looking for typical MPEG-4 Audio applications?

Here are three audio applications for MPEG-4 audio:

MPEG-4 language and abbreviations

What is an Audio Object in MPEG-4?

MPEG-4 defines audio objects as "real-world" objects.

A "real-world" audio object can be defined as an audible semantic entity (voice of one or more speakers, one or more instruments, etc.). It can be recorded with one microphone in case of a mono recording or with more microphones, at different positions, in case of a multichannel recording. Audio objects can be grouped or mixed together, but objects can not easily be split into sub-objects.

One single audio object can consist of more than one audio channel, if we define audio channels as information, specific for one loudspeaker position. For example, one MPEG-1 audio bitstream will be the coded representation of one object in MPEG-4. This object contains either 1 channel (mono mode) or 2 channels (dual, stereo or joint stereo mode).

What is "structured audio"?

Structured audio formats use ultra-low bit-rate algorithmic sound models to code and transmit sound. MPEG-4 standardizes an algorithmic sound language and several related tools for the structured coding of audio objects. Using these tools, algorithms which represent the exact specification of a sound scene are created by the content designer, transmitted over a channel, and executed to produce sound at the terminal. Structured audio techniques in MPEG-4 allow the transmission of synthetic music and sound effects at bit-rates from 0.01 to 10 kbps, and the concise description of parametric sound post-production for mixing multiple streams and adding effects processing to audio scenes.

Technicalities of MPEG-4

I´ve heard there is a "Version 2" of MPEG-4 Audio - what´s all this about?

The first version of the MPEG-4 Audio Standard was finalised in 1998 and provides tools for coding of natural and synthetic audio objects and composition of such objects into an "audio scene". Natural audio objects (such as speech and music) can be coded at bitrates ranging from 2 kbit/s to 64 kbit/s and above using Parametric Speech Coding, CELP-based Speech Coding or transform-based General Audio Coding. The natural audio coding tools also support bitrate scalability. Synthetic audio objects can be represented using a Text-To-Speech Interface or the Structured Audio synthesis tools. These tools are also used to add effects, like echo, and mix different audio objects to compose the final "audio scene" that is presented to the listener.

Because of the very tight schedule of the MPEG-4 standardisation, several promising tools proposed for MPEG-4 were not mature enough to be included in the first version of the standard. Since many of these tools provide desirable functionalities not available in MPEG-4 Version 1, it was decided to continue the work on these tools for an extension of the standard, MPEG-4 Version 2.

With this extension, new tools are added to the MPEG-4 Standard, while none of the existing tools of Version 1 is replaced. Version 2 is therefore fully backward compatible to Version 1, as depicted in Figure below.

In the area of Audio, new tools are added in MPEG-4 Version 2 to provide the following new functionalities:

In the Systems part of MPEG-4, Version 2 specifies a file format to store MPEG-4 encoded content. Besides other new tools, also a backchannel for dynamic control and interaction with a server is specified.

Which additional functionalities are covered by MPEG-4?

While "high coding efficiency" is of course a major functionality of MPEG-4, here are some examples of so-called "additional functionalities": speed control, pitch change, error resilience, scalability. MPEG-4 addresses several types of scalability:

  1. Bit rate scalability allows a bitstream to be parsed into a bitstream of lower bit rate that can still be decoded into a meaningful signal. The bit stream parsing can occur either during transmission or in the decoder.
  2. Bandwidth scalability is a particular case of bit rate scalability whereby part of a bitstream representing a part of the frequency spectrum can be discarded during transmission or decoding.
  3. Encoder complexity scalability allows encoders of different complexity to generate valid and meaningful bitstreams.
  4. The decoder complexity scalability allows a given bitstream to be decoded by decoders of different levels of complexity. The audio quality, in general, is related to the complexity of the encoder and decoder used.
  5. Error robustness provides the ability for a decoder to avoid or conceal audible distortion caused by transmission errors.

What is "audio scene description"? How does it work in MPEG-4?

As video scenes are made from visual objects, audio scenes may be usefully described as the spatiotemporal combination of audio objects. An "audio object" is a single audio stream coded using one of the MPEG-4 coding tools, like CELP or Structured Audio. Audio objects are related to each other by mixing, effects processing, switching, and delaying them, and may be spatialized to a particular 3-D location. The effects processing is described abstractly in terms of a signal-processing language (the same language used for Structured Audio), so content providers may design their own empirically, and include them in the bitstream.

Why should I use an MPEG-4 speech coder instead of an ITU codec?

The ITU-T speech coders currently operate at 6.3/5.3 (G.723), 8 kbps (G.729),16 kbps (G.728), 32 kbps (G.721) 48/56/64 Kbps (G.722). MPEG-4 speech coders operate at bit rates between 2 - 24 kbps for the 8 kHz mode and 14-24 kbps for the 16 kHz mode.

Currently ITU-T coders do not operate at bitrates as low 2 kbps for the 8 kHz mode, or 14-24 kbps for the 16 kHz mode. Furthermore, MPEG-4 speech coders provide bitrate scalability, complexity scalability and multi-bitrate operation from 2 - 24 kbps. The coding quality of the coder is comparable to that of the ITU coder at corresponding bitrates. MPEG-4 is standardising a speech coder which can operate down to 2.0kbps. This will be the lowest bit rate international standard. ITU standards do not support this low bit rate. The quality at 2.0 kbps is "communication quality" and could be used for usual conversation, and better than FS1016 4.8kbps coder.

You could utilise the benefits of the MPEG-4 speech coder in various applications. The MPEG-4-based internet-phone system will offers robustness against packet loss or change in transmission bitrates. When applied in an audio-visual system, the coding quality is improved by assigning the audio and the visual bitrates adaptively based on the audio-visual contents. The MPEG-4 speech coder could be also used in a radio communication system, where the error robustness is enhanced by changing the bit-allocation between speech coding and error correction depending on error conditions. These features have not been realized by any other standards.

Low bit rate is, furthermore, useful for "Party talk". Even though 10 people are making conversation simultaneously over Internet, one terminal only has to receive 18kbps of bitstream for 9 other people's talk.

Another thing is that usually ITU-T is focusing on real time communication. However, MPEG-4 low bit rate speech coder is also looking at storage media. The coder has speed change (without changing pitch nor phoneme) capability and it's very useful for very fast speech database search/access, where people can identify the content of speech even when the speed is doubled. The coder also has pitch change capability. These features are not supported by other speech coding standards.

Finally, as the MPEG-4 speech coders are standardised within the MPEG-4 audiovisual framework, they interoperate with all of the other MPEG-4 capabilities, such as transport from distributed servers, synchronisation with synthetic music or visual images, and parametric effects post-processing. These capabilities are not provided by standards whose only domain is speech coding.

What is HVXC?

HVXC stands for "Harmonic Vector eXitation Coding", which is a very efficient speech coding algorithm for low bit-rate speech coding at 1.5 to 4.0 kbit/s. MPEG-4 Version 2 adds a variable Bitrate mode (VBR) mode to HVXC's capabilities.

What are WB-CELP and NB-CELP in MPEG-4 Audio?

The CELP speech coding algorithm in MPEG-4 can operate at 2 sampling rates: 8 kHz and 16 kHz. The former one is often referred to as Narrowband (NB) CELP, the latter one as Wideband (WB) CELP.

What is TwinVQ?

TwinVQ (Transform domain Weighted INterleave Vector Quantization) ) is a name of audio coding technique and is one of the object types defined in MPEG-4 Audio Version 1. This object type is based on a general audio transform coding scheme which is integrated with the AAC coding frame work, a spectral flattening module, and a weighted interleave vector quantization module. This scheme has high coding gain for low bitrate and potential robustness against channel errors and packet loss, since it does not use any variable length coding and adaptive bit allocation. It supports bitrate scalability, both by means of layered TwinVQ coding and in combination with the scalable AAC.

Note that some commercialized products such as Metasound (Voxware), SoundVQ (YAMAHA), and SolidAudio (Hagiwara) are also based on the TwinVQ technology, but the configurations are different from the MPEG-4 TwinVQ.

What is BSAC?

BSAC stands for bit sliced arithmetic coding that provides one of the forms of scalability in MPEG-4 audio. In order to make the bitstream scalable, BSAC uses an alternative to AAC noiseless coding module, although the other coding modules are identical to AAC. A bitstream encoded by AAC can be transcoded to an BSAC bitstream noiselessly. BSAC is capable of generating a bitstream with a precise bit rate control in the range of 16kbps to 64kbps per channel. This bit rate enables the decoder to stop anywhere between 16kbps and the encoded bit rate with a 1kbps step size. Through use of this scalablity, the user can experience nearly transparent sound quality at 64kbps and graceful degradation at lower bit rates. BSAC is best performed in the range of 40kbps to 64kbps, though its operating range is 16kbps and 64kbps.

What is "parametric audio coding"?

Parametric audio coding utilizes a very compact, parametric representation of the audio signal which is based on the decomposition of the input signal into basic components, such as sinusoids and noise. In MPEG-4, HVXC and HILN coders are combined to provide very efficient parametric coding of speech and music at very low bit-rates, respectively. Both speed and pitch change functionality is provided by MPEG-4 parametric audio coding.

What is HILN?

HILN stands for "Harmonic and Individual Lines plus Noise", which is an efficient parametric audio coding algorithm for very low bit-rate audio coding at 4.0 to 16.0 kbit/s.

What is "silence compression"?

When silence compression is being applied, silent/almost silent parts of the input signals are coded more efficiently than active parts. Silence compression can be applied in MPEG-4 Version 2 CELP coding. The bit rate for these silent parts becomes less than 200 bps when silence compression is being used.

What is a low delay coder?

A low delay audio coder provides data compression while keeping the delay caused by the encoding / decoding chain low enough to enable two-way communication applications, such as telephony and videoconferencing (typically around 30 ms for ideal processing). To this end, MPEG-4 provides both speech coders and the Low-Delay AAC coder. The latter is a derivative of the standard AAC coder which is defined in MPEG-4 Version 2 to enable high quality low delay audio coding for general audio signals including speech.

What is error robustness?

Error robustness means that the encoded audio data can be transported over error prone channels without unacceptable loss in signal quality. MPEG-4 Version 2 realizes this important functionality with two kinds of techniques. One is called "Error Resilience". The MPEG-4 Audio Version 2 coding algorithms have the ability to generate such error resilient bitstreams that can be decoded with minimal degradation even if they are partially corrupted, if the corrupted part is distinguished. The other technique is called "Error Protection" (see below).

What is error protection?

MPEG-4 Version 2 contains the Error Protection (EP) Tool which can be used by all MPEG-4 audio coding algorithms and is able to adapt to a wide range of channel error conditions. The main features of the EP tool are as follows:

MPEG-4 Audio Version 2 coding algorithms provide a classification of each bitstream field according to its error sensitivity. Based on this, the bitstream is divided into several classes, which can be separately protected by the EP tool, such that more error sensitive parts are protected more strongly. This technique is known as UEP.

Can MPEG-4 be used on error-prone transmission channels?

Yes. MPEG-4 Version 2 specifically addresses this problem by providing means for "Error Robustness" for each of the MPEG-4 audio coders. See "error robustness".

What is TTS?

TTS is an abbreviation of the Text-To-Speech conversion system. Originally, TTS synthesizes speech as its output from input text data. In other words, the TTS translates the text into a string of phonetic symbols and the corresponding basic synthetic units are retrieved from a database. Then the TTS concatenates the synthetic units to synthesize the output speech with the rule-generated prosody.

What is MPEG-4 TTS interface?

The MPEG-4 TTS interface (TTSI) is designed to pass TTS data to a speech synthesizer within the MPEG-4 framework. Beyond synthesizig speech according to the input data with a rule-generated prosody, it also enables several other functions. They are, 1) speech synthesis with the original prosody from the original speech, 2) synchronized speech synthesis with Facial Animation (FA) tools, 3) synchronized dubbing with moving pictures not by recorded sound but by text and some lip shape informations, 4) trick mode functions such as stop, resume, forward, backward without breaking the prosody even in the applications with Facial Animation (FA)/ Motion Pictures (MP), and 5) users can change the replaying speed, tone, volume, speaker's sex, and age.

Can MPEG-4 TTSI support various languages in the world?

Yes. We adopted the concept of the ISO 639 language code. In addition to this, we include the International Phonetic Alphabet (IPA) code to send phoneme symbols.

Why does MPEG-4 TTS interface use IPA (International Phonetic Alphabet) as its standard representation for phoneme symbols?

Because there are a lot of different languages in our world, no single language's phoneme symbols can cover all possible human speech sounds. The only available method to represent all various phonemes in this world is to use IPA.

What are the application areas of MPEG-4 TTSI?

Some application areas for MPEG-4 TTSI are as follows:

What is standardized for MPEG-4 TTSI?

qFor MPEG-4 TTSI, only the interface bit stream profiles are the objects of standardization. Because there are already many different types of TTS and each country has several or a few tens of different TTSs synthesizing its own language, it is impossible to standardize all the things related to TTS. However it is believed that almost all TTSs can be modified to accept MPEG-4 TTS interface bit stream profile in an hour or two by a TTS expert because of the rather simple structure of the MPEG-4 TTS interface bit stream profiles.

How can MPEG-4 TTS interface operate with FA tools synchronously?

MPEG-4 TTS interface passes phoneme symbols with their duration and average pitch information to the phoneme-to-FAP (Facial Animation Parameter) converter. Then the phoneme-to-FAP converter generates FAPs with its duration for the corresponding phoneme and passes the information to FA tools. From this information FA tools can generate face images in synchronization with synthesized speech.

How do I encode into an MPEG-4 Structured Audio format?

You can't, today. The techniques required for automatically producing a Structured Audio bitstream from an arbitrary sound are beyond today's state of the art, although they are an active research topic. These techniques are often called "automatic source separation" or "automatic transcription"; there are many references within the audio processing literature on the capabilities of today's methods. In the mean time, content authors will use special content creation tools to directly create Structured Audio bitstreams. This is not a fundamental obstacle to the use of MPEG-4 Structured Audio, because these tools are very similar to the ones that content authors use already; all that is required is to make them capable of producing MPEG-4 output bitstreams.

What synthesis method does Structured Audio use for music synthesis?

MPEG-4 does not standardize a synthesis method, but a signal-processing language for describing synthesis methods. Using this language, any current or future synthesis method may be described by a content provider and included in the bitstream. This language is entirely normative and standardized, so that every piece of synthetic music will sound exactly the same on every compliant MPEG-4 decoder, which is an improvement over the great variety in MIDI-based synthesis systems.

What is the complexity of a Structured Audio decoder?

There is no fixed complexity which is adequate for decoding every conceivable Structured Audio bitstream. Simple synthesis methods are very low-complexity, and complex synthesis methods require more computing power and memory. Since the description of the synthesis methods is under the control of the content provider, the content provider is responsible for understanding the complexity needs of his bitstreams. Past versions of structured audio systems with similar capability have been optimized to provide multitimbral, highly polyphonic music and post-production effects in real-time on a 150 MHz Pentium computer or simple DSP chip.

One "level" of capability in the Synthetic Object Audio profile of MPEG-4 Audio provides for simpler and/or less normative synthesis methods in RAM- or computing-limited terminals.

What is SAOL?

SAOL, pronounced "sail", stands for "Structured Audio Orchestra Language" and is the signal-processing language enabling music-synthesis and effects post-production in MPEG-4. It falls into the music-synthesis category of "Music V" languages; that is, its fundamental processing model is based on the interaction of oscillators running at various rates. However, SAOL has added many new capabilities to the Music V language model which allow for more powerful and flexible synthesis description.

There is a WWW page (http://sound.media.mit.edu/~eds/mpeg4) for composers and sound designers interested in SAOL.

What is the Structured Audio Sample Bank Format (SASBF)?

SASBF is part of the Structured Audio tool in MPEG-4. It provides for powerful wavetable (or sampling) synthesis capabilities. Wavetable synthesis produces very good results when synthesizing timbres of natural instruments. The technique uses a relatively small number of PCM recordings of notes, called "wavetables", as the basis for synthesizing all the notes required for rendering. In synthesis, the wavetables are pitch shifted, filtered and enveloped. The particular types of pitch shifting, amplitude modulation (envelopes) and dynamic filtering used in SASBF allow shifting over a broad range of pitches.

SASBF provides wavetable synthesis features such as:

Three main features provide data compression:

SASBF can work either as an element ("opcode") in SAOL or in a stand-alone profile more suitable for ASIC implementation.

Can MPEG-4 give me 3-D Audio spatialization?

Yes. MPEG-4 Version 2 provides 3-D sound positioning, modeling of source directivity, propagation of sound in the air, and the room acoustic response. Room acoustic modeling can be done according to two different approaches, namely physical or perceptual. The former relies on physical (geometrical) description of acoustic characteristics of the space, such as, reflectivity of walls or reverberation time within a specified region in the space. In the latter, each sound source is characterized by perceptually relevant parameters (such as source presence, room envelopment and late reverberance) which are used to synthesize a room acoustic response for that source.

What does Audio-Composition means?

MPEG-4 contains mechanisms of handling the functionality of the compositor for both 'Basic composition' and 'Advanced effects'. 'Profile 1' handles just synchronisation and routing. 'Profile Full composition' covers additionally mixing, tone control and sample rate conversion. 'Profile Advanced effects' includes reverb, spatialisation, flanging, filtering, compression, limiting, dynamic range control etc.. Profiles 1 and Full can both be handled within the system part of MPEG-4, i.e. outside the audio decoder. Profile Advanced effects is handled in the audio/SNHC compositor, the Structured Audio/Effects (SAFX) box.

Are there new implementations of MPEG-4 Stuctured Audio?

Yes. John Lazzaro and John Wawryznek of the Computer Science division of the University of California have been working on a translator from MPEG-4 Structured Audio to C, and it's stable enough now for a developer release. You can download it from

http://www.cs.berkeley.edu/~lazzaro/sa/index.html

Here are a few excerpts from the README:

Implementing MPEG-4 audio software

Under what circumstances can I use MPEG-4 reference software?

First, it should be understood that this software has the goal of describing and explaining the MPEG-4 standard.

The decoder software accurately implements one method of decoding MPEG-4 bit-streams. In this sense it is normative. However, there are other ways of correctly decoding MPEG-4 bit-streams. As long as the resulting difference in output from the output signal generated by this reference software is not larger than the limits set in the conformance part of the MPEG-4 standard, these other methods are also considered compliant decoders.

For the encoder, the purpose of this software is purely informative. It describes one example of possible encoding software producing bitstreams which comply to the MPEG-4 standard. This software is not optimized, neither in the speed of execution nor in the quality of the output signal. For some of the algorithms the picture or sound quality is similar to what is possible according to state-of-the-art encoding For other algorithms, (like the AAC t/f-encoding of audio signals) the encoder is just an illustration of techniques to be used and delivers an output quality far below the one which has been demonstrated in verification tests of MPEG-4.

The following copyright disclaimer is attached to the software modules in the reference software:

"This software module was originally developed by <FN1> <LN1> (<CN1>) and edited by <FN2> <LN2> (<CN2>), <FN3> <LN3> (<CN3>), … in the course of development of the <MPEG standard>. This software module is an implementation of a part of one or more <MPEG standard> tools as specified by the <MPEG standard>. ISO/IEC gives users of the <MPEG standard> free license to this software module or modifications thereof for use in hardware or software products claiming conformance to the <MPEG standard>. Those intending to use this software module in hardware or software products are advised that its use may infringe existing patents. The original developer of this software module and his/her company, the subsequent editors and their companies, and ISO/IEC have no liability for use of this software module or modifications thereof in an implementation. Copyright is not released for non <MPEG standard> conforming products. CN1 retains full right to use the code for his/her own purpose, assign or donate the code to a third party and to inhibit third parties from using the code for non <MPEG standard> conforming products. This copyright notice must be included in all copies or derivative works. Copyright 199_".

In the text <MPEG standard> should be replaced with the appropriate standard, e.g. MPEG-2 AAC (ISO/IEC 13818-7), MPEG-4 System (ISO/IEC 14496-1), MPEG-4 Video (ISO/IEC 14496-2), MPEG-4 Audio (ISO/IEC 14496-3). <FN> should be replaced by First Name, <LN> by Last Name, and <CN> by Company Name.

I heard about an MPEG-4 player, what is it?

The MPEG-4 player is the name used to describe the reference software that will interface and demonstrate the tools developed by all the MPEG subgroups. This software will be updated at each meeting, and will eventually include all the functionalities of MPEG-4. This software will become available as Part 5 of the standard.

Where can I obtain test pattern and compliance bitstreams for verifying the performance of an implementation of an MPEG audio tool?

Please see a web site being hosted by AT&T at http://www.research.att.com/projects/mpegaudio

Relation to other standards / methods

What does MPEG-4 Audio Version 1 provide above MPEG-2 Audio?

MPEG-4 Audio Version 1 integrates the worlds of synthetic and natural coding of audio. The synthetic coding part is comprised of tools for the realisation of symbolically defined music and speech. This includes MIDI and Text-to-Speech systems. Furthermore, tools for the 3-D localisation of sound are included, allowing the creation of artificial sound environments using artificial and natural sources. MPEG-4 Audio also standardises natural audio coding at bit rates ranging from 2 kbit/s up to 64 kbit/s. In order to achieve the highest audio quality within the full range of bit rates, three types of codecs have been defined: a parametric codec for the lower bit rates in the range, a Code Excited Linear Predictive (CELP) codec for the medium bit rates in the range, and Time to Frequency (TF) codecs, including MPEG-2 AAC and Vector-Quantiser based, for the higher bit rates in the range. Furthermore, a number of functionalities are provided to facilitate a wide variety of applications which would range from intelligible speech to high quality multichannel audio.

What is the difference between MPEG-2 AAC and MPEG-4 AAC?

MPEG-4 AAC has all the tools and functions of MPEG-2 AAC plus the Perceptual Noise Substitution (PNS) tool, a Long Term Predictor (LTP) as well as extensions to support scalability. Therefore all MPEG-4 AAC decoders can decode MPEG-2 AAC bitstreams, and conversely MPEG-2 AAC decoders can decode MPEG-4 AAC bitstreams if none of the MPEG-4 extensions are signalled.

What is MPEG-4 Version 2?

Compared to Version 1, additional tools are added in Version 2. These are: Error robustness which consists of Error Resilience (ER) and Error Protection (EP), Bit Slice Arithmetic Coding (BSAC) for fine-grain bitrate scalability, Low Delay AAC (AAC-LD) for coding of general audio signals with low delay, Harmonic and Individual Lines plus Noise (HILN) in stand alone mode as well as in combination with the parametric speech coding scheme HXVC .

Bibliographic references

See also: MPEG Audio - General Documents, Publications, and Presentations

Organisational details

What is the status of the MPEG-4 Version 1 Standard?

MPEG-4 Audio Version 1 was finalized in October 1998 and published in 1999.

What is the status of the MPEG-4 Version 2 Standard?

MPEG-4 Audio Version 2 was finalized in December 1999 and will be published in 2000.

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Heiko Purnhagen 10-Mar-2000